Check the DPI of any image.

DPI stands for dots per inch. In an image file it is a single number stored alongside the pixels that says how many of them should be packed into each printed inch. It changes nothing about the pixels themselves. A 3000 × 2400 photo tagged 300 DPI and the same photo tagged 72 DPI contain byte-for-byte the same picture; they only differ in the size a printer will assume you wanted.

PPI (pixels per inch) describes the image; DPI (dots per inch) describes the printing device, which lays down many tiny ink dots to make one pixel. A photo lab press might run at 1440 DPI while printing your 300 PPI file. In everyday use the two words have collapsed into one — file metadata, Photoshop, and every print shop order form all say DPI when they mean PPI — so this tool says DPI too.

300 pixels per printed inch is roughly where a normal eye stops resolving individual pixels at reading distance, so it became the standard target for photographic prints. It is also what most professional labs specify. To find the pixels you need, multiply the print size by 300: an 8 × 10 wants 2400 × 3000 pixels, a 5 × 7 wants 1500 × 2100. If your file has those pixels, it prints cleanly at that size no matter what number the metadata claims.

When the print is large and nobody looks at it from ten inches away. Banners, gym posters, senior night signage, and 16 × 20 wall pieces hold up fine at 150 to 200 DPI because viewing distance grows with print size. Below 150 the softness starts to show at any distance, which is why Batch Relay treats 150 as the floor for a print rather than a preference.

No. 72 DPI is the fallback almost every camera, phone, export dialog, and screenshot tool writes when it has no reason to write anything else, and many files carry no density tag at all — software then assumes 72 on your behalf. What matters is the pixel count. A 6000 × 4000 file tagged 72 DPI is a flawless 20 × 13 inch print at 300 DPI; the tag is a note in the margin, not a measurement of quality.

Change the metadata, not the pixels. In Photoshop open Image → Image Size and uncheck Resample before typing 300 into Resolution: the pixel count stays identical and only the printed size recalculates. In Affinity Photo it is Document → Resize Document with resampling set to None. If you leave resampling on, the software invents or discards pixels — upsampling a 72 DPI file to 300 DPI at the same inches makes the file bigger and the picture softer, which is the one move worth avoiding.

On a Mac, open the file in Preview and press Command-I for the inspector, which lists image DPI along with pixel dimensions. On Windows, right-click the file, choose Properties, and look under Details for Horizontal and Vertical Resolution. Both read the same metadata this page reads, one file at a time; the checker above is faster when you have a card full of images and want to see them together.