Online Ruler

A true-to-size on-screen ruler. Pick mm, cm or inches. Calibrate using your screen's size or match a credit card. Works horizontally or vertically with fullscreen mode for precise measurements.

Units

Orientation

Viewport fit

Only affects the on-screen length, not the scale.

Tick density

Calibration

📺 Calibrate by screen size

Common diagonals

Desktop and laptop primary sizes.

Or enter exact diagonal (in)

Uses your device's reported viewport to set scale.

Current resolution

× CSS px
DPR

💳 Credit card calibration (Most Accurate)

Match a credit or bank card (85.60 mm wide)

Adjust until this outline exactly matches your card

Fine tune

Nudge scale ±20%

Set by known length

Type a target length and we set the scale so the on-screen bar below measures exactly that.

Scale

px per mm

Approx PPI

px per inch

Drop-in presets

Actions

Share link keeps your calibration and layout.

Ruler Preview

Reference bar

Drawn below at exact size

💡 Instructions

For best accuracy:

• Use the credit card method for calibration
• Try fullscreen mode for measuring objects
• Recalibrate if your display scaling changes

Fullscreen features:

• Click and drag to measure distances
• Use entire screen as measuring surface
• Press Escape to exit fullscreen

📏 Screen Ruler

Units

Orientation

Detail

Click and drag to measure

Scale: px/mm

Tip: If your OS display scaling changes, recalibrate. For the tightest match, use the credit card method. Use fullscreen mode to measure physical objects against your screen.