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Measurement

Online Ruler

Measure small objects on screen after matching the ruler to a physical card. Screen measurements are approximate and require calibration.

Match the card

Place a standard payment card over the outline. Adjust the width until both edges match, then confirm. No photo or card details are needed. If the whole card does not fit, turn your phone to landscape or use a larger screen.

Calibration required

Recalibrate after browser zoom, changing displays or rotating the device. Measurements are estimates; use a physical ruler when precision matters.

A 171 px card outline gives 171 Ă· 8.56 ≈ 19.98 px/cm. A 5 cm length then spans about 99.88 px.

Practical guides

Reproducible examples that explain a tool's result, not just its buttons.

Calibrate an online ruler and check its error

A web page cannot automatically know whether its displayed centimetre is a physical centimetre. Apps24 matches an on-screen rectangle to an object known to be 85.60 mm wide. The table verifies implementation calculations, not a physical measurement.

What is Online Ruler?

A CSS centimeter is not necessarily a physical centimeter on your monitor. Apps24 converts a reference card’s on-screen width into a scale: pixels per centimeter = card width in CSS pixels Ă· 8.56. The reference is an 85.60 × 53.98 mm card; a differently sized card will produce incorrect measurements.

When to use this tool?

Use this for a quick estimate of a small object when a physical ruler is unavailable. Do not use it for manufacturing tolerances, medical measurements or other precision-critical decisions.

Examples

  • A reference width of 171 px gives about 19.98 px/cm. A 5 cm span then occupies about 99.88 px.
  • At 1 inch = 2.54 cm, the inch scale uses the same calibration. Small divisions are decimal tenths of an inch, not sixteenths.

How to use Online Ruler

1. Hold a standard-size card gently against the screen without entering or uploading card details. 2. Adjust the outline until its width matches the card, then confirm calibration. 3. Place the object at zero and read the endpoint. Double-click the ruler to move zero. 4. Recheck calibration after zooming, changing displays or reopening the page.

References

Frequently Asked Questions

Q: Does a saved setting guarantee accuracy?

A: No. Only the previous outline width is saved locally. Display scaling and zoom can change, so the page asks you to confirm the match again.

Q: Why does the result differ from a real ruler?

A: Card alignment, screen scaling and viewing angle introduce error. Use a physical ruler if the measurement matters.

How it works

The ruler draws calibrated tick positions in a browser canvas. It cannot automatically determine your screen’s physical pixel density.