Resize Photos for Printing on Mac (DPI and Print Sizes)

The rule is pixels = inches x DPI. Print labs commonly ask for 300 DPI, so use 300 as the target. Check your lab’s upload page, because some ask for different values.

The numbers are arithmetic, not lab rules. Most modern phone and camera files already exceed these pixel counts for the common sizes, so you often resize nothing and only check the numbers.

DPI (also PPI) is a tag stored in the file. It tells a program how many pixels to spread across one inch. It does not add or remove pixels. A 3000 x 2000 photo is 3000 x 2000 whether it is tagged 72 or 300.

What changes is the default print size. 3000 x 2000 at 300 DPI is 10 x 6.7 inches. The same file at 72 DPI is 41.7 x 27.8 inches. Print labs and layout apps usually ignore the tag and use the pixel count against your chosen paper size, so the pixels are what matter.

If you only need to turn a 72 tag into 300 and the pixels already cover your print, set the DPI without resampling. If you do not have enough pixels, see how to increase image resolution on a Mac.

1. Open the photo in Preview and choose Tools > Adjust Size.
2. Set the unit to inches. Leave Scale proportionally on.
3. Type 300 in Resolution (pixels/inch).
4. To keep the pixels exactly as they are, uncheck Resample image. Width and height in inches then update to match.
5. To change the pixels, leave Resample image checked and enter the print width, for example 6 inches. Preview computes the pixels from your DPI.
6. Choose File > Export to save a copy.

Check the result in Tools > Show Inspector, where the pixel dimensions appear. Never resample upward unless you must. Going down is safe. Going up softens the image, as the AI image upscaler comparison explains.

# fit inside 1800 x 1200 pixels (a 4x6 print at 300 DPI)
sips -Z 1800 photo.jpg --out photo_4x6.jpg

# set the DPI tag only; pixels stay the same
sips -s dpiWidth 300 -s dpiHeight 300 photo.png --out photo_300.png

# read the size back
sips -g pixelWidth -g pixelHeight -g dpiWidth photo_300.png

I tested this on a 600 x 400 fixture. On a PNG, the tag was written as 300 pixels per inch and exiftool confirmed the unit as inches. On a JPEG, sips reported the DPI as 72 on read-back and exiftool showed a 300 value with the resolution unit set to None. So verify a JPEG with exiftool, or use Preview, and do not trust a single tool’s read-out.

-Z scales the longest edge and keeps the proportions. It does not crop, so a photo at 3:2 will not match a 5x7 (a 7:5 shape) without cropping. More flags are in the sips command guide.

Photo shapes and paper shapes differ. A camera frame is usually 3:2 or 4:3, a 4x6 is 3:2, a 5x7 and 8x10 are not. The lab either crops to fill or leaves white borders. Decide this yourself. Crop in Preview with Tools > Rectangular Selection, drag to the print ratio, then Tools > Crop, and resize afterward.

Phone shots are often 4:3, so an 8x10 print loses part of the edge. Leave room around faces when you shoot.

Usually not by much. A lab upload page may have a file size limit, but a print file should stay high quality. Keep JPEG quality high and the pixel count at the print size. Compression artifacts show on paper more than on a phone.

For emailing a print to someone, a smaller copy is fine. Keep the large original for the lab. For a PDF going to print, use compressing a PDF for print instead.

For one print, Preview does everything above, free. For a folder, a sips loop is faster than Smol. Smol is not the right tool to set a DPI tag or crop to a print ratio.

One warning if you do use Smol on print files: by default it resizes images to fit inside 2000 x 2000 pixels. That is below the 2400 x 3000 you need for an 8x10. Raise the resize limit or switch resizing off before you run a print batch. Smol also strips metadata by default and keeps no lossless mode, so for archival masters keep your originals. Smol earns its $29 (once, one Mac) when you have many photos in mixed formats to make smaller for sharing or a web gallery, nothing uploaded. See pricing.

For photographer-specific workflows, read image compression for photographers.