What "accurate" means for an on-screen ruler, where errors come from, how to verify your own setup in a minute, and when not to use these tools
Last updated 2026/09/07
For the screen ruler, accuracy is how closely a distance read off the screen matches the same distance measured with a physical ruler. The software scale factor is governed by the CSS pixels per inch that calibration produced: if it is 3 % off, every scale reading is 3 % off. Endpoint placement, parallax and the object itself add separate uncertainty. The methodology page shows how the scale is derived.
We do not publish a blanket figure such as "accurate to 1 %", because we have no way of knowing your display, your calibration or your viewing angle. What we can do is tell you how to measure the error yourself.
Ctrl/Cmd + 0)
and calibrate with a bank card if you have not already.A reading of 85–86 mm is a useful repeatability check, not a general accuracy guarantee. A reading of 88 mm means the saved CSS PPI is too low by about 3 %. Return browser zoom to 100 %, realign the physical card to the outline, save again, and repeat the check. Repeating the check with the card held vertically (53.98 mm) confirms the vertical rulers too.
| Source | How the effect is determined | What to do |
|---|---|---|
| No calibration | Fine-pointer desktops start from the 96 CSS px/in reference; touch screens use a device-based estimate. Neither reveals your display's physical size or a known error. | Calibrate with a card |
| Browser zoom not 100 % | Exactly the zoom factor (125 % zoom → 25 % error) | Ctrl/Cmd + 0; the ruler warns when the pixel ratio changed since calibration |
| Rounded diagonal size | A "27-inch" panel that is really 26.9 in → about 0.4 % | Prefer the card method, or use the exact active-area size from the datasheet |
| Card outline off by one slider step | 0.5 CSS px over 85.6 mm (about 0.15 % at 96 CSS px/in) | Adjust until the outline just covers the card edges |
| Parallax | Depends on viewing angle and cover-glass thickness; there is no universal offset | Look straight down at the edge you are reading |
| Curved screens, thick protectors | Varies across the panel | Measure near the center of the display |
| Tick resolution | Depends on your calibrated CSS PPI; a 96 CSS px/in pixel is 0.26 mm | Not correctable; use the millimeter scale for small objects |
The photo tool is only as good as the reference you mark and the geometry of the shot:
Unit conversions use exact definitions and are rounded for display. Only the current-screen-pixel result depends on the estimated or saved screen mapping.
Do not rely on any of these tools where a wrong number could cause injury, financial loss or legal consequences: engineering tolerances, medical measurements, legal evidence, commercial trade, cutting expensive material. Use a certified physical instrument and treat the screen ruler as a convenience for everyday checks, as the terms of service state.
If the card check fails after a careful recalibration, tell us through the contact page with your device, browser, screen size and the readings you got. A report we can reproduce is the most useful feedback we receive.