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Calibration

Make the ticks match the real world

Screens differ in pixel density and OS scaling. Calibration teaches ruleronphone how many CSS pixels equal one millimeter on your display. Open ruler.

01

Credit card (recommended)

  1. Choose Card in the calibration sheet.
  2. Hold an ISO/IEC 7810 ID-1 card against the screen along the active axis.
  3. Match the overlay to the card long edge (85.60 mm), then Apply.

02

A4 / Letter

Use a known paper sheet when you have one handy. Align the reference edge and Apply.

03

Custom length

Enter a known length (for example 100 mm or 4 in), drag the overlay to match a trusted mark, then Apply.

04

Device / diagonal / auto

Diagonal uses advertised inches as a rough start. Device and auto-detect helpers can seed calibration - card or custom length is usually better for handheld work.

05

Fine adjust & zoom

Use +/- to nudge density until ticks lock. Work at ~100% browser zoom; recalibrate after display or zoom changes.

Open calibrationFAQ