A ruler that matches the real thing.
Calibrate once with a bank card or a coin. Then measure objects, draw boxes and circles, lay a slide ruler at any angle, or scale a photo. It all runs in your browser.
Drag on the stage to measure. Hold Shift to snap lines to 15° or boxes to squares. Keys: L line, B box, O circle, U unit, G grid, R slide ruler, Delete removes the last measurement. You can also drop a photo onto the stage.
Check it against real things
Set a coin, card or ring on these outlines. If it fits edge to edge, your calibration is right.
Convert a length
Type a size once and see it in every unit, with an actual-size bar underneath.
Why most screen rulers are wrong
Browsers draw in CSS pixels and assume 96 of them make an inch. Real screens disagree. A 27 inch QHD monitor fits about 109 CSS pixels into each inch, so a 10 cm bar drawn at 96 dpi measures about 8.8 cm. On a modern phone, with roughly 153 CSS pixels per inch, the same bar measures about 6.3 cm.
Calibration finds the true pixels per millimeter for your screen. TrueRule stores that number in your browser and rescales it when you zoom, so a centimeter stays a centimeter.
Read a ruler from the 0 mark, not from the edge of the screen. Look straight at the glass to avoid parallax, and keep the page at its normal zoom when you calibrate.
| Method | Best for | Typical accuracy |
|---|---|---|
| Card or coin | Any screen, including external monitors | About 1% |
| Custom length | When you have a real ruler to match | About 1% |
| Screen size | A quick start when you know the diagonal | A few percent |
| No calibration | Pixel work for design and web | Often 10% or more off in real units |
Questions
How accurate is it?
With a card or coin calibration, expect readings within about 1 mm over 10 cm on most screens. The screen size method can drift by a few percent because makers round their diagonals. Use calipers when you need engineering tolerances.
Does browser zoom break it?
No. TrueRule reads your zoom level and keeps lengths physical. If you move the window to a screen with different scaling, run Calibrate again. Pinch zoom on a phone is not tracked, so reset it before measuring.
How do I measure a photo?
Choose Photo, or drop an image on the stage. Draw a line over something with a known length, type that length, and press Set scale. Every measurement after that reads in real units. Shoot straight on, because an angle skews the result.
Can I measure something longer than my screen?
The ruler is as long as your screen. Measure long objects in sections with the slide ruler, or use a tape measure.
Is anything uploaded?
No. Your calibration, measurements and photos stay in this browser.
Which calibration method should I use?
Use a card or coin whenever you have one. Bank cards follow a single international size, 85.60 by 53.98 mm. Use the screen size method only as a fallback.