Check Image DPI — Free, Fast & Private
DPI (dots per inch) determines how sharp an image looks when printed. A 3000x2000 pixel photo at 300 DPI prints beautifully at 10x6.67 inches. The same photo at 72 DPI would need to print at 41.7x27.8 inches to use all its pixels — fine for a billboard, terrible for a business card.
Here is the part that confuses most people: DPI is a print concept. On screens, only pixel dimensions matter. A 1920x1080 image displays identically at 72 DPI and 300 DPI on a monitor. The DPI metadata only affects how a printer or design application interprets the image size. So when someone says "make this image 300 DPI," what they usually mean is "make sure it has enough pixels to print sharply at the intended size."
To calculate what you need: multiply the print size in inches by the required DPI. A 4x6 inch photo at 300 DPI needs 1200x1800 pixels. An A4 page (8.27x11.69 inches) at 300 DPI needs 2481x3507 pixels. If your image has fewer pixels than required, it will look soft or pixelated when printed.
Phone cameras produce more than enough pixels for most print needs. A 12MP phone camera captures 4032x3024 pixels — that is enough for a 13.4x10 inch print at 300 DPI. The file will likely say 72 DPI in its metadata, but that is just a default tag. The actual pixel count is what matters.
The checker above reads both numbers from the file itself: the DPI it states — from the JFIF header and EXIF in a JPEG, the pHYs chunk in a PNG, the resolution tags in a TIFF, EXIF inside a HEIC, AVIF or WebP, and the BMP header — and the pixel size its header declares. If a JPEG states two different values, it shows both, because programs differ in which one they read. If the file states none, it says so rather than printing 72, which is a viewer's default and not something the file said. Type a print width and it does the division for you. For web use, the pixel size is all you need.
DPI, and why it does nothing on screen
DPI — dots per inch — is a tag, not a property of the pixels. An image is a grid of a certain width and height. DPI only says how large that grid should be when printed. Change a file from 72 to 300 DPI without resampling and every pixel is untouched; you have edited a number in the header that tells a printer to lay the same pixels down more densely, on smaller paper.
This is why “save it at 300 DPI” is meaningless advice for anything shown on a screen. A monitor renders pixels; it never consults the DPI tag. A 1200×1600 image is exactly as sharp on screen at 72 DPI as at 300.
For print it matters, but as arithmetic: pixels ÷ DPI = printed inches. That 1200×1600 image at 300 DPI prints at 4×5.33 inches. Want it at A4? You need roughly 2480×3508 pixels. Government forms often specify dimensions in centimetres for the same reason — CTET asks for 3.5 × 4.5 cm and leaves the pixel count to you, which is only resolvable by choosing a DPI.