CM Ruler: an actual-size centimetre ruler on your screen

Not calibrated yet. Screens differ, so calibrate once for true actual size, it takes about 30 seconds with any bank card.
screen: 96.0 PPI
Between markers–
In inches–
Tape fraction–
Pixels–
Ruler length–
Hold an object against the screen inside the box, drag the box to move it, drag the dot to resize.
240 × 120 px

Any standard bank, credit, debit, or ID card works: the ISO/IEC 7810 ID-1 format is exactly 85.60 mm wide everywhere in the world. Hold the card flat against the screen, match the width, then apply.

True actual size after calibration Inch fractions to 1/16, like a tape measure Corner ruler & measure box Free, private, works offline
Metric, clean

A centimetre ruler without the clutter

This page loads the ruler in metric-only mode: numbered centimetres, mid-length ticks at every half centimetre, and a fine tick for every millimetre, the layout of a standard school ruler, rendered at your screen's true size once calibrated. Drag the two orange markers and the readout reports the span in centimetres, switching to millimetres automatically below one centimetre, with decimal inches alongside if you ever need the conversion. The CM + IN button restores the dual scale at any time.

Reading the scale

How to read a centimetre ruler

Three tick lengths carry the whole system: the longest, numbered ticks are whole centimetres; the medium tick midway between them is the half centimetre, 5 mm; and each of the small ticks is one millimetre. A length of 7.3 cm therefore means the 7 mark, plus three small ticks. There is nothing else to memorize, which is precisely why the metric scale is taught first.

Familiar objects in centimetres, from their governing standards, useful as on-screen sanity checks after calibrating.
ObjectSize in cmSource
Bank card width8.56 cmISO/IEC 7810 ID-1
A4 paper21.0 × 29.7 cmISO 216
US Letter paper21.59 × 27.94 cmANSI, 8.5 × 11 in exactly
US one-dollar bill15.6 × 6.6 cmBureau of Engraving and Printing
CD / DVD diameter12.0 cmDisc format standards
FAQ

CM ruler questions, answered

How long is the on-screen cm ruler?
As long as your display is wide: typically 25 to 35 cm on a laptop and 50 to 70 cm on a desktop monitor, with the fullscreen button using every pixel. The ruler-length readout shows the current capacity, and for longer measurements the printable ruler or staged measuring with the markers takes over.
Is the cm ruler accurate?
After the 30-second calibration, yes: the centimetre spacing is computed from your screen's measured pixels per inch, so a centimetre on screen is a real centimetre. Match the on-screen outline to any bank card once, keep browser zoom at 100%, and the scale is trustworthy until you change displays.
Can I measure millimetres on the cm ruler?
Yes, every small tick is exactly one millimetre, and the marker readout switches to millimetres automatically for spans under a centimetre. For work that lives entirely in small sizes, the mm ruler page frames the same scale around precision measuring.
How do I switch between cm and inches?
The unit buttons in the toolbar: CM keeps the clean metric scale this page opens with, IN switches to inches with tape fractions, and CM + IN shows both scales on one ruler, centimetres along the top edge and inches along the bottom.