MM To CM Converter

1. Type the length in millimeters. Use the direction box to switch to centimeters to millimeters.
2. Pick what you are measuring: a general length, a ruler reading, a rainfall total or a ring size. This only changes the note under the result.
3. Read the centimeters, then the meters, the ruler split and the other forms below it.

What each result shows:

A centimeter is 10 millimeters. Both are decimal parts of the meter, the SI base unit of length, and they differ only in their prefix.

The BIPM, which maintains the International System of Units, lists centi (symbol c) as 10^-2 and milli (symbol m) as 10^-3. So 1 cm is 0.01 m and 1 mm is 0.001 m. Dividing one by the other gives exactly 10. NIST states the same chain for length: 10 millimeters make 1 centimeter, and 10 centimeters make 1 decimeter.

Millimeters suit small, precise sizes: screws, paper thickness, rainfall and jewelry. Centimeters suit everyday sizes such as body height, clothing and school rulers. Converting between them never needs a rounded factor, unlike metric to inch work.

You divide the millimeters by 10, which moves the decimal point one place to the left.

1. Write the millimeters: 127 mm.
2. Move the decimal one place left: 127.0 becomes 12.70, so 12.7 cm.
3. For meters, move it three places left: 0.127 m.

The prefix table explains the step size. Milli to centi is one power of 10. Milli to the plain meter is three powers of 10, and milli to kilo is six. For larger jumps, the scientific notation calculator keeps the exponents straight.

A rain gauge rim. The Met Office storage rain gauge has a rim 5 inches across, which it gives as 127 mm. 127 / 10 = 12.7 cm. As a cross-check, 127 / 25.4 = 5.0 in exactly.

A ruler reading. A pencil line sits at 85 mm. 85 / 10 = 8.5 cm, which on the ruler is the 8 cm mark plus 5 small mm marks.

A rainfall total. A forecast says 25 mm of rain. That is 2.5 cm of water depth. Over 1 square meter, 1 mm of depth is 0.001 cubic meters, which is 1 liter, so 25 mm puts 25 liters on every square meter.

A ring size. A ring has an inner circumference of 54 mm, which is 5.4 cm. ISO 8653 names ring sizes by this inner circumference in millimeters, so it is size 54. Its inner diameter is 54 / pi = 17.19 mm.

A room length, the other way. A plan shows 350 cm. 350 x 10 = 3,500 mm, which is 3.5 m. The meters to cm converter handles plans drawn in meters.

The factor of 10 never changes. What changes is how many digits make sense for the job.

Ruler Resolution

A school ruler marks every 1 mm, so its finest reading is 0.1 cm. A value such as 12.35 mm needs a caliper, not a ruler.

Rain Gauge Steps

The Met Office tipping-bucket gauge records rain in steps of 0.2 mm, or 0.02 cm. A daily total is therefore a multiple of 0.2 mm, and more decimals add no real information.

What the Ring Measure Is

A ring size in ISO 8653 is an inner circumference. A ring sizer that gives a diameter needs a pi step: circumference = diameter x pi.

Unit Choice for Very Small or Large Values

Below 1 mm, micrometers read better: 0.2 mm is 200 micrometers. Above 100 m, meters or kilometers are clearer than thousands of centimeters. For long distances, see the cm to km converter.

The metric prefixes step by powers of 10, which is why these conversions are exact.

Prefix names and factors follow the BIPM SI prefix table and the NIST prefix chart.

School and Homework

Metric unit questions often ask you to change mm to cm and back. The decimal-shift result shows the rule, not just the answer.

Weather and Gardening

Rain totals come in mm. Converting 38 mm to 3.8 cm helps when a garden guide gives water depth in cm.

Jewelry and Crafts

Ring sizes under ISO 8653 are circumferences in mm, while craft patterns may use cm. For very long runs in feet, use the mm to feet converter.

Common millimeter values in centimeters and meters, with the ruler reading. Computed from 1 cm = 10 mm and 1 m = 1,000 mm.

1. Moving the Decimal the Wrong Way

Millimeters are smaller, so the cm number must be smaller. 45 mm is 4.5 cm, not 450 cm.

2. Dividing by 100

Dividing by 100 gives decimeters, not centimeters. 45 mm / 100 = 0.45 dm, which is 4.5 cm.

3. Reading a Ruler from the Case Edge

Some rulers start the scale a few millimeters in from the end. Line up the zero mark, not the edge.

4. Mixing Up Ring Diameter and Circumference

A 17 mm inner diameter is about 53.4 mm around. ISO ring sizes use the circumference.

5. Writing the Symbols in Capitals

SI prefix symbols are case sensitive. The prefix m means milli, while M means mega, a factor of 10⁶. Write mm and cm in lower case.

The conversion is exact, because centi and milli are defined powers of 10. Only your measurement adds uncertainty.

One millimeter is 0.1 centimeter. Ten millimeters make exactly one centimeter.

Move the decimal point one place to the left. 250 mm becomes 25.0 cm, and 3 mm becomes 0.3 cm.

The prefix centi means one hundredth and milli means one thousandth. A hundredth is ten times a thousandth, so 1 cm = 10 mm.

Twenty-five millimeters of rain is 2.5 cm deep. Weather services report rain in mm, and 1 mm equals 1 liter on each square meter.

The Met Office tipping-bucket gauge logs rain in steps of 0.2 mm, which is 0.02 cm.

Under ISO 8653, a ring with a 54 mm inner circumference is size 54. That is 5.4 cm around and about 17.19 mm across the inside.

Count the numbered centimeter marks, then the small marks after the last one. Each small mark is 1 mm, so 8 cm plus 5 marks is 85 mm.

A meter has 1,000 millimeters and 100 centimeters. The BIPM prefix table gives milli as 10^-3 and centi as 10^-2.

No. Both are exact units. Accuracy comes from the measuring tool, so 12.7 cm and 127 mm carry the same information.

Yes. The centimeter is the meter with the SI prefix centi. The BIPM allows any SI prefix to form decimal multiples and submultiples of SI units.

• SI prefixes: decimal multiples and submultiples (BIPM, International System of Units).
• Metric (SI) prefixes chart (NIST Office of Weights and Measures).
• SI units of length: 10 millimeters = 1 centimeter (NIST).
• How rainfall is measured: 0.2 mm tipping bucket and 127 mm gauge (Met Office).
• Ring size defined as inner circumference in mm (ISO 8653 catalogue entry; the 1986 edition, since replaced by ISO 8653:2016).