How We Measure an Inch

What is an inch?

One inch is exactly 2.54 centimetres, or 25.4 millimetres. This has been the international definition since the International Yard and Pound Agreement of 1959. Source: National Institute of Standards and Technology (NIST).

How the on-screen ruler works

A screen shows length in pixels. To turn pixels into inches, the site needs to know your screen's PPI (pixels per inch). The ruler length in pixels = inches x PPI.

How calibration works

Hold any standard credit card against the on-screen card and adjust the slider until they match. A standard credit card is 85.60 mm x 53.98 mm (3.375 in x 2.125 in). Once you save, the site uses your calibrated PPI for every ruler on the site. Every ruler is then drawn at Calibrated Actual Size on your screen.

Why the first size you see is approximate

The site starts at 96 PPI, which is an old Windows default. Most modern laptop and phone screens are 150-300+ PPI, so the ruler can look wrong before calibration. That is why the site does not claim the uncalibrated ruler is exact.

What we claim, honestly

  • After calibration: the ruler is true to size on your screen.
  • Before calibration: the size shown is approximate.
  • Object comparisons are approximate, not exact. Some objects are exact by definition, most are close — for example, a US quarter is 0.955 inches across, not exactly 1 inch. Each inch page says which objects are exact and which are close.

What you need

You only need a web browser. No app, no install, no sign-up. It works best on desktop, and it also works on mobile.

Who made this

Chetan Kumar, creator of How Long Is an Inch?

Hi, I'm Chetan Kumar — an SEO specialist from India, and I own and maintain this site. I built it after ordering a 7-inch burrito and realising I had no idea how long 7 inches actually was. Everything runs in your browser — no app, no install, no sign-up — and every measurement method is documented. Spotted a mistake? Email me and I'll fix it. More: About | How We Measure | Email | GitHub

Browse every size from 1 to 21 inches on the Measurements page.

Last updated: November 2026