PixelCalc answers five questions people keep re-googling: how dense is this screen, how many megapixels is this image, how big will this file print, what aspect ratio is this really, and why does the browser report half my resolution? One page, five modes, answers in plain units — ppi, megapixels, inches and centimeters, reduced ratios, and live CSS-pixel readouts.

Built around arithmetic you can audit

Every mode reduces to a formula printed on the page: pixel density is the hypotenuse over the diagonal, print size is pixels over dpi, aspect ratio is the pair reduced by its greatest common divisor. Nothing is estimated, sampled or approximated — when the result says ≈461.4 ppi and the spec sheet says 460, the difference is vendor rounding, and we say so rather than hiding it. The same honesty applies to the awkward cases: 1366×768 is not 16:9, and a “21:9” ultrawide is usually 64:27 or 43:18. The calculator shows the reduced truth plus the nearest named standard.

One person deep, on purpose

The site is a static page — HTML, a stylesheet and one JavaScript file — so the arithmetic executes in the tab you have open and never involves a server round-trip. There is nothing to sign into because no sign-in exists, no usage counter keyed to your inputs, and nothing persisted between visits. You can unplug the network after the page loads and every mode behaves identically.

Where the reference numbers come from

Preset devices use published panel resolutions and advertised diagonals; print references use exact definitions — ISO paper sizes in millimeters, US sizes in inches, converted at 1 in = 25.4 mm. The PPI vs DPI, print-resolution, 4K and CSS-pixel guides were written against the actual standards (DCI, ITU BT.2020, the CSS spec), not against each other.

Corrections

Caught a preset that disagrees with the panel in front of you, a ratio that should match a standard, or a guide that gets a number wrong? That’s the mail we most want to get — see the contact page.