The DPI calculator for printing and scanning dpi is on the previous page, but there are some basics that can be a concern, which is this page. It was split up here because the one page was getting too big.

The image dimension in pixels (Image Size) is the important detail for using any image. Around 300 pixels per inch is the optimum and standard proper printing goal for photographs. 200 dpi can sometimes be acceptable printing quality, but more than 300 dpi is not of much use to printers (for color photos), because our printers are not designed to do more (for color work) and our eyes cannot see greater detail (color work). Many local 1-hour photo lab digital machines are usually set to print at 250 pixels per inch, but it won't hurt to always provide pixels to print 300 dpi. 250 to 300 dpi is a reasonable and optimum printing resolution for photos.

FWIW, I'm old school, and I learned the term for printing resolution as "dpi", so that's second nature to me, dpi has simply always been the name of it. Some do call it ppi now, same thing, pixels per inch, which is what it is. But pixels are colored dots, and the jargon name of Dots Per Inch is what it has always been called. Ink jet printers do have their own other thing about ink drops per inch which they also call that dpi (but which is about the quality of dithering colors (to color each pixel), not about image resolution). But here, we're speaking about printing and scanning resolution of image pixels, which ink jets also have to do to scan or print it. Ink jet printers used to call dithering drops of ink per inch as dpi, but that seems to be called a "good, better, best" type rating now.

Things to be sure you know about printing

There are some background concerns. There are two situations when printing images, depending on if using print shops or home printing.

• The one hour photo printing shops where we order prints will offer a paper size, and will fill that paper with your image. They will ignore your dpi number already in the image file, and will recompute their own necessary "pixels per inch" value, to scale your image to their paper size. If you order 8x10 inches, you will get 8x10 print size, but if your image does not match the paper shape, a bit of it might be cut off. And you do need to give them enough pixels to print well. Planning ahead to avoid surprises by first cropping the image to match that paper shape, and also to provide enough pixels so that the result will be 250 to 300 pixels per inch will be a very good plan.

  If your image dimensions (pixels) are too large, the photo shop will first resample it to this smaller requested size. That's not necessarily a problem, except a far too-large image will be slow to upload. Or, if too small (insufficient pixels provided), the print quality will be lower, and the lab may refuse worst cases. But if the provided image shape does not match the paper shape, the excess image outside the paper edges simply disappears, results may not be what you expected.

• If printing yourself at home, the Print menu in your photo editor normally does use the file's scaled image dpi number (pixels per inch) to size the images on paper (regardless if it matches the paper size at all). But it typically will also allow changing that dpi, called scaling (to fit the paper size). For example, if an image dimension is 3000 pixels, then specifying that file number as 300 dpi printing resolution will print it to be 3000/300 = 10 inches print size (even if the paper is only 4x6). But the dpi number that your digital camera initially stores in the image file, unless you have reset it to your planned value, is otherwise far from meaningful, it is just some arbitrary number, which will print SOME size, but not likely to be your own printing goal. Hopefully, you have already properly scaled the image for your selected paper size.

  Most photo editors will also provide an option to "Scale to fit media" or "Best fit to page", which will scale (one dimension of) the image to fit on the specified paper size (similar to the labs above). This can be borderless if so specified in the printer Properties. This scaling will print at a new dpi which will fit the paper size. But it will not necessarily fit the paper "shape", which requires cropping attention done by you.

Any digital image has four sizes for different purposes:

• Image Size is in pixels and is the size it is (and is the necessary and most important and useful thing to know about how you can use the imaage).
• Data Size (when uncompressed in computer memory) is in Bytes (megabytes, MB).
• File Size (often compressed, always in JPG files, then becoming tremendously smaller for example) is in Bytes (MB). Too much JPG compression is NOT a good thing. Too much JPG compression can damage the image quality, unrecoverable unless you go back to the archived original camera image.
  File Size is about storage, and possibly is a hint about image quality, but IT IS NOT a measure of useful Image Size. You need to know pixels. Image size is in pixels.
• Print Size on paper is in Inches or mm of paper. Printing or Scanning is about Image Size in Pixels. Image resolution DPI is pixels per inch, which distribution rate determines the inches or mm a certain number of pixels will cover on paper. For example, 3000 pixels printed at 300 pixels per inch will cover 10 inches of paper (or 20 inches at 150 dpi). But your selected print paper SHAPE may not match it. It is just that simple.

  But image size in screen video is only about pixels. The screen is also dimensioned in pixels to accept it, and dpi is Not used in video.

