Accuracy depends on calibration. Calibrate with a card or an object of known length before measuring anything that matters.
An inch ruler with 1/16-inch marks. Readings are shown as a fraction, such as 2 3/8 in, and as a decimal like 2.375 in. Calibrate first with a credit card, which is 3.37 inches wide; one inch is exactly 2.54 cm.
How to use
- Tap Calibrate and hold a bank card (85.6 × 53.98 mm) flat against the screen. Drag the edge or use − and + until the outline matches the card, then tap Done. You can also use any object of known length.
- Line up the object with the zero mark at the left edge of the ruler.
- Drag the marker (or tap anywhere on the ruler) to the other end of the object and read the length. Both markers can be moved.
- Switch between cm and inches, turn the ruler vertical for longer objects on a phone, or go full screen to use the whole display.
- Protractor: drag either arm. The angle between them is shown in degrees and radians.
- Browsers don’t know the physical size of your screen, so a ruler is only exact after calibration. The calibration is saved for this screen and browser and is used again next time.
- Before calibrating, the size is estimated from typical screens and can be off by 10–30%.
- Bank cards follow ISO/IEC 7810 ID-1: 85.60 × 53.98 mm. Any card of that size works: credit, debit, many ID and loyalty cards.
- Browser zoom changes the scale. Keep the page at 100% zoom, or calibrate again after zooming.
- Fractions are rounded to the nearest 1/16 inch; the decimal value shows the exact reading.
Why calibration matters
A web page cannot ask a screen how big its pixels really are. Browsers use a nominal “CSS inch” of 96 pixels, which matches a physical inch on almost no phone or laptop, so an uncalibrated ruler is typically off by 10 to 30%.
Calibration fixes that. Hold any ID-1 card, the size of a credit or debit card defined in ISO/IEC 7810 as 85.60 × 53.98 mm, against the screen and adjust the outline until it matches. The ruler then knows how many pixels make a millimeter on your screen and saves that value for next time.
Getting an accurate reading
Keep the page at 100% browser zoom, since zooming changes the scale. Line up one end of the object with the zero mark, look straight at the screen to avoid parallax, and move the marker to the other end. In inches, the reading is rounded to the nearest 1/16 inch and the exact decimal is shown too. For items longer than the screen, measure in parts, or turn a phone's ruler vertical for more length.
Frequently asked questions
Is this online ruler actual size?
After calibration, yes, to within about a millimeter on most screens. Before calibration the size is estimated and can be off by 10 to 30%.
How do I calibrate the ruler without a credit card?
Use any card of the same size, such as a debit, ID or loyalty card, or any object whose length you know, like a coin or a banknote.
How big is a credit card?
85.60 × 53.98 mm, about 3.37 × 2.13 inches. This ID-1 size is set by the ISO/IEC 7810 standard.
How many centimeters are in an inch?
Exactly 2.54 cm, fixed by international agreement in 1959. One centimeter is about 0.3937 inch, so 10 cm is roughly 3 15/16 inches.
Why is the ruler the wrong size after zooming?
Browser zoom scales everything on the page. Go back to 100% zoom or calibrate again at the new zoom level.