Free Mm to Cm Converter
Published: 14 November 2026 · Updated: 14 November 2026
Type a length in millimetres and the exact value in centimetres appears instantly. Rain gauges, screws, drill bits and print sizes are all quoted in millimetres, while everyday lengths use centimetres — this converter moves between the two. It starts with 150 mm, which is 15 cm. For the reverse tool, use the Free Cm to Mm Converter.
1 mm is exactly 0.1 cm, so divide any millimetre value by 10. That makes 150 mm equal to 15 cm. Dividing by 10 only moves the decimal point one place to the left — 42 mm becomes 4.2 cm — with no rounding and no fractions.
Researched by MixKhazana Team · Last updated 14 November 2026 · Data: NIST SP 811; BIPM SI Brochure; International Yard and Pound Agreement 1959
What is a millimetre?
A millimetre (mm) is one thousandth of a metre: 1 mm = 0.001 m, and 10 mm = 1 cm. It is the finest metric unit in everyday use.
Millimetres are the working unit for rain gauges, fasteners, drill bits, watch cases, print and packaging, and engineering drawings. A millimetre is about the thickness of a credit card, which is 0.76 mm.
What is a centimetre?
A centimetre (cm) is one hundredth of a metre: 1 cm = 0.01 m, and 1 cm = 10 mm. It is the everyday metric unit for heights, clothing, furniture and paper across Europe.
Centimetres suit lengths you judge by eye: a phone is 15 cm tall, a book is 2 cm thick. When the number would need decimals, millimetres take over — which is why both units appear on the same ruler.
How to use this converter
- Type your length in the millimetres box — centimetres appear as you type.
- Whole numbers and decimals both work: 42 mm becomes 4.2 cm, and 7 mm becomes 0.7 cm.
- To convert back, type in the centimetres box or press Swap direction, which flips the converter and carries the converted value over.
- The result line also shows the reverse rate: 1 cm = 10 mm.
Millimetres to centimetres formula
Let Lmm be the length in millimetres and Lcm be the same length in centimetres:
- Centimetres from millimetres: Lcm = Lmm ÷ 10
- Millimetres from centimetres: Lmm = Lcm × 10
Both directions are exact, because the millimetre is defined as one tenth of a centimetre. See our metric vs imperial conversion guide for Europe for how metric units relate to inches and feet.
Worked example
A rain gauge collected 42 mm overnight, and the weather report quotes centimetres:
- Divide by 10: 42 ÷ 10 = 4.2 cm
- Split it up: 40 ÷ 10 = 4, and 2 ÷ 10 = 0.2, giving 4 + 0.2 = 4.2 cm
- Check: 4.2 × 10 = 42 mm, back where we started
So 4.2 cm of rain fell — and since 1 mm of rain equals 1 litre of water per square metre, that is 42 litres per square metre. Try 42 in the converter above.
Millimetres to centimetres conversion table
| Millimetres (mm) | Centimetres (cm) |
| 1 | 0.1 |
| 2 | 0.2 |
| 5 | 0.5 |
| 8 | 0.8 |
| 10 | 1 |
| 15 | 1.5 |
| 20 | 2 |
| 25 | 2.5 |
| 40 | 4 |
| 50 | 5 |
| 65 | 6.5 |
| 80 | 8 |
| 100 | 10 |
| 125 | 12.5 |
| 200 | 20 |
| 300 | 30 |
Centimetres are shown to four decimal places; dividing by 10 only moves the decimal point.
Everyday benchmarks: millimetre sizes in centimetres
These millimetre sizes appear on packaging, gauges and hardware every day:
- M6 bolt, 40 mm long — 4 cm. The 40 in M6 × 40 is the length in millimetres.
- Drill bit, 10 mm — 1 cm. A 10 mm bit drills a 1 cm hole for wall plugs.
- Watch case, 40 mm — 4 cm across. A 40 mm watch is a medium case size.
- Heavy rainfall, 25 mm — 2.5 cm. A day with 25 mm of rain is a very wet day in most of Europe.
- A4 paper width, 210 mm — 21 cm. Printer paper is specified in millimetres, 210 × 297 mm.
- Wood screw, 65 mm — 6.5 cm. A 65 mm screw suits 18 mm boards fixed into a frame.
For units beyond the metric length scale, the Free Unit Converter covers more units in one place, including inches, feet, weight and temperature.
Frequently asked questions
How do you convert mm to cm?
Divide the millimetre value by 10. For example, 80 mm ÷ 10 = 8 cm. The conversion is exact: a millimetre is defined as one tenth of a centimetre, so it only moves the decimal point one place.
What is 150 mm in cm?
150 mm is 15 cm. Divide by 10: 150 ÷ 10 = 15. One hundred and fifty millimetres is also about 5.9 inches.
How many cm is 10 mm?
10 mm is exactly 1 cm. Ten millimetres make one centimetre, just as one hundred centimetres make one metre.
What is 1 mm in cm?
1 mm is 0.1 cm. A millimetre is one tenth of a centimetre, so single millimetres always convert to a decimal number of centimetres.
How do I read a rain gauge in cm?
Divide the millimetre reading by 10. A gauge showing 42 mm means 4.2 cm of rain. Rain is usually reported in millimetres because 1 mm equals 1 litre of water per square metre, which is easy to picture.
Why are screws and bolts sold in mm?
Millimetres give whole numbers for small parts: a 40 mm bolt is clearer than a 4 cm bolt when the next size is 45 mm, not 4.5 cm. Metric fastener sizes — M6, M8, 40 mm long — are all specified in millimetres.
What is 250 mm in cm?
250 mm is 25 cm. Divide by 10: 250 ÷ 10 = 25. A 250 mm ruler marking is the 25 cm mark.
Can I convert cm to mm here too?
Yes. Type in the centimetres box and the millimetre value updates instantly, or press Swap direction. The dedicated reverse tool is the Free Cm to Mm Converter.
Sources
- NIST Special Publication 811 — Guide for the Use of the International System of Units (SI), conversion factors for the inch, foot and yard.
- BIPM SI Brochure — the International System of Units, definitions of the metre, centimetre and millimetre.
- International Yard and Pound Agreement, 1959 — 1 yard = 0.9144 m exactly, giving 1 in = 25.4 mm and 1 ft = 0.3048 m exactly.
Disclaimer: conversions use the exact defined factor (1 cm = 10 mm); for fasteners and drill sizes, confirm against the product datasheet.
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