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Calibrate
Match this screen to the real world. Choose a method below.
Starts from the CSS reference density (~96 CSS px/inch). Device pixel ratio does not multiply this — high-DPI screens pack more physical pixels into the same CSS inch. Use as a starting point only; confirm with a credit card or your display diagonal.
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Spec PPI is physical. This app converts it to CSS PPI for your current display scale (÷ devicePixelRatio) so ticks match on-screen CSS pixels.
Enter your display’s diagonal size in inches (from specs or the bezel label). Uses the browser’s CSS screen width×height — correct for on-screen CSS measurements when zoom is 100%.
Place a real object against the blue outline. Adjust CSS PPI until they match, then Done. The outline is drawn at true size (not shrunk to fit) — if it is larger than the screen, match the visible long edge, or use Card / Custom.
How to use
Calibrate once, then measure with edges, guides, or crosshair.
Basics
- Calibrate — open Calibrate and pick Auto-detect, Device, Diagonal, or a physical Reference (Card / A4 / Letter / Custom).
- Edges & guidelines — toggle Top / Bottom / Left / Right (all may be off). Click the scale or measuring area to drop a crosshair (horizontal + vertical pair). Multiple clicks add multiple pairs. Click a guide or its mark on any scale to select it, then Remove in the tooltip (Delete/Backspace also works). Esc or Clear removes all.
- Overlay grid — add one or more tick-aligned grids; each new grid is denser. Esc or the clear control removes guides & grids.
- Distance guides — tool button with two lines; drag to read the gap.
- Crosshair — follows the pointer with live X,Y readout.
- Hide Info — clears the marketing panel for a clean canvas.
Keyboard
- H
- How to use
- C
- Toggle crosshair
- G
- Show / hide guidelines
- Esc
- Close modal · clear guides & grids · off crosshair
- F
- Fullscreen
- D
- Dark / light
- 1 / 2 / 3
- Units mm / cm / in
- + / −
- Fine-tune calibration ±0.5%
Prefs key: ruleronphone:v3. If the UI glitches, clear that localStorage key and reload. Use Restart tour below to replay the first-visit tour anytime; Skip and Done keep the choice saved.