How big can you print this photo?

How print size is worked out

A print's sharpness depends on how many of the photo's pixels land on each inch of paper, a figure called pixels per inch, or PPI. To find the largest print, divide the photo's width and height in pixels by the PPI you want. The answer is in inches; multiply by 2.54 for centimetres.

Take a 12-megapixel phone photo of 4032 x 3024 pixels. At 300 PPI it prints at 13.4 x 10.1 in (34.1 x 25.6 cm). At 150 PPI the same pixels stretch to 26.9 x 20.2 in, each pixel twice as wide, which looks fine on a wall but soft up close.

It works the other way too: multiply the print size by the PPI to get the pixels you need. A 4 x 6 inch print at 300 PPI needs 1200 x 1800 pixels, and an 8 x 10 inch print needs 2400 x 3000.

How many pixels per inch you need

• 300 PPI and up: Sharp. The usual standard for photo prints, albums and anything looked at in the hand.
• 200 to 299 PPI: Good. Still looks sharp at arm's length, so most prints framed on a desk or a shelf hold up.
• 150 to 199 PPI: Fair. The usual floor: fine for posters and wall prints seen from a few feet away.
• Below 150 PPI: Too low. Edges start to look soft or blocky, and many online print services warn that the photo is too small for the size you picked.

These are rules of thumb, not hard limits. The farther away people stand, the fewer pixels per inch they can make out, which is why billboards are printed at far lower densities than photos. Print services also set their own minimums, so heed the warning yours shows when you order.

PPI, DPI and the 72 DPI label

DPI, dots per inch, describes how finely a printer lays down ink. PPI describes how densely a photo's pixels are spread over the paper. The two get mixed up all the time, and when a print service asks for 300 DPI it means 300 pixels per inch of print.

Most photos also carry a DPI number in the file, often 72 or 96, which some apps show as the image's resolution. It is only a label telling software what size to print at by default. Changing it without resampling leaves every pixel as it was, so it cannot make a print sharper. The calculator ignores it and uses the pixel count, which is what decides the print.

Why the photo's shape matters

Photos and prints come in different shapes. Phones usually save 4:3 photos, most DSLR and mirrorless cameras shoot 3:2, and an 8 x 10 print is 5:4. When the shapes differ, the print is trimmed along the longer side to fill the paper, so the side that runs out first decides the sharpness. The calculator works out the pixels per inch that way, and says how much of the photo each size trims.

A 6000 x 4000 camera photo printed at 8 x 10 inches loses about 17% of its long side to the trim and prints at 500 PPI. On 4 x 6 inches, the same 3:2 shape, nothing is trimmed.

If your photo is too small

• Find a bigger copy. A photo saved from a chat, a social network or a web page is often a smaller copy of the original. The original file, from the phone or camera that took it or from the person who sent it, can have several times the pixels.
• Print smaller, or match the shape. A size that matches the photo's shape trims nothing, so every pixel goes to the print.
• Upscale it. An AI upscaler redraws the image larger and fills in plausible detail. Pixelperfect returns about 4 megapixels for most images (about 1 for very small ones), so a 1000 x 750 photo that prints sharp only up to 3.3 x 2.5 in comes back large enough for a sharp 5 x 7 inch print. It cannot make a poster: about 4 megapixels in a 3:2 shape (about 2510 x 1670 pixels) prints sharp up to about 8.4 x 5.6 in. The added detail is a prediction, not a recovery, so small text and faces can change slightly. How AI upscaling works