Free Online Tools & Utilities

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0.5mm Accuracy

Calibrated with your physical credit card for near-perfect millimeter accuracy.

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Full-Screen Ruler

Extend the ruler across your entire screen — up to 400+ mm on large monitors.

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mm ↔ cm ↔ inches

Instantly convert between units using the built-in converter.

Common Object Sizes in Millimeters

Object	Size (mm)	Standard
Credit Card	85.6 × 53.98	ISO 7810
A4 Paper width	210	ISO 216
US Quarter	24.26 ⌀	US Mint
1 Euro Coin	23.25 ⌀	EU
AA Battery length	50.5	IEC
USB-A port width	~12	USB IF
Passport width	125	ICAO
Adult thumb width	~20	Avg

mm → Inches

1 mm0.039"

5 mm0.197"

10 mm0.394"

25 mm0.984"

50 mm1.969"

100 mm3.937"

Inches → mm

¼"6.35 mm

½"12.7 mm

1"25.4 mm

1.5"38.1 mm

2"50.8 mm

4"101.6 mm

Why Use a Millimeter Ruler Online?

Millimeters are the preferred unit for precision measurement worldwide. A millimeter is small enough for exact results (jewelry, crafts, phone accessories) yet readable without magnification. Our screen ruler shows tick marks at every 1mm, bold marks every 5mm, and labeled marks every 10mm — identical to a physical ruler. After one credit card calibration, it renders at true physical size on your specific screen.

Practical Uses for an Online MM Ruler

Measuring phone case or screen protector dimensions
Checking drill bit width without calipers
Verifying envelope or packaging dimensions
Beading, jewelry, and craft measurement
Comparing object sizes side by side

Frequently Asked Questions

How accurate is an online ruler in mm?

Within 0.5mm when calibrated with a credit card. Suitable for everyday and craft use.

How many mm is an inch?

25.4mm exactly. Use the inline converter above for any value.

Can I use this without printing?

Yes — entirely screen-based. No PDF or printer required.

How long is the ruler?

As wide as your screen. Up to ~420mm on a 24" monitor. Use Full-Screen Ruler for maximum length.

Does my calibration save?

Yes — saved in your browser localStorage. Works instantly on return visits.