A ruler reading has two parts: the whole units before the endpoint and the smaller divisions after them. This guide explains both with examples you can try on the online ruler. The illustration below teaches the marks; its size on this page is not a calibrated physical measurement.
Set up the ruler before taking a reading
- Open the ruler on the screen you will use. Reset browser zoom to 100% with Ctrl+0 on Windows or Command+0 on Mac.
- Open Calibrate. Match a standard payment card, select your device, or enter the full screen diagonal. Check the result against a physical reference. See the screen calibration guide if the scale looks wrong.
- Select cm, mm or in, and choose the ruler edge nearest your object. For a long object, try a side edge or fullscreen.
- Align the object's starting point with the zero tick. Read at its other end, looking straight at the display. The screen bezel is not the zero mark.
Read centimeters and millimeters
There are ten millimeters in one centimeter. In centimeter mode, a short tick represents 1 mm, the halfway tick represents 5 mm, and numbered ticks mark whole centimeters. If an object finishes four short ticks after 3 cm, its length is 3.4 cm = 34 mm.
In millimeter mode, read the numbered millimeter marks directly. An endpoint at 34 means 34 mm. To convert millimeters to centimeters, divide by ten: 34 ÷ 10 = 3.4. Do not count the zero line as the first millimeter; count the spaces from it.
Read inches and sixteenths
The inch scale divides each inch into 16 parts. Whole-inch ticks are longest. Between two whole-inch numbers, the middle tick is 1/2 inch, the quarter ticks mark 1/4 and 3/4, and the shortest divisions are sixteenths.
For an endpoint six short divisions beyond the 2-inch mark, start with 2 + 6/16. Simplify 6/16 to 3/8: the result is 2 3/8 inches, or 2.375 inches. Multiplying 2.375 by 25.4 gives 60.325 mm. That conversion is exact arithmetic; it does not make the screen reading equally precise.
| Inches | Decimal inches | Millimeters |
|---|---|---|
| 1/16 | 0.0625 | 1.5875 |
| 1/8 | 0.125 | 3.175 |
| 1/4 | 0.25 | 6.35 |
| 3/8 | 0.375 | 9.525 |
| 1/2 | 0.5 | 12.7 |
| 3/4 | 0.75 | 19.05 |
| 1 | 1 | 25.4 |
Measure when the object cannot start at zero
Use two readable tick positions and subtract the first from the second. If the object starts at 1.2 cm and ends at 5.8 cm, its length is 5.8 − 1.2 = 4.6 cm. Clicking the ruler places A and then B as visual guides. Read each endpoint on the scale; the letters identify the two positions.
Switch between physical units and pixels
One inch equals exactly 2.54 cm or 25.4 mm. Multiply inches by 2.54 for centimeters, or divide centimeters by 2.54 for inches. A 12-inch length is 30.48 cm, so a phone cannot show the whole length at actual size.
Pixel mode uses CSS pixels for on-screen positions. A CSS pixel is not a fixed physical length across devices. Use calibrated cm, mm or inches for an object; use pixels when you need screen coordinates.
Check the result
Measure the same object a second time from the same zero mark. If your known reference reads incorrectly, correct the calibration rather than adding a guessed offset. Avoid sharp or heavy objects on the display. Use a physical ruler or caliper when a small error would matter.
Open the ruler and try a measurement → · Troubleshoot your screen scale