Image Size is absolutely only about pixels, but file compression or the image mode of Color, or Grayscale, or Line art, or Indexed color, or Raw, all will make a big File size difference (bytes). See another calculator that will compute these four sizes.

Basic First Printing and Scanning Necessary Facts

This is what you need to know about preparing your image for printing or viewing, and you do need to know. It's easy, and you will become Near Expert (it is certainly more than most people know about printing). Some people will be very interested to know how to manage their images, but others can't be bothered with it.

Printing existing digital images: Aspect Ratio is a SHAPE, NOT A SIZE. Aspect is merely the ratio of the image dimensions, simply the long side dimension divided by the short side, which is a SHAPE. Print paper of course needs to be the same SHAPE.
For example, a 6x4 inch print is Aspect Ratio 6/4 = 1.5, meaning 1.5:1 ratio of the two sides, and 6/4 reduced fractionally is also commonly "reduced" to be called 3:2 Aspect Ratio (is the same thing, 6/4 = 3/2 = 1.5:1).
Compact cameras and cell phone cameras typically make images of Aspect Ratio 4:3 (1.333:1) which is 5.33x4 inch or 8x6 inch paper (5.33 / 4 is 1.333:1). The larger DSLR cameras typically make aspect ratio 3:2 (1.5:1) images, which does match the shape of 6x4 inch or 12x8 inch paper (6/4 is 1.5:1).
A 4x5 or 8x10 inch print paper is Aspect 5/4 = 1.25, which is 1.25:1 ratio.
A 5x7 inch or 10x14 inch print paper is Aspect 7/5 = 1.4, which is 1.4:1 ratio.
Your wallet sleeve size accepts wallet size prints that were conventionally half the dimensions of 5x7 paper, or 3.5x2.5 inches. A few better shops do still print that size (5x7 inch images still print those well without cropping off areas), but today, most local one hour shops print 3x2 inch wallets, sized for 6x4 images to print them. It is advised to ask walled print size before cropping your images to fit them.

Very few available print paper sizes will match the camera image shape, so the necessary and good plan for Printing is next:

1. FIRST, (First is important) crop a COPY of the image to the aspect ratio shape to match the print paper shape. The huge advantage is that you can see and choose what content will be cropped away, instead of letting the automatic one hour machines to just chop off some edge regardless of content.There are no surprises if you do it yourself. And you can also improve your picture by cropping away any unwanted content, such as any wasted blank space (with nothing to show) around the object you want to show better (will be larger). The free editors never seem to have this better crop tool (to crop to an aspect ratio shape), but it is necessary to match the print paper. The better photo editors do have it (Adobe Elements is one, good and not too expensive). These have a crop feature so that any crop you mark will be the specific aspect ratio that you declared for your paper size (or for your HD TV screen shape, which is 16:9 aspect). Then you simply slide it around and resize the crop box for your subject you want to show, and adjust it so it shows best. This is extremely important to the best display.

   I can think of only two warnings: 1. NEVER overwrite your original image, because then it's gone, period. Archive the original and only work on a copy of the original file. And 2. Cropping too extreme could leave too few pixels to print well. Our large 24 megapixels will accept significant cropping, but be sure you will have enough pixels left to print. For example, an 8x10 inch 300 dpi image needs (8 inches x 300 dpi) x (10 inches x 300 dpi) = 2400x3000 pixels. So starting from a 6000x4000 pixel is surely extremely adequate, but there can always be extremes.

2. Then SECOND, after cropping to shape, resample the too large camera image to correct printing size. Specifying 300 dpi in the Resample tool will now produce that size. For example, if you have cropped it to 3:2 Aspect shape for a 6x4 inch print but the image is still too is still large (excessive pixel dimensions), resampling the 3:2 image to 300 dpi gives (6 inches x 300 dpi) x (4 inches x 300 dpi) = 1800 x 1200 pixels, which is perfect to print 6x4 at 300 dpi. Or if any trouble, you can just resample either side to its appropriate 1800 or 1200 pixel dimension, and because it is cropped to shape, it will be prefect. Or if it is the correct shape (for 6x4), you can just tell the one hour printing service you want 6x4 inches, and their automation will do it.
3. CROP is a SHAPE. Then RESAMPLE is a SIZE. Both are important. Crop a COPY of the image to shape first, and then resample to size. Size need not be exact, and 250 dpi is acceptable too. But very much less starts looking unsharp (too fuzzy). Or more than 300 dpi does still work, but 300 dpi is all that can help printing, because neither the printer nor our eyes are designed for more (300 dpi is best for viewing at 10 inches or 250 mm. Viewing at further distances need less dpi).

