What mm means (millimetre, a thousandth of a metre), how many mm in a cm (10), in a metre (1,000), in an inch (25.4), the length units chart (mm, cm, m, km against inches, feet, yards and miles), how to read the small lines on a ruler, when to use which unit, and a printable chart with practice

"mm" is one of the most-searched three-letter queries in the world, about 224,000 times a month in the US, 17,000 in the UK, 14,000 in Canada and 11,000 in Australia, and "how many mm in a cm" another 11,000 in the US alone. Behind most of them is a child with a ruler, a parent with a drill bit, or a form that wants a measurement in a unit the family does not use at home. This page says what mm means, gives the whole length chart in both systems with the exact factors from the US National Institute of Standards and Technology (NIST Special Publication 811), shows how to convert by moving the decimal point, explains how to read the small lines on a ruler, and ends with a printable chart and practice set, because measuring and converting length is a Year 3 to 6 objective in England, a Grade 2 to 5 standard in the US and a Year 3 to 6 one in Australia.

Download the length units chart with practice (PDF) (A4, the metric and imperial charts with a printed ruler on page 1, the practice set with answers on page 2, free).

What mm means

mm is the symbol for the millimetre, one thousandth of a metre. The metre is the SI base unit of length (see the seven SI base units), and the prefixes do the rest:

A millimetre is about the thickness of a credit card, the width of the wire in a paper clip, or the gap between the small lines on a ruler. A grain of rice is 6 to 7 mm long; a pencil is 7 mm across; rain is measured in mm (10 mm of rain means a 10 mm deep layer over the ground); camera lenses, drill bits, screws, paper (A4 is 210 × 297 mm) and phone screens are all specified in millimetres, which is why the unit is everywhere even in countries that still drive in miles.

The symbol rules, which examiners mark: lowercase, no full stop, no plural s (5 mm), a space between the number and the symbol, and never "MM" (which would mean mega-metre, a thousand kilometres).

The short answers

The metric chart and the decimal-point rule

Every metric length conversion is a move of the decimal point:

• mm to cm: ÷ 10 (one place left). 45 mm = 4.5 cm. 8 mm = 0.8 cm.
• cm to mm: × 10 (one place right). 7.2 cm = 72 mm. 0.5 cm = 5 mm.
• cm to m: ÷ 100 (two places left). 350 cm = 3.5 m. 85 cm = 0.85 m.
• m to cm: × 100. 1.62 m = 162 cm (a person's height).
• mm to m: ÷ 1,000 (three places left). 2,500 mm = 2.5 m.
• m to km: ÷ 1,000. 1,500 m = 1.5 km. 400 m = 0.4 km.
• km to m: × 1,000. 3.2 km = 3,200 m.

The check that catches most mistakes: a bigger unit gives a smaller number. 45 mm must become 4.5 cm, not 450 cm, because a centimetre is bigger than a millimetre.

A worked example from the Year 5 and Grade 5 papers: a shelf is 1.2 m long and three boxes 35 cm wide are placed on it. 1.2 m = 120 cm; 3 × 35 = 105 cm; 120 − 105 = 15 cm = 150 mm of shelf left.

The imperial chart and the exact factors

Since 1959 the inch has been defined as exactly 25.4 mm (NIST, SP 811 Appendix B, marked exact), so every imperial length has an exact metric value:

Going the other way: 1 mm = 0.03937 in, 1 cm = 0.3937 in, 1 m = 39.37 in = 3.281 ft = 1.094 yd, 1 km = 0.6214 mi.

• Inches to mm: × 25.4. A 6-inch sandwich is 152 mm; a ¼-inch bolt is 6.35 mm.
• mm to inches: ÷ 25.4. A 100 mm tile is 3.94 inches, just under 4.
• Feet to cm: × 30.48. A 6-foot person is 183 cm.
• cm to feet and inches: ÷ 2.54 for total inches, then ÷ 12. 170 cm = 66.9 in = 5 ft 7 in.
• Miles to km: × 1.609 (or, roughly, × 8 ÷ 5). 26.2 miles (a marathon) = 42.2 km.
• km to miles: × 0.6214 (roughly × 5 ÷ 8). 10 km = 6.2 miles.

The 5-to-8 trick for miles and kilometres is good enough for the Year 6 objective "convert between miles and kilometres" in England and for reading a road sign.

How to read the millimetres on a ruler

A 30 cm school ruler has three sizes of line. The long lines with numbers are centimetres. The medium line halfway between two numbers is 5 mm (half a centimetre). The short lines are single millimetres, nine of them between each pair of centimetre lines (ten gaps). To read a length:

1. Line the object's end up with the 0 mark, not the end of the ruler (there is usually a small gap before 0).
2. Read the last whole centimetre the object passes: say, 7.
3. Count the small lines past it: say, 4.
4. Write 7.4 cm or 74 mm, both correct.

Most wrong answers in Year 3 and Grade 2 come from starting at the ruler's edge, counting the lines instead of the gaps, or reading "7 cm and 4 mm" as 7.04 cm. On a tape measure with inches, the small lines are sixteenths of an inch, which is why millimetres are easier: ten equal gaps and a decimal point.

Which unit for what

Engineering drawings, carpentry, sewing patterns and 3D printing use millimetres throughout and avoid centimetres, so that every dimension is a whole number and no decimal point can be misread. Medicine measures tumours and babies' heads in mm and cm; maps use km and miles.

Where it sits in each curriculum

Practice (answers in the PDF)

1. Write 68 mm in centimetres, and 3.5 cm in millimetres.
2. A door is 2.04 m tall. How many centimetres? How many millimetres?
3. A pencil is 17.5 cm long. A pencil case is 180 mm long. Does it fit?
4. Convert 8 inches to millimetres, and 50.8 mm to inches.
5. A running track is 400 m. How many laps make 5 km?
6. A car journey is 150 miles. Roughly how many kilometres is that?

Download the chart and the practice set (PDF)

Where to get help

A 1-on-1 online maths class with a tutor who holds a master's in mathematics and teaches KS2 and KS3, the Australian Years 3 to 8, CBSE Class 3 to 8 and US Grades 2 to 6 every week: measurement taught with a real ruler on camera, the decimal-point moves and the "bigger unit, smaller number" check practised until they are automatic, and both systems kept straight for families who live between them. The subjects page lists every maths course; the free 30-minute first class is a real lesson on this week's homework. The other pages in this set are the mass conversion chart (kg in a ton) and the US kitchen conversion chart.

Questions parents ask

See how we teach this, 1-on-1 online →