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Reading guide
Worked exampleEnd 47 mm − start 12 mm = length 35 mm
Subtract an offset start
Enter two readings from the same millimeter edge. This calculation does not need calibration; taking the readings does.
| Reading | Equivalent | How to read it |
|---|---|---|
| 5 mm | 0.5 cm | Middle tick in the first interval |
| 23 mm | 2.3 cm | Three small intervals after 20 |
| 35 mm | 3.5 cm | Difference between 12 and 47 mm |
| 100 mm | 10 cm | One tenth of a meter |
Read mm directly, not as a decimal centimeter
In millimeter mode, the numbered major marks show 10, 20, 30, and so on. Every small interval is one millimeter; five intervals reach the longer middle mark. An endpoint three intervals after 20 is 23 mm. The same point on a centimeter scale reads 2.3 cm. Millimeter mode changes the labels, not the physical distance between ticks.
Measure from an offset starting point
You do not need to place every object exactly at the origin. Note the reading at each end, then subtract start from end. A paper strip spanning 12 mm to 47 mm is 35 mm long, not 47 mm. Enter those two readings in the helper below the ruler. Keep both readings on the same edge, with the object parallel to the scale.
A small unit does not certify high precision
Displaying one-millimeter marks does not prove one-millimeter measurement accuracy. Calibration, screen viewing angle, endpoint alignment, and tick rendering all contribute uncertainty. A result displayed with decimals is a numerical readout, not a certified tolerance. For a tight fitting, machining task, or safety-critical measurement, verify with an appropriate physical instrument.
How this ruler works
The scale uses your current calibration in CSS pixels per inch. Metric ticks divide that density by 25.4; inch ticks divide it by 16. Conversions use the exact definition 1 inch = 25.4 mm. The target keeps its entered length rather than stretching to fill the display.
Keep browser zoom at 100%, verify with a known reference, and recheck after changing screens. Inputs stay in your browser; no camera, account, or upload is required. For critical dimensions, use a suitable calibrated physical instrument.
Methodology, calibration help, and measurement support: Accuracy methodology · Calibrate screen · Contact us. Unit definitions: NIST.