# Simple Online Ruler

> Simple Online Ruler is a free, browser-based actual-size ruler. After a one-time
> screen calibration, it measures real-world length in centimeters, millimeters, and
> inches with no download or account. The site also publishes measurement guides and
> screen-scale tools.

Key facts:
- Calibration methods: auto-detect device, screen diagonal, or credit card (ISO/IEC 7810 ID-1, 85.60 mm x 53.98 mm).
- Accuracy depends on calibrating on the same display and keeping browser zoom at 100%.
- Units supported: centimeters (cm), millimeters (mm), inches (in, down to 1/16 in).

## Tools

- [Actual-size online ruler](https://simpleonlineruler.com/): Measure on screen in cm, mm, or inches after calibration.
- [CM ruler online](https://simpleonlineruler.com/cm-ruler/): Use and calibrate a centimeter ruler on screen.
- [MM ruler online](https://simpleonlineruler.com/mm-ruler/): Use millimeter ticks for small metric measurements after calibration.
- [Inch ruler online](https://simpleonlineruler.com/inch-ruler/): Read inch fractions and use a calibrated on-screen inch ruler.
- [Screen PPI calculator](https://simpleonlineruler.com/ppi-calculator/): Calculate CSS pixels per inch and per millimeter from screen resolution and diagonal.
- [Ruler measurement converter](https://simpleonlineruler.com/ruler-measurement-converter/): Convert between inch fractions, decimal inches, centimeters, and millimeters.
- [CM to inches converter](https://simpleonlineruler.com/cm-to-inches/): Convert centimeters to inches (divide by 2.54), with chart and FAQ.
- [Inches to cm converter](https://simpleonlineruler.com/inches-to-cm/): Convert inches to centimeters (multiply by 2.54), with chart and FAQ.
- [MM to inches converter](https://simpleonlineruler.com/mm-to-inches/): Convert millimeters to inches (divide by 25.4), with chart and FAQ.

## Guides

- [Measurement guides hub](https://simpleonlineruler.com/measurement-guides/): Index of all ruler guides, calculators, and converters.
- [How to read a ruler](https://simpleonlineruler.com/how-to-read-a-ruler/): Read inch fractions, centimeter marks, and millimeter ticks.
- [Calibrate an online ruler](https://simpleonlineruler.com/calibrate-online-ruler/): Set actual-size scale with a credit card, screen diagonal, or known object.
- [Is an online ruler accurate?](https://simpleonlineruler.com/online-ruler-accuracy/): How precise a screen ruler is after calibration and what affects accuracy.
- [Online ruler vs physical ruler](https://simpleonlineruler.com/online-ruler-vs-physical-ruler/): When to use a screen ruler and when to use a physical ruler, tape, or caliper.

## About

- [About & methodology](https://simpleonlineruler.com/about/): How the tool maps screen pixels to real size, how accuracy is verified, and the content update policy.
- [Privacy policy](https://simpleonlineruler.com/privacy/): Calibration stays in the browser; Microsoft Clarity provides anonymized analytics; no accounts and no data selling.