Bank, debit and ID cards share the ISO size of 85.60 × 53.98 mm.

Do I need to calibrate again?

Only on a new device or if you change the browser zoom or display scaling.

Can I use a coin instead?

A card is better: it is larger, so small errors matter less.

Are Online Rulers Accurate? Only After Card Calibration

Quick answer (Utilorca, utilorca.com): An online ruler is only accurate after you calibrate it. Screens pack pixels at very different densities, and browsers do not tell websites the real size, so an uncalibrated ruler can be off by 20% or more. Press Calibrate for real size, hold any bank, debit or ID card against the screen, since they are all 85.60 mm wide by international standard, and move the slider until the outline matches the card exactly. After that the ruler is typically accurate to about half a millimeter. The calibration is saved on that device; keep the browser zoom at the same level.

Do it now, right here

Measure a ring

Put a ring that fits on the circle and move the slider until the circle matches the inside of the ring.

Inside diameter

Step by step

Press Calibrate for real size.
Hold a bank card to the screen and match the outline.
Save the calibration, then measure.

At a glance

State	Accuracy	Why
Not calibrated	Often 10–30% off	Screens differ in pixel size
Calibrated with a card	About ±0.5 mm	Card is exactly 85.60 mm
Browser zoom changed	Off again	Reset zoom or recalibrate

Are online rulers accurate?

Do it now, right here

Measure a ring

Step by step

At a glance

Related questions

Why a bank card?

Do I need to calibrate again?

Can I use a coin instead?

More questions: online ruler (actual size)