What is Photo Size & DPI Calculator?
Photo Size & DPI Calculator answers the questions that come up before every print: how many pixels a 35×45 mm or 4×6 in print needs, how large a 3000×4000 photo can be printed, and what any size is in millimetres, centimetres, inches and pixels at a given DPI. Enter a photo's pixel size (or pick the file and only its dimensions are read, on your device) and the print size you want to see its effective resolution with a plain-language quality note. Every result shows the formula it uses, and a calibration page lets you check that your printer prints at 100%.
Key features
- Live conversion between mm, cm, inches and pixels at any DPI (72–1200)
- Pixels needed for a print size, and the largest print size for a photo
- Effective resolution (PPI) of a photo at your print size, with a quality note
- Formulas shown for every result
- Optional: read a photo's width and height from its file header, locally
- Printable calibration page with mm and inch rulers and reference squares (PDF)
Popular uses
- Check whether a phone photo is sharp enough for a 4×6 or A4 print
- Find the exact pixels an online form asks for (e.g. 300 dpi at 35×45 mm)
- Convert a size given in inches into millimetres or pixels
- Check that a printer is not scaling prints
Supported formats
Units: millimetres, centimetres, inches, pixels. Optional input file: JPG, PNG, WebP, AVIF, HEIC/HEIF, GIF, BMP, TIFF (only the header is read). Output: on-page results and a calibration PDF.
Troubleshooting
- The pixel fields stay empty after picking a photo Some files don't store their size in a readable header. Enter the width and height yourself — your photo viewer shows them in the file details.
- The calibration squares print too small or too large Your printer or its dialog is scaling the page. Choose 100% / actual size and turn off "fit to page".
Also known as: DPI Calculator, PPI Calculator, Pixels to mm Converter, Print Size Calculator, Image Resolution Calculator.
How it works
- Enter a size in mm, cm, inches or pixels and the DPI (pixels per inch) — every other unit updates as you type.
- To check a photo, enter its width and height in pixels (or pick the file — only its size is read, on your device) and the print size you want.
- Read the effective resolution and the quality note, and download a calibration page to test your printer's scaling.
FAQ
What DPI should I print photos at?
300 dpi is the usual target for photos viewed up close, such as passport photos and 4×6 prints. 200–300 dpi still looks good; below 150 dpi prints usually look soft.
Is DPI the same as PPI?
For this purpose, yes: both mean how many image pixels are printed per inch. Printers also use "DPI" for ink dots, which is a separate setting.
How many pixels does a 35×45 mm photo need?
At 300 dpi: 35 ÷ 25.4 × 300 = 413 pixels wide and 45 ÷ 25.4 × 300 = 531 pixels high.
Is my photo uploaded if I pick it?
No. Only the width and height are read from the file's header, in your browser. The picture itself is never decoded or sent.
Why does my photo look sharp on screen but soft in print?
Screens show a photo at about 100–160 pixels per inch; a print viewed up close needs around 300. A photo that fills your screen may only be enough for a small print.
Does a higher DPI number in the file improve the print?
No. The DPI stored in a file only tells software the intended print size. Quality depends on how many pixels you have for the size you print.