   Another big plus possible during this cropping is that you can often significantly improve your picture composition considerably by optionally also cropping out any objectionable or wasted areas (devoid of any content you want to show) at the same time. Assuming you care about your pictures, that is a big deal. Better editors have a crop tool option to to crop to an aspect ratio... that is, any crop box you can draw the will be the selected aspect

4. Enlargement: Don't go crazy with high dpi on the scanner. High dpi is used for enlargement, to create enough necessary pixels. High dpi is used for detail, but without enlargement, you can't see it. Use what is necessary for the job, but more is simply an unnecessary burden.

   To print at 300 dpi:
   A 6x4 inch print needs 1800×1200 pixels, 2.16 megapixels
     That idea is (6 inches×300 dpi)×(4 inches×300 dpi) = 1800x1200 pixels.
   A 5x7 inch print needs 1500×2100 pixels, 3.15 megapixels
     That idea is (5 inches×300 dpi)×(7 inches×300 dpi) = 1500x2100 pixels.
   A 8x10 inch print needs 2400×3000 pixels, 7.2 megapixels
     That idea is (8 inches×300 dpi)×(10 inches×300 dpi) = 2400x3000 pixels.
   A 12x18 inch print needs 3600×5400 pixels, 19.44 megapixels

   A 1920x1080 pixel image for the HD TV is 2.07 megapixels (video screens are dimensioned in pixels, with no concept of dpi).  Crop first to the 16:9 aspect ratio to show what you want to show, and then resample smaller to the 1920x1080 pixel size.

   Dpi is about spacing the pixels over the size.
   Pixels are colored dots and dpi (dots per inch) is jargon for pixels per inch.

   Scanning uses (inches scanned × scan dpi) to create the pixels.
     6 inches scanned at 300 dpi is 6×300 = 1800 pixels.
     Each inch is covered with 300 pixels.
     You can scan at higher dpi to have more pixels to print it enlarged.
   And it's a good plan to scan at a little higher dpi, maybe at the next even scanner menu number, to have a choice of pixels to crop away for the best image composition.

   Printing uses (pixels / printing dpi) to determine the inches printed.
     Printing 1800 pixels at 300 dpi fills 6 inches of paper.
     Each one inch of print paper is covered with 300 pixels.
     You can enlarge an image by printing at lower dpi but that reduces the resolution.
     Better to scan it larger first, to have the pixels to enlarge it.

   Easy so far, and Video is even easier, but very different than scanning or printing.
   Printing and scanning use paper, which is dimensioned in inches or mm.
   Video uses a screen dimensioned in pixels. Video has no use for inches or dpi.
   Regardless if the video screen is a cell phone 2 inches wide,
   or a wall TV 70 inches wide, both show the same 1920x1080 pixel video images.
   Video just places the image pixels on its screen of pixels without using dpi.
     A half-size 920×540 pixel image directly fills 1/4 of screen area of a
     1920×1080 pixel screen (half of width, half of height = 1/4)
     The pixels are reproduced one for one (images too large are reduced to fit).
   If scanning 6x4 inches for the HDTV, then 1920 pixels / 6 inches = 320 dpi minimum.
   And 4 inches at 320 dpi is 1280 pixels, but video is only 1080 pixels tall, so 200 pixels (15%) must be cropped off of the vertical dimension. That's easy in some images, and impossible in others.

   When you get it (it's very easy), you're ready to go with digital images.

   Qualifying video images just a bit, the images prepared for video were of course scanned, likely by a camera, and size of the camera sensor needed dpi (pixels per inch) to make the 1920x1080 pixel dimensions. That will include you too, if scanning a 6x4 inch photo to show on a 1920x1080 pixel video screen. Scanning uses inches and dpi, but video only cares about the image size in pixels. The cell phone screen likely was constructed using 400 or 500 pixels per inch, and the large wall TV screen uses maybe 30 pixels per inch, but both only show the 1920x1080 pixel images in whatever space they have. The space does not matter, they just have 1920x1080 pixels to show.

