Screen ruler converts your display into a millimeter-accurate measuring surface. Place a physical object of known size on the screen to calibrate once, and the tool calculates the pixel-to-millimeter ratio for that screen. After calibration it enters full-screen mode with rulers on all four edges and a live crosshair coordinate readout as you move the mouse.
How Calibration Works
Calibration determines the pixel-per-millimeter ratio of your screen. Drag the red markers to align with the ends of a reference object — for example, a credit card at 85.6 mm — and the tool divides pixel count by physical length to get the ratio (e.g. 342 px ÷ 85.6 mm ≈ 4.0 px/mm). Every coordinate displayed afterward uses this ratio.
Typical pixel densities by screen type:
- Standard desktop monitors: ~2–4 px/mm
- High-res laptops: ~4–8 px/mm
- Retina / high-DPI displays: ~8–12 px/mm
The same object will be placed at different pixel positions on different screens — that is expected behavior, not a calibration error.
Reference Objects by Device
The tool automatically shows different reference object lists depending on screen width:
Desktop reference objects
- Credit card long edge: 85.6 mm
- A4 paper width: 210 mm
- A4 paper height: 297 mm
- US dollar bill: 155.956 mm
Mobile reference objects
- Credit card short edge: 53.98 mm
- AA battery: 50.5 mm
- AAA battery: 44.5 mm
- SIM card long edge: 25 mm
- USB-C port width: 8.4 mm
On mobile, use objects that fit entirely on screen. A4 paper extends beyond the screen edge on phones and will cause calibration error.
What Reduces Accuracy
After calibration, typical precision is ±0.5 mm. These conditions degrade it:
- Browser zoom level is not 100% — the most common source of error
- Object has physical thickness (not flat against the screen)
- Reference object is non-standard (e.g. a card slightly larger or smaller than 85.6 mm)
- Phone case or screen protector adds distance between the glass and the object