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CM Ruler Online
Measure anything on your screen with this free CM ruler online. Calibrate it once with a credit card or your screen DPI, then drag across the ruler to measure in centimetres, millimetres, and inches — no download, no sign-up, and it works on phones, laptops, and desktop monitors.
Step 1 — Calibrate your screen
Screens differ, so calibrate first or measurements will be wrong. Pick one method:
Method A — credit card: hold a standard credit card (85.6 mm wide) against the blue bar below. Drag the right edge of the bar — or use the slider — until the bar is exactly as wide as the card.
Bar width: 323 px = 85.6 mm card · 37.7 px per cm
Method B — screen DPI: if you know your screen's DPI, type it here instead.
Current calibration: 96 DPI (37.8 px per cm) — not yet calibrated
Step 2 — Measure
Drag across the ruler below, from one end of the object to the other. The blue band shows your drag.
| Centimetres | — |
| Millimetres | — |
| Inches | — |
| Pixels dragged | — |
Step 3 — Show a length (optional)
Type a length and the ruler highlights it, so you can hold an object up and compare.
How it works
An on-screen ruler has one problem: the browser knows how many pixels wide your screen is, but not how big those pixels are in real life. A centimetre on a dense laptop screen might be 50 pixels, while on an old desktop monitor it is 38. That is why this CM ruler online starts with calibration instead of pretending one size fits all screens.
Calibration is simple because a credit card is a fixed, known size almost everywhere in the world: 85.6 mm wide and 53.98 mm tall. Hold a card flat against your screen, line its left edge up with the left edge of the blue calibration bar, and drag the bar's right edge — or move the slider underneath it — until the bar ends exactly where the card ends. Press "Set calibration" and the tool divides the bar's pixel width by 8.56 cm to get your screen's pixels per centimetre. If you would rather not use a card, type your screen's DPI instead: pixels per centimetre is DPI divided by 2.54, because there are 2.54 cm in an inch.
Once calibrated, the ruler itself is drawn to scale. The top edge is marked in centimetres with longer ticks every 5 cm, and the bottom edge is marked in inches, so both scales sit on one ruler. To measure, press and drag across the ruler: a blue band marks the distance you dragged, and the result shows it in centimetres, millimetres, and inches, plus the raw pixel count. Dragging again replaces the old measurement. The "Show a length" box works the other way round — type a length such as 12 cm and a green band highlights exactly that distance on the ruler, which is handy when you want to check whether an object, a photo, or a printed label is the right size.
Two things keep the measurement honest. First, calibrate at the zoom level you will measure at, and do not pinch-zoom afterwards, because zooming changes how many screen pixels a centimetre covers. Second, hold the card flat and square against the screen while calibrating; a card tilted or held at an angle reads wider than it really is. Done carefully, the ruler is usually accurate to about a millimetre over a 10 cm span — good enough for checking sizes, craft work, and comparing objects, though a real steel ruler is still the right choice when a measurement has to be exact.
Worked example
Say you want to measure a pencil, and your screen is a typical laptop display.
Step 1: Hold a credit card against the blue calibration bar and drag the bar until it matches the card. The bar reads 323 px wide. Pixels per cm = 323 ÷ 8.56 = 37.7 px per cm. Press "Set calibration".
Step 2: Drag across the ruler from the pencil's tip to its end. The drag covers 454 px.
Step 3: Length in cm = 454 ÷ 37.7 = 12.0 cm. In millimetres that is 120 mm, and in inches 12.0 ÷ 2.54 = 4.7 in.
The tool shows the same result: 12.0 cm, with 120 mm and 4.7 inches underneath. If you then type 12 cm into "Show a length", the green highlight band lands exactly on the same span you dragged, confirming the measurement from the other direction.
FAQ
How do I calibrate the on-screen ruler?
Hold a standard credit card against your screen and drag the blue calibration bar until it is exactly as wide as the card, then press Set calibration. A credit card is 85.6 mm wide, so the tool works out how many screen pixels make a centimetre. If you know your screen DPI, you can type it in instead — the ruler updates right away.
How accurate is an on-screen ruler?
After careful calibration with a credit card it is usually accurate to about 1 mm over a 10 cm span, which is good enough for checking sizes, comparing objects, and rough craft work. It is not a replacement for a real ruler when a measurement has to be exact, because screens differ and a card held at a slight angle changes the result.
How do I measure something with the ruler?
Press and drag across the ruler from one end of the thing you want to measure to the other. A blue band marks the distance you dragged, and the result shows the length in centimetres, millimetres, and inches. You can also type a length into the Show a length box and the ruler highlights exactly that distance.
Why does the ruler look the wrong size before I calibrate it?
Every screen packs a different number of pixels into a centimetre, so a browser cannot know your screen's real size until you tell it. The ruler starts at the common 96 DPI setting, which is right on many desktop monitors but wrong on most laptops and phones. Calibrating once fixes it for your screen.
Does the ruler work on a phone?
Yes. Drag with one finger to measure, and use the credit card calibration the same way. Phones have very dense screens, so the default 96 DPI setting will be far off — calibrating with a card is essential on a phone. Pinch-zooming the page after calibration will throw the measurement off, so keep the page at its normal zoom.