5. Copy work:   Color photos scanned at 300 dpi and then printed at 300 dpi will print Optimally as a Copy at the Same Original size.  Greater scan resolution is only needed to create sufficient pixels for greater enlargement. However, black and white text pages will copy better scanned and printed at 600 dpi. Film and digital camera files enlarge well, but photo prints simply don't enlarge much. Printing of color is designed for 300 dpi. For example to print 8x10 inches from a 35 mm film slide scan size is near a 8x enlargement, needing 8x more scan dpi to create sufficient pixels to cover the 8x10 inch area. 8x10 and 10x8 are the same size in all respects, you simply rotate the paper. Never seen a 35 mm film slide? They are the size of a wide postage stamp. The purpose of the very high scan resolution is to enlarge tiny things.

   The vast majority of your color photo Copy work should be at scanned and printed at 300 dpi. Greater scan resolution is only needed to create sufficient pixels for greater enlargement.  But don't expect photo prints to enlarge much very well, because they don't. Printing 2x size can only do 1/2 the resolution. A 2x sized copy of a photo print possibly might be marginally acceptable (viewed at a little distance), but 3x won't look good. A film image is designed for enlargement and does vastly better than from a printed photo. The problem is that a color printer is not designed for its print to be enlarged, but film is. The movies showing newspaper photos solving a murder case when viewed greatly enlarged are just fiction, newspapers certainly do NOT work that way (but they might try enlarging the original picture film though). But an original 24 megapixel image file has lots of pixels that can be enlarged quite a bit (and still have 300 dpi resolution). One good exception is that black & white text documents (or other line art work) are better scanned and printed at 600 dpi (but not photos). Your home printer can pretty nearly do 300 dpi color or 600 dpi line art. But a scanned color photo print paper itself generally can't do more than 300 dpi, and many of the local one-hour print shops don't do more than 250 dpi.

Or if showing images on the HD TV screen, that is 16:9 aspect ratio and 1920x1080 pixel size (2.07 megapixels). Images larger than 1920x1080 pixels can be too slow to load in the TV, and the TV must resample them smaller. Image dpi has no meaning on a video screen (the video screen only shows pixels and totally ignores any dpi number in the image file). FWIW, a 1920x1080 pixel image on a 24 inch diagonal screen shows at 92 dpi (screen pixels), and on a 65 inch diagonal screen is 34 dpi (but we sit further back — We do not view TV from 10 inches.) The 1920x1080 pixel image shows properly on any size HD video screen. The originating TV station does not know which size screen you use, but they will transmit 1920x1080 pixel images (or 1280x720 pixels in some cases, depending on source).

Reducing a COPY of your images to 1920x1080 pixels will each load much faster when showing on the HD TV.

For either printing or HD TV, the important thing to know and remember is to check your printing results this way:
Fill in any appropriate numbers, but prints are to be prepared this way:

6x4 inches is (6 inches x 300 dpi) x (4 inches x 300 dpi) = a 1800 x 1200 pixel image — 3:2 or 1.5x1 Aspect
5x7 inches is (5 inches x 300 dpi) x (7 inches x 300 dpi) = a 1500 x 2100 pixel image — 7:5 or 1.4x1 Aspect
8x10 inches is (8 inches x 300 dpi) x (10 inches x 300 dpi) = a 2400 x 3000 pixel image — 5:3 or 1.25x1 Aspect
8x8 inches square is (8 inches x 300 dpi) x (8 inches x 300 dpi) = a 2400 x 2400 pixel image — 1x1 Aspect
All metric sizes are (Width x 1.414 * 300 dpi) x (Height x 1 * 300 dpi) — 1.414x1 Aspect (1.414 is √2)
The 3x2 inch wallet paper prints well with 6x4 inch images (both 3:2)
The 3.5x2.5 inch wallet paper prints well with 5x7 inch images (both 5:7)

These are important numbers. Aspect ratio is normally expressed as Width x Height, but 2100x1500 and 1500x2100 pixels is the same thing, you simply rotate the paper properly, up and down or sideways (which called portrait or landscape orientation).

Enlargement For example (in general - speaking of any size original):

Scan at 600 dpi, print at 300 dpi, for 600/300 = 2X size (to print double size or 200% size)
Scan at 300 dpi, print at 300 dpi, for 300/300 = 1X size (to print original size or 100% size)
Scan at 150 dpi, print at 300 dpi, for 150/300 = 1/2X size (to print half size or 50% size)

Or scan small film at 2700 dpi, print at 300 dpi, for 2700/300 = 9X size. If from full frame 35 mm film (roughly 0.9 x 1.4 inches), then 9X is about 8x12 inches (near A4 size). Film is typically small, requiring more scan resolution for more pixels for more print enlargement. The reason to scan at high resolution is for "enlargement", specifically to create enough pixels to print a larger print at about 300 pixels per inch. Scanning larger than any reasonable future use is likely pointless.

