About and methodology

About Simple Online Ruler

Simple Online Ruler is a free, browser-based actual-size ruler and a set of measurement tools. This page explains how it produces real-world measurements, how we check accuracy, and how we maintain the content.

How the measurement works

Pixels are mapped to millimeters

Calibration stores a pixels-per-millimeter value for your display. Every tick and readout is drawn from that single saved value, so all units share one physical scale.

Calibration uses a known reference

Auto-detect estimates from device signals, screen-diagonal uses the display size in inches, and credit card calibration matches a fixed ISO/IEC 7810 ID-1 width of 85.60 mm. The card method is the most dependable because the reference is physical and exact.

Constants are exact, not rounded

Conversions use the defined relationships: 1 inch = 25.4 mm = 2.54 cm. These are definitions, so converter outputs are exact to the displayed precision.

Zoom and display are held constant

Because browser zoom and OS display scaling change CSS pixel size, a saved calibration is only valid on the same display at the same zoom. The tool prompts you to keep zoom at 100%.

How we treat accuracy

  • After credit card calibration at 100% zoom, everyday measurements are typically accurate to within about 1 millimeter.
  • Uncalibrated readings rely on a default 96 CSS pixels per inch and can be off by 10% or more on high-density screens.
  • For sub-millimeter tolerances we recommend a physical caliper; we say so plainly rather than overstating screen accuracy.

Editorial and update policy

Each page carries a visible last-updated date that we change only when content meaningfully changes.

Conversion math and physical constants are checked against published SI and ISO definitions.

We correct errors when found and note material changes in the page's update date.

We do not publish fake reviews, fabricated authorship, or AI-generated pages that lack original value.

Who maintains this site

Simple Online Ruler is built and maintained by the Simple Online Ruler team. Questions, corrections, and accuracy reports are welcome and help keep the tools trustworthy.

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