Online Ruler
No install • Works on mobile, tablet, desktop • <1% with calibration

Measure anything.
Right here.

Your screen, turned into a true-size ruler. Precise to 0.5 mm. Calibrate once with a bank card and measure in cm, mm & inches — directly on your screen.

Start measuringHow to calibrate
✓ 250+ devices auto-detected✓ Credit card • Diagonal • Device✓ PWA • Fullscreen • Print
12.3 cm · 4.84 in · 123 mm
Drag box to measure • Drag edges to resize •
Using default 96 PPI • Calibrate for better accuracy
inches
Why righello online?

More accurate than any fixed-pixel ruler.

Most online rulers assume 96 PPI. A 2024 iPhone is 460 PPI, a MacBook 254 PPI — that’s 58% to 380% error on a fixed ruler. We solve it with 5 calibration methods and show your exact PPI.

96 PPI
fixed rulers
-4% → +380%
PiliApp
monitor picker
±3 → ±8%
Righello
card calibrated
<1%
Calibration methods
5 ways
  • ◎
    Auto-detect
    250+ phones, tablets, laptops — instant factory PPI
  • ▦
    Pick your device
    Choose exact model — ±1% excellent
  • ⧉
    Screen diagonal
    Enter inches — we compute PPI from native resolution
  • ◆
    Credit card (most accurate)
    85.60×53.98 mm ISO ID-1 — sidesteps zoom & scaling
All settings saved locally. Re-calibrate when you switch monitors or change browser zoom.

How to use the online ruler

Three steps. No app. No signup. Works on iPhone, iPad, Android, MacBook, Windows — any browser.

1
Calibrate once
Tap Calibrate and place a bank card on the blue guide. Drag slider until it matches. That’s your true PPI — saved for next time.
Perfect <1% error
2
Pick units & orientation
Toggle cm / inch. Ruler shows both — metric on top, imperial below. Rotate for tall objects on phones.
cminchmm
3
Drag to measure
Place object on screen (or hold near). Drag the white box — live readout in cm, inch & mm. Arrow keys nudge 0.5 mm. Click label to copy.
Tip: Fullscreen gives you the longest edge. Print locks at 100% scale.
Supported devices
Works on every modern browser. Auto-detect for 250+ models — Apple iPhone/iPad/MacBook, Samsung Galaxy, Pixel, Surface, plus any monitor via diagonal.
  • • Chrome / Edge / Firefox
  • • Safari iOS / macOS
  • • Opera • Samsung Internet
  • • Phone, tablet, desktop, TV
Tips for best results
  • ✓ Keep browser zoom at 100% (we warn you if not)
  • ✓ Re-calibrate when you switch screens
  • ✓ JavaScript enabled — ruler is canvas-drawn
  • ✓ Share: copy link with ?ppi=142.3

FAQ — is the ruler accurate?

Why is it off at first?
Browsers default to 96 PPI (old web standard). A 27″ 4K monitor is ~163 PPI, iPhone 460 PPI. Without calibration an “inch” is just a guess. Calibration tells us how many CSS pixels = one real inch.
How accurate after calibration?
Credit card method: <1% error (about 1 mm on 10 cm) on any screen, because ISO cards are exactly 85.60 mm worldwide and you match visually. Diagonal method: ~2%.
Does it stay calibrated?
Yes — saved to localStorage + URL ?ppi= share. Put it on your phone via QR in one tap.
Can I measure larger than my screen?
Mark, slide object, add lengths. Or print our printable ruler (30 cm / 12 inch, A4 & Letter, 100% lock).
Quick links — more rulers
CM Ruler
30 cm • mm • actual size
Inch Ruler
12 inch • 1/32 fractions
MM Ruler
300 mm • 0.5 mm
PPI Calculator
diagonal → PPI
Printable rulerPaper sizes
Share your calibration
Your current PPI is saved as ?ppi=96.0. Send it to your phone or teammate — they’ll get the same scale instantly.