This Enlargement is a concept called "scaling", and this enlargement concept is true for scanning anything of any size: photo prints, documents, film, etc.

This is so easy, it does seem trivial, but it is importantly basic. And the above FIRST and SECOND is the easy way to do it. But having a proper crop tool that can crop to an aspect ratio is all important to make it be trivially easy.

Most paper print sizes do not match camera image shapes, so printing will usually require cropping. The print paper shape will of course crop it in some random way, but if you care about what is left to see, you'll want to do it yourself first.

Basic facts are that common aspect ratio usages are:

• 3:2 (1.5:1) prints 6x4 inches or 12x8 or 18x12 or 3x2 inches, etc. Or metric sizes too, 15x10 cm. 35 mm film and DSLR camera images are also 3:2.
  Printing 6x4 inches at 300 dpi is (6 inches x 300 dpi) x (4 inches x 300 dpi) = 1800x1200 pixels.
• 5:4 (1.25:1) prints 5x4 inches or 10x8 inches, etc. You can say this as 10x8 or 8x10, it's the same paper shape, rotated.
  Printing 10x8 inches at 300 dpi is (10 inches x 300 dpi) x (8 inches x 300 dpi) = 3000x2400 pixels.
• 7:5 (1.4:1) prints 7x5 inches or 3.5x2.5 inch conventional wallet size. SIZE does not matter yet, this crop is about SHAPE. However much local one hour printing wallet size is 3x2 inches (3:2), Not 5x7 shape.
  Printing 7x5 inches at 300 d(pi is (7 inches x 300 dpi) x (5 inches x 300 dpi) = 2100x1500 pixels.

• 16:9 (1.78:1) are movie video images. HD TV is 1920 x 1080 pixels regardless if a cell phone or a 72 inch wall TV. All are 16:9 Aspect, 1920x1080 pixels, 2.07 megapixels. DPI has no use in video, video shows pixel for pixel. I'm not aware of any matching paper print sizes. Some camcorders can take single frame photo images much larger.

• √2:1 (1.414:1) are metric paper sizes, any of them, A2, A4, A6, etc.
  A4 is 210x297 mm or 8.27x11.69 inches and is (8.27 inches x 300 dpi) x (11.69 inches x 300 dpi) = 2481x3507 pixels.
• 4:3 (1.33:1) images are from compact and cell phone cameras. It's not a regular print size, most print paper is NOT 4:3, but some do offer 4x5.33 or 6x8 inches.
• 1:1 (1:1) square paper like 5x5 or 8x8 inches.

Again, some editor programs offer a very easy procedure to crop a COPY of the image to paper shape. They can assign the print paper aspect ratio to the crop box. Then any crop box size you can draw will be the correct SHAPE to match that paper shape of any size (dpi resolution will vary with size as pixels / inches, so your crop needs to have enough remaining pixels for the size). You can simply resize and move that crop box as desired to best select and show your image content best, but it still retains the right shape to fit the paper shape. See easy procedure with more detail. And then Resample is also shown as the last part of That page (it will still retain the new SHAPE).
Please realize that a "this easy crop to fit paper shape", and then the simple resample calculation (first small calculator on previous page) is all you need to know about resizing to print photos well. And you really don't need this or any calculator. Your photo editor will very easily crop to match the paper aspect ratio, and then its resample tool can specify (for example as) 8x10 inches at 300 dpi, which will compute to resample to 2400x3000 pixels (if it has been cropped to match the paper shape).

If you have no tools yet, a Free choice is the IrfanView viewer/edit program with menu Edit - Create Custom Selection that crops to print paper aspect ratio shape. Specify the paper aspect ratio and then draw that crop box however you want it (you can resize and move that crop box for the proper subject appearance in that crop box, but it still remains same paper shape). And Adobe Elements and Lightroom and Photoshop are particularly good for this, and surely most others do this too. See that procedure with more detail. I'm trying to make this clear, because it is a basic and very necessary procedure.

