How do the inch, centimetre and millimetre scales differ?

Inches have quarter-inch marks and a reading rounded to the nearest sixteenth. At a 1:1 drawing ratio, centimetres have a tick every millimetre, with centimetre numbers when space permits. Millimetres use the same physical ticks and nominal 10 mm numbers. Crowded screen numbers are thinned while every tick remains. A supplied drawing ratio changes the represented numbers; calibration and the printable ruler keep their physical lengths. Changing units keeps the calibration.

How can a screen ruler be actual size?

Match the calibration marker to a physical reference before measuring. CSS pixels do not report the physical size of your display, so the ruler shows pixel counts until you calibrate. The readout and tick spacing then use your measured scale.

Millimetre Ruler Online, Calibrated to Your Screen

This ruler starts in millimetres, with a tick each millimetre at a 1:1 drawing ratio and nominal 10 mm numbers, thinned when crowded. Match a physical reference before measuring.

Hold a bank, credit or ID card against the rule and match the marker to its right edge.ISO/IEC 7810 ID-1, a standard bank card 85.60 mm wide

Not calibratedA zoom or a screen change clears it

Units
Calibrate against
Selected: 0 px, not a length yet

Target length (mm)

Screen targets need calibration and enough visible width. The printable SVG has physical dimensions: open it, print at 100% or Actual size on paper large enough, and check its marks with a physical ruler. Printer scaling can change the size.

Drawing scale

Scale ratio (1:n)

At 1:100, 1 cm on a drawing represents 100 cm. After calibrating, drag the marker over a drawing displayed on this screen. Numbered marks and represented length use the chosen ratio; the physical reading, calibration references, targets and printable SVG stay at actual size. Confirm the drawing's displayed scale before measuring. Represented numbers are rounded to eight significant digits.

Calibrate before reading a represented drawing length.

Not calibrated yet, so these marks are not a measurement. Lay a bank card or a sheet of paper against the rule, match the marker to its edge, and the marks become real inches and centimetres.

Click or drag anywhere on the rule to move the edge. With the marker focused, the arrow keys move it one pixel at a time and shift with an arrow moves ten, so you can land on an exact figure. Nothing you do here leaves your browser: the calibration is kept on this device only.

An object of a known width, matcheda bank card is 85.60 mm under ISO/IEC 7810 ID-1
sets the scale for every mark on the rule
Nothing matched yetno reference given
marked uncalibrated, and no length is shown
The page zoom changesthe reference no longer holds
the calibration is cleared and the rule asks again

Exact to the reference you calibrate it against, and marked uncalibrated until you do. A browser cannot ask your display how large it is, so an uncalibrated screen ruler is a guess, and a page zoom or a screen change clears the calibration because the reference no longer holds.

Common questions

How do the inch, centimetre and millimetre scales differ?: Inches have quarter-inch marks and a reading rounded to the nearest sixteenth. At a 1:1 drawing ratio, centimetres have a tick every millimetre, with centimetre numbers when space permits. Millimetres use the same physical ticks and nominal 10 mm numbers. Crowded screen numbers are thinned while every tick remains. A supplied drawing ratio changes the represented numbers; calibration and the printable ruler keep their physical lengths. Changing units keeps the calibration.
How can a screen ruler be actual size?: Match the calibration marker to a physical reference before measuring. CSS pixels do not report the physical size of your display, so the ruler shows pixel counts until you calibrate. The readout and tick spacing then use your measured scale.