The dimension in pixels (Image Size) is the important detail for using any image. Around 300 pixels per inch is an 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). Many 1-hour photo lab digital machines are usually set to print at up to 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. Printers do have their own other thing about ink drops per inch (but which is about the quality of dithering colors, not about image resolution), but here, we're speaking about printing resolution of image pixels.
The dpi calculator is below, but first, some background concerns. There are two situations when printing images, depending on if print shops or home printing.
If your image dimensions 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.
However, most photo editors will also provide an option to "Scale to fit media" or "Best fit to page", which will scale the image to fit 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.
Images have both size and shape properties. The image "shape" (which is width / height, called Aspect Ratio) likely rarely matches the paper "shape", which always needs attention first. When the print and paper shapes differ, a print shop "fit" typically fills all of the paper, fitting one dimension to leave no unfilled white space border in the other. A home photo editor "fit" typically is the opposite by default, not cropping at all, but leaving thin white space in one dimension if it doesn't fit precisely.
Either way, it is good if your plan properly prepares the image for printing. Sufficient pixels is important, but first cropping the image so that the image SHAPE actually matches the selected paper SHAPE is also a very important concern. Different paper sizes are different shape. And we need to provide the necessary pixels. The simple calculation for that acceptable image size for printing is:
(The actual dpi calculator is below). This first simple calculator will serve two general purposes:
Scanning to print a copy at the same size is a very common goal. It's important to realize that an area scanned at 300 dpi will create the pixels necessary to also print the same size at 300 dpi. The concept either way is pixels per inch. And 300 dpi is likely what you want for a photo copy job. The one-hour print shops accept larger images, but many machines are set to use 250 dpi.
But this dpi number does NOT need to be exact, 10% or 15% variation won't have great effect. Just scale it to print size. But planning size to have sufficient pixels to be somewhere near the size ballpark of 240 to 300 pixels per inch is a very good thing for printing.
Long dimension fitted
Short dimension fitted
Preparing the image shape to fit the paper shape is necessary, because paper and image are often different shapes.
Aspect Ratio is the "shape" of the image — the simple ratio of the images long side to its short side, which is a shape, maybe long and thin, or short and wide. And every paper size seems to be a different shape too. Shape and size are two different properties. To print an image, we can always enlarge the Size, but the image shape needs to match the paper shape (which is done by cropping). If this Aspect Ratio subject is new, see Image Resize - Aspect.
It's important to crop the image shape to fit the paper shape. Size is easily adjusted, but shape can only be cropped. You could wait for the printer machine to simply trim the image automatically (leaving whatever actually fits on the paper shape, without regard to the subject), but you will like the results better if you first choose the cropping yourself. It should be obvious that the shape of an 8x12 inch image simply cannot be fully fitted onto 8x10 inch paper. See Image Resize about how to plan this necessary resize and/or crop (specifically the second page there which has specific instruction about cropping image to match paper shape). That usual procedure is, FIRST crop image to paper SHAPE, and then resample image size to fit paper SIZE. Otherwise, printing will just cut off the part of the image that won't fit on the paper. Which could be a surprise you probably won't like. :) But this is all easy to do right.
If the image was previously cropped to be the same aspect ratio as the selected paper shape, then great, that's the idea. If not, the calculator will advise what the optimum cropped size should have been. Most one hour print shops won't leave any white space, and this calculator does the same. However, before you print it, it would always be a really good thing if you had first prepared the image to fit the paper properly, both shape and size.
Scanning common film and paper print dimensions will be in the Scanner "Area" box below (metric paper shown in blue there). If you need the dimensions of film sizes or metric A paper sizes, you can find them there (in the film list box). Or you can specify any scan or print size. However, the scanned Input area size that you mark on the scan Preview can be smaller than the actual film or print or document (if you crop it there). Or you can crop it to match the print paper size later.
Clicking a Compute button scrolls the screen to that button's results, but if this jumping is objectionable, you can turn scrolling off.
If the Result text might not be meaningful yet, then start at this: Cropping, Resampling, Scaling. It's the basics of something we all need to know about printing images. The idea is not to simply compute some numbers, but to try to explain how you can already know this yourself. It's actually pretty simple.
If the image aspect ratio does happen to match the print paper aspect ratio, but the megapixel count is excessive, then it will suggest resample to smaller usable 300 dpi size. That is about "size", but aspect ratio is about "shape". And generally image and paper shapes do not match at first.
Caution: When cropping and resamping your image for printing purposes, you should always save your original image for any future plans, because we do change our minds, but there's no going back. Always save such edits into a new file name. Don't overwrite your original file, because then the original image is gone.
If your image aspect ratio does Not match the paper aspect ratio, then more results are offered:
A possible green text suggestion in the scanning options: The computed scan resolution for film possibly can result as like 2540 dpi, closely missing one of the scanners default multiples of 300, 600, 1200, 2400, 4800 dpi. It will show and use that number, but if the miss is pretty close and might be considered negligible, the calculator might also suggest for example, that scanning at 2400 dpi (instead of 2540) would still print the same size at perhaps 283 dpi, which likely cannot be distinguished from 300 dpi (see more below). Many one hour labs limit printing to 250 dpi anyway (but their continuous tone is better quality than an inkjets dithered reproduction). Again, it is just an alternate suggestion to be aware and possibly consider. You can also get the same information for different multiples in Option 3 by just trying a couple of values of available resolution.
Both Size and Shape are important, and while you're dealing with "crop to shape", why not also resample it to a much more reasonable size first? (see next Blue step).
Again, some editor programs offer a very easy procedure to crop 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 the paper shape. You can still resize and move that crop box as desired to best 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 calculator near top of this 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.
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 good 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). Specifically, 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. It is a small and hard-to-see difference (maybe it's a geeky thing, and more important in the early days when scanner resolution was much lower), however the photo editor can do this resample better, after it has all the image data.
So scan and then for printing preparation, FIRST crop 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.
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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