Scanning resolution

Just to be sure you are aware, Scaling is an option in the scanners menu that is a multiplier for resolution that scales output size. If you set the scan to 4x6 inches at 300 dpi at 200% scale, it will scan the 4x6 inches at 600 dpi (will create 2400x3600 pixels), but will set the image files dpi resolution value to the specified 300 dpi so that it will print 2x size or 8x12 inches size on paper at 300 dpi. That's the meaning of Scale, and the scanners meaning of Input and Output (what we scan, and what we get). While most scanner menu boxes don't show the 600 dpi number, it shows the 200%, and should show all of these inch and pixel numbers (scaling discussed more). This scaling is mentioned in the calculator Button 2 and 3 results, but below, I am speaking of 100% scale, which is NOT multiplied (100% scale multiplies scan resolution by 1, which has no effect).

When a scanner scans at 300 dpi, it creates 300 pixels per inch of dimension scanned. Scanning 8x10 inches at 300 dpi creates a 2400x3000 pixel image.

For scanning, we can calculate the exact numbers required. And we are able to scan at any non-standard numbers like say 375 dpi, or at 214% scale, and that does work well enough when we want it, but purists consider it better to scan a little larger, specifically at one of the scanner default menu resolution settings, like the 150, 300, 600, 1200, 2400, 4800 dpi values offered in the selection menu (assuming at 100% scale). Normally the next offered step larger than your desired size, and at 100% scale (no non-standard numbers). Then crop as desired, and resample smaller to the desired size. The reason is that the scanner hardware (the sensor pixels and especially the carriage stepping motor) can only do certain steps, only those specific offered standard values in the menu. Any other value is approximate resampling as the carriage moves, not exactly precise sampling. Then resampling in the photo editor already sees the larger pixel dimensions and is assumed to be able to do a better job. It is a small and hard-to-see difference (maybe it's a geeky thing, and maybe more important in the early days when scanner resolution was much lower), however it's reasonable that the photo editor can do this resample better, after it has all the image data.

So scan and then for printing preparation, FIRST crop a COPY of the image to paper shape. Crop as desired to both fit paper shape and also to adjust crop size and location to improve artistic composition — keep important detail, and crop away only the unimportant - Duh. 😊  But it is a choice that you can make while you are seeing it. You can make this crop be the best size on the image, and placed at the best location, but the shape will be fixed, matching the declared print shape. Then SECOND, resample that cropped image to be the smaller desired size to print (pixels, for example 3000 pixels for 10 inches at 300 dpi). Cropping to match paper shape is normally about trivial to do (see procedure). We must choose this ourself.

Printing resolution

Normal desired photo printing resolution is considered to properly be 240 to 300 dpi. Many one hour machines are set to print 250 dpi, but you can send 300 dpi. An exception is that large wall poster images often don't have that many pixels available, so are printed at significantly lower resolution, but a compensation is that they are viewed from a greater distance, not up under our nose. Another exception is that line art images (all black ink or white paper, no gray tones) will look a bit better printed at 600 dpi, and commercial work will prefer 1200 dpi for line art. But it won't help photos. Printing dpi is dependent on the capabilities of the printing process, see a Printing Guidelines page.

When a printer prints at 300 dpi, it spaces the pixels onto paper at 300 pixels per inch of paper. Printing 3000 pixels at 300 dpi prints a 10 inch image on paper.

The straight-forward way to scale for printing is to simply compute "pixels per inch" for the inches scanned, and then recompute those pixels over the inches printed (called scaling, as mentioned in the scanning Results). The scanner will have its Input and Output dimensions to show this. Also we have photo editor tools to make this resize be easy. See Image Resize.

A shortcut for the same scaling concept is this:

The ratio of (printing size / scanning size) is the enlargement factor.
The ratio of (scanning resolution / printing resolution) is the enlargement factor.

For example (in general - speaking of any size original):

Scan at 600 dpi, print at 300 dpi, for 600/300 = 2X size (to print double size or 200% size)
Scan at 300 dpi, print at 300 dpi, for 300/300 = 1X size (to print original size or 100% size)
Scan at 150 dpi, print at 300 dpi, for 150/300 = 1/2X size (to print half size or 50% size)

Or scan small film at 2700 dpi, print at 300 dpi, for 2700/300 = 9X size. If from full frame 35 mm film (roughly 0.92 x 1.41 inches), then 9X is about 8x12 inches (near A4 size). Film is typically small, requiring more scan resolution for more pixels for more print enlargement. The reason to scan at high resolution is for "enlargement", specifically to create enough pixels to print a larger print at about 300 pixels per inch. Scanning larger than any reasonable future use is likely pointless.

This is a concept called "scaling", and this enlargement concept is true for scanning anything of any size: photo prints, documents, film, etc.

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