An online ruler in actual size — measure in cm, mm and inches right on the screen, and calibrate it with a bank card so the millimetres are real.
What is this?Hide details▾
What is Online Ruler?
A ruler drawn straight onto your screen, with centimetres and millimetres along one edge and inches down to 1/16 along the other. No browser can know how big your display really is, so you calibrate once by matching an outline to a bank card — or by typing a length you already know — and from then on the marks are true to size. Drag the two markers to measure anything you can lay on the glass; it works offline and remembers the calibration on this device.
ruler.js
CalibrationUncalibrated — using a 96 DPI guess
Your screen's pixel ratio changed since you calibrated — probably a different monitor or a zoom change. Calibrate again for marks you can trust.
Browser zoom is on — sizes will be wrong. Reset zoom (Cmd/Ctrl+0).
Sized from your device's known display. Calibrate with a bank card if you need it exact.
The ruler opens full screen, running down the left edge with zero at the top. Lay your object against the zero line, drag the pink line to its far end, and read the length.
Accuracy depends entirely on calibration: a browser has no way to know your display's true physical size, which is exactly why the bank card step exists. Calibrate once per screen and the marks stay honest.
A
B
Zero — line the top of your object up with this line
Length—
Drag anywhere to move the line. The reading runs from the zero line at the top.
Browser zoom is on — sizes will be wrong. Reset zoom (Cmd/Ctrl+0).
Sized from your device's known display. Calibrate with a bank card if you need it exact.
Uncalibrated — real-world sizes will be wrong. Calibrate with a bank card.
Calibrate to your screen
Lay any bank, ID or loyalty card flat on the screen and resize the outline until the two match exactly. Standard cards are 85.60 × 53.98 mm.
85.60 × 53.98 mm
Set the measuring line against something whose exact size you know, then type that real length in millimetres.
Pick your screen's diagonal, or type it. It's the number on the spec sheet — 15.6 inch, 27 inch, and so on.
