Online Ruler
Measure small objects with a true-to-scale ruler on your screen in centimeters, millimeters or inches. Calibrate it once with any bank card or your screen size; the calibration is saved on this device and nothing leaves your browser.
Line up one end of the object with 0 at the left (or top) edge of the ruler, then drag the marker or click the ruler at the other end. Arrow keys move the marker by 1 mm or 1/16 in (Shift for 10×).
Calibrate with a credit card
Hold any bank, ID or loyalty card (ISO ID-1, 85.60 × 53.98 mm) flat against the screen with its top-left corner in the corner of the outline, then resize the outline until it matches the card exactly.
…or use your screen size
How to use the Online Ruler
- Calibrate: hold a credit, debit or ID card against the screen and drag the size slider until the outline matches the card exactly, then press Save calibration. Or enter your screen diagonal.
- Choose a scale: centimeters, inches, or both, and pick horizontal or vertical.
- Place the object with one end exactly at the 0 mark on the left (or top) edge of the ruler.
- Drag the marker (or click on the ruler) to the other end of the object and read the length in cm, mm and inches.
Why an on-screen ruler needs calibration
Web pages are laid out in CSS pixels. The CSS standard defines 96 CSS pixels per inch, but that "inch" is only a reference: a 13-inch laptop, a 27-inch monitor and a phone all show 96 CSS pixels at very different physical sizes. That is why this ruler asks for a quick calibration. Matching a real card tells it the true number of pixels per millimeter on your screen:
- Card method: pixels per mm = card outline width in pixels ÷ 85.60 mm. Example: an outline 323.5 px wide gives 3.779 px/mm, which is exactly the 96 px/inch CSS default.
- Diagonal method: pixels per inch = √(width² + height²) ÷ diagonal. Example: a screen reported as 1536 × 864 CSS pixels with a 15.6-inch diagonal gives √(1536² + 864²) ÷ 15.6 = 1762.3 ÷ 15.6 ≈ 113 px per inch.
The calibration is stored in your browser for this site only. Calibrate again if you change browser zoom, move the window to another monitor or change your display scaling. You can check your scaling with the screen resolution checker.
Common reference sizes for checking the ruler
| Object | Size (mm) | Size (inches) |
|---|---|---|
| Credit / debit card (ISO ID-1) | 85.60 × 53.98 | 3.370 × 2.125 |
| US quarter (diameter) | 24.26 | 0.955 |
| US penny (diameter) | 19.05 | 0.750 |
| 1 euro coin (diameter) | 23.25 | 0.915 |
| A4 paper | 210 × 297 | 8.27 × 11.69 |
| US Letter paper | 215.9 × 279.4 | 8.5 × 11 |
| Standard business card (US) | 88.9 × 50.8 | 3.5 × 2 |
Reading the scales
On the metric scale every small tick is 1 millimeter, the medium tick marks 5 mm and the numbered long ticks are whole centimeters (10 mm). On the inch scale the marks get shorter as the fraction gets smaller: inch, 1/2, 1/4, 1/8 and finally 1/16 inch. The readout rounds inch values to the nearest 1/16 inch (about 1.6 mm) and also shows the decimal value to three places.
Useful conversions: 1 inch = 2.54 cm = 25.4 mm exactly; 1 cm ≈ 0.3937 in; 1/16 in = 1.5875 mm; 1/8 in = 3.175 mm. For accurate results, look straight at the screen rather than at an angle, place the object flat against the glass, and remember that an on-screen ruler is limited to the size of your display. To convert a measurement afterwards, use the cm to inches converter or the inches to cm converter; for anything longer than your screen, use a tape measure.
Frequently asked questions
Is this online ruler actual size?
It is actual size once calibrated. Screens have very different pixel densities, so no website can know the physical size of a pixel by itself. Matching the on-screen card to a real card, or entering your screen diagonal, tells the ruler how many pixels are in a millimeter on your display.
What size is a credit card?
Credit, debit and most ID cards follow the ISO/IEC 7810 ID-1 format: 85.60 mm × 53.98 mm (3.370 × 2.125 inches). Because this size is standardised worldwide, any bank card, driver's license or loyalty card of that format works for calibration.
Why is the ruler wrong when I zoom the page?
Browser zoom changes the size of everything on the page, including the ruler. Calibrate and measure at the same zoom level, ideally 100%. If you change the zoom, open a different monitor or rotate a tablet, calibrate again.
How accurate is the screen-diagonal calibration?
It is usually accurate to within one or two percent, but it relies on the browser reporting the full screen size. Some laptops use display scaling or report a slightly different resolution, so the credit-card method is more reliable.
How many centimeters are in an inch?
Exactly 2.54 centimeters, or 25.4 millimeters, are in one inch. One centimeter is about 0.3937 inches.
How do I read 1/16 inch marks?
The longest marks are whole inches, then 1/2, then 1/4, then 1/8, and the shortest marks are sixteenths. Count the small marks after the last whole inch: for example 3 inches plus 5 short marks is 3 5/16 inches.