Millimeter ruler

What is a millimeter?

A millimeter is one thousandth of a meter and one tenth of a centimeter — the smallest unit most people comfortably measure by eye. Anything thinner than a millimeter (paper is about 0.1 mm) belongs to calipers and micrometers; everything from about 1 mm up to a few centimeters is millimeter territory.

Reading the mm scale

In mm mode, every small tick is one millimeter and the long numbered ticks come every 10 mm, labeled 10, 20, 30… Read the numbered mark below the object’s end, then count the small ticks past it: four ticks past 40 is 44 mm. The medium-length tick at each 5 is the half centimeter — a handy landmark when counting.

The scale is identical to cm mode; only the labels differ. Switch freely — 44 mm and 4.4 cm are the same mark.

Handy size references

• Credit / bank card thickness — 0.76 mm (an exact standard)
• US dime thickness — about 1.35 mm
• SD card thickness — about 2.1 mm
• Standard pencil diameter — about 7 mm
• US quarter diameter — 24.26 mm

Conversions

From	To	Multiply by
mm	cm	0.1
mm	m	0.001
mm	inches	0.03937
inches	mm	25.4 (exact)

Getting precise results

• Calibrate with the card method. Millimeter accuracy lives or dies on calibration — the physical-card method is the most accurate.
• Zoom the page to 100% and leave it there. Any zoom rescales the ticks.
• Use guides for small objects. Drop a guide at each end of the object and subtract the readouts — steadier than eyeballing a tick.
• View straight on. Parallax at an angle can shift an apparent edge by a millimeter or more.

Frequently asked questions

How small is a millimeter?

A millimeter is one thousandth of a meter — roughly the thickness of a credit card (0.76 mm) or a little less than half the thickness of a US dime (1.35 mm). It is the smallest unit most people measure by eye.

Should I use mm or cm mode?

They show the same scale with different labels. Use mm mode when you want a single precise number like 47 mm; use cm mode when you think in centimeters like 4.7 cm. For anything under about 5 cm, mm mode is usually easier to read.

How accurate is the on-screen millimeter scale?

As accurate as your calibration. With the credit-card method, expect accuracy within about half a millimeter on a typical display. For sub-millimeter work — jewelry, electronics — use proper calipers; a screen ruler is a quick check, not a metrology tool.