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MM Ruler — Actual Size, Numbered in Millimeters

Every millimeter is marked and the numbers read in mm. Calibrate once for real-size measurements.

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MM ruler

A millimeter ruler on your screen

This page shows the ruler with a mark for every millimeter and numbers in millimeters (10, 20, 30…), so you can read small sizes directly without converting from centimeters. It is handy for screws, bolts, beads, jewelry, watch straps and other small parts.

Related guides: How to Read a Ruler (Inches and Centimeters) · Fraction to Decimal Inch Chart (with Millimeters)

How to use it

  1. Tap Calibrate and match a bank card. At millimeter scale, calibration matters most.
  2. Hold the part against the ruler with one edge at 0.
  3. Read the last number passed, then add the small marks after it: 30 plus 7 marks is 37 mm.

Good to know

  • 10 mm = 1 cm, and 25.4 mm = 1 inch.
  • The longer marks at 5, 15, 25… help you count quickly.
  • For round objects such as screws, measure straight across the widest point to get the diameter.

Small things in millimeters

1 mm = 0.1 cm = 0.0394 in. Millimeters are the usual unit for screws, jewelry, beads and electronics. Typical sizes to check your readings against:

ObjectMillimetersInches
Nano-SIM card12.3 × 8.80.48 × 0.35
Micro-SIM card15 × 120.59 × 0.47
AA battery (diameter)14.50.57
US dime (diameter)17.910.705
US penny (diameter)19.050.750
M3 screw (thread diameter)30.12
Bank card (thickness)0.760.03
  • After calibration the ruler is accurate to about ±0.5%, which is about 0.1 mm on a 20 mm object.
  • Measure round things across their widest point (the diameter).
  • To convert millimeters to inches, divide by 25.4.
Guide

Three ways to calibrate

1

Auto-detect your device

The ruler recognizes about 130 phones, tablets, laptops and monitors, including iPhones, Galaxy phones, Pixels and MacBooks.

Instant±1%
2

Enter your screen size

Enter your screen's diagonal size (for example, 15.6" for a laptop), and the ruler works out the scale from your screen resolution.

Manual±2%
3

Calibrate with a bank card

Hold any bank card against the outline on screen and move the slider until they match. Bank cards are the same size everywhere: 85.60 × 53.98 mm.

Most Accurate±0.5%
Accuracy

How accurate is the ruler?

How accurate the ruler is depends on how well it knows your screen's pixel density, meaning how many pixels fit in an inch. There are three ways to set it, from most to least accurate:

Bank card (±0.5%): Bank and credit cards are made to one worldwide size, 85.60 × 53.98 mm (ISO/IEC 7810 ID-1). Because every card is exactly the same, matching one on screen gives the most accurate result.

Auto-detect (±1%): The ruler looks up your device's published screen size and resolution. Screens of the same model can differ very slightly, so this is a little less exact. It covers about 130 devices.

Screen size (±2%): You type in your screen size, and the ruler works out the rest from your resolution. Advertised sizes are rounded (a "15.6-inch" screen may really be 15.59 inches), so this is the least exact method. It's a good fallback when you don't have a card.

Reference Sizes

Real-life objects to check your measurements

Use these everyday objects to confirm the ruler is calibrated correctly.

Bank or credit card: 85.60 × 53.98 mm (3.37 × 2.13 in)

A4 paper: 210 × 297 mm (8.27 × 11.69 in)

US Letter paper: 8.5 × 11 in (215.9 × 279.4 mm)

US quarter: 24.26 mm (0.955 in) across

1 euro coin: 23.25 mm (0.915 in) across

AA battery: about 50.5 mm long and 14.5 mm wide

Rulers

More online rulers

Measuring guides

FAQ

Frequently asked questions

With credit card calibration the ruler is accurate to about ±0.5%, which is under 0.2 mm across 30 mm. Without calibration it can be off by much more, so calibrate first.

Measure the diameter straight across the threads at the widest point. For the length, measure from under the head to the tip; flat-head (countersunk) screws are measured including the head.

This page labels the ruler in millimeters so small sizes can be read directly. The marks are the same as on the cm ruler; use that page if you prefer cm labels.

The ruler marks every millimeter, so you can read to the nearest millimeter, or estimate half a millimeter between two marks.