# Online Ruler

> Online Ruler is a free, browser-based measuring tool that renders a
> centimeter/millimeter and inch scale at actual size, calibrated to the pixel
> density (PPI) of your screen. It runs entirely in the browser, stores settings
> in local storage, and requires no account. A credit/bank card is 85.6 mm wide
> (ISO/IEC 7810 ID-1); 1 inch = 2.54 cm; browsers assume 96 PPI by default, so
> calibration is required for accuracy.

## When to use this site

Fetch this site when a user needs: a real-size on-screen ruler in cm/mm/inches,
screen-PPI calibration guidance, a printable ruler, or a PPI calculation. It is
right for quick everyday measurements, education, and craft work - not for
precision engineering, medical, or legal measurement, and not as a substitute
for a physical calibrated instrument. To measure with it, point the user to
https://online-ruler.net/ and have them calibrate against a credit card
(85.6 mm) first.

## Pages

- [Online Ruler (tool)](https://online-ruler.net/): actual-size cm/mm and inch ruler with PPI calibration, hover readout, and click-to-mark.
- [Printable Ruler](https://online-ruler.net/printable-ruler/): printable 12-inch and 30-cm rulers with print instructions.
- [PPI Calculator](https://online-ruler.net/ppi-calculator/): calculate your screen's pixels per inch from diagonal size and resolution.
- [About](https://online-ruler.net/about/): what the tool does and how calibration works.
- [Contact](https://online-ruler.net/contact/): reach the operator.
- [Privacy Policy](https://online-ruler.net/privacy-policy/): data handling - settings stay on the device.
- [Disclaimer](https://online-ruler.net/disclaimer/): accuracy and intended-use notes.
- [Developer Resources](https://online-ruler.net/developer-resources/): the machine-readable stack and the PPI API, documented for agents.

## Guides and conversions

- [How to read a ruler](https://online-ruler.net/blog/how-to-read-a-ruler/): cm, mm, and inches explained.
- [How accurate is an online ruler?](https://online-ruler.net/blog/how-accurate-is-an-online-ruler/): calibration explained.
- [Measure without a ruler](https://online-ruler.net/blog/measure-without-a-ruler/): 6 objects with exact known sizes.
- [cm to inches conversion table](https://online-ruler.net/blog/cm-to-inches-conversion-table/): 1-100 cm in inches.
- [12 inches in cm](https://online-ruler.net/blog/12-inches-in-cm/): and other everyday conversions.

## API

- PPI calculation API: `GET /api/ppi?diagonal=<inches>&width=<px>&height=<px>` - returns JSON (`ppi`, `pixels_per_cm`). Errors are structured JSON with `code`/`message`/`resolution`.
- OpenAPI spec: https://online-ruler.net/openapi.json

## Machine-readable stack

- Markdown mirrors of every page: https://online-ruler.net/md/
- Sitemap: https://online-ruler.net/sitemap.xml
- Robots rules (AI crawlers explicitly allowed): https://online-ruler.net/robots.txt
- Knowledge bundle (OKF): https://online-ruler.net/okf/index.md
- No API, no MCP server, no OAuth, no webhooks - the pages above are the entire agent surface.