This online ruler is accurate only after you calibrate it. A web page cannot know how big your screen really is, so you match the grey shape to a bank card, which is 85.6 mm wide, and the ruler scales from that. After that it shows centimetres and millimetres or inches in sixteenths, and your setting is remembered in this browser.
| Units | Centimetres with millimetre marks, or inches in 1/16 steps |
|---|---|
| Calibration | Match a bank card (85.6 mm wide) once; saved in your browser |
| Length | Up to 30 cm or 12 in, scroll sideways for the rest |
| Cost | Free, no sign-up, nothing is uploaded |
By Ship AI Lab. Last updated .
Why an online ruler needs calibrating
Web pages draw in CSS pixels, and a CSS pixel is only defined as a fixed fraction of an inch on paper. On a real phone or laptop, the physical size of a pixel varies with the screen, the zoom level and the display scaling. That is why a ruler that assumes a standard pixel size is often off by several percent.
A bank card gives you a known length that you can hold against the glass. A standard ID-1 card is 85.60 mm wide and 53.98 mm tall, the same everywhere. Once you stretch the grey shape to match it, the page knows how many screen pixels make a millimetre on your device.
Reading centimetres, millimetres and inches
In centimetre mode the long marks are whole centimetres with numbers, the medium marks are half centimetres and the short marks are millimetres. In inch mode the long marks are whole inches, and the shorter marks go down through half, quarter, eighth and sixteenth of an inch. One inch is exactly 25.4 millimetres, so you can check a reading either way.
Hold the object flat against the screen with its edge on the zero mark, then read where the other end falls. A screen is glass over a display, so look straight down at the mark to avoid reading it at an angle.
How to use the online ruler
- Calibrate with a cardLay a credit or debit card on the screen. Drag the slider until the grey shape is exactly as wide as the card.
- Pick your unitSwitch between centimetres with millimetres, or inches in sixteenths.
- MeasurePut the edge of the object on zero and read the other end. Scroll sideways for anything longer than the screen.
What this ruler cannot do
- It is only as accurate as your calibration. Check it against a real ruler before you rely on it for anything that has to fit.
- Changing browser zoom or rotating a tablet changes the pixel size, so calibrate again if you do.
- It cannot measure anything longer than a couple of metres without help. For a room or a table, use the AR tape measure in the app.
- It is for reference measurements. For cutting, building or anything that must be exact, use a physical tape or ruler.
Online Ruler questions, answered
How do I get an actual-size ruler on my screen?
Calibrate it with a bank card. Lay the card on the screen, drag the slider until the grey shape matches the card’s width, and the ruler then shows real centimetres and inches on that device.
Is an online ruler accurate?
Only as accurate as the calibration. A card is a good reference, but you should check the result against a real ruler, and calibrate again after changing zoom or rotating the screen.
How wide is a credit card?
A standard bank card is 85.60 mm wide and 53.98 mm tall (ISO/IEC 7810 ID-1), which is about 3.37 by 2.13 inches.
Can I use it on my phone?
Yes. It works in a phone browser. Because phone screens are small, scroll the ruler sideways to measure longer objects.
Does the online ruler store anything?
It remembers your calibration in your browser so you do not repeat it. Nothing is uploaded, and no account is needed.