Millimeters to Centimeters (mm to cm): Formula and Chart

Quick answer: 1 mm = 0.1 cm. To convert, multiply the mm value by 0.1; to go back, divide.

Millimeters to centimeters is a ÷10: 45 mm = 4.5 cm, 8 mm = 0.8 cm — move the decimal point one place left. Product specs, rainfall, drill bits and engineering drawings quote mm; rulers, sewing patterns and body measurements quote cm.

The formula

cm = mm × 0.1 · mm = cm ÷ 0.1

Common values

mm	cm	also
1 mm	0.1 cm	0.039 in
2.5 mm	0.25 cm	0.098 in
4.5 mm	0.45 cm	0.177 in
5 mm	0.5 cm	0.197 in
10 mm	1 cm	0.394 in
12.5 mm	1.25 cm	0.492 in
15 mm	1.5 cm	0.591 in
20 mm	2 cm	0.787 in
25 mm	2.5 cm	0.984 in
30 mm	3 cm	1.181 in
45 mm	4.5 cm	1.772 in
50 mm	5 cm	1.969 in
75 mm	7.5 cm	2.953 in
100 mm	10 cm	3.937 in
250 mm	25 cm	9.843 in

4.5 mm to cm: 0.45 cm

4.5 mm = 0.45 cm — 0.177 in. Working: 4.5 × 0.1 = 0.45.

4.5 mm is 0.45 cm — a shade under 3/16 of an inch (0.177 in). It is a common drill-bit and screw size (M4.5), the thickness of standard window glass and many phone bezels, and a 4.5 mm knitting needle is a UK size 7 / US size 7.

The other way: cm to mm

cm	mm
1 cm	10 mm
2 cm	20 mm
5 cm	50 mm
10 cm	100 mm
12 cm	120 mm
15 cm	150 mm
20 cm	200 mm
25 cm	250 mm
50 cm	500 mm
100 cm	1,000 mm

Common mistake

Moving the decimal two places (that is mm→dm or cm→m). Between mm and cm it is exactly one place.

Why this one is easier than it looks

Millimeters to centimeters is the gentlest conversion in the metric system, because both units are defined against the meter by powers of ten. A centimeter is a hundredth of a meter and a millimeter a thousandth, so there are exactly 10 mm in 1 cm.

That means no multiplication is needed at all — you are moving the decimal point one place left. 45 mm becomes 4.5 cm; 4.5 mm becomes 0.45 cm; 4 mm becomes 0.4 cm. Going the other way, move it one place right.

This is the whole appeal of the metric system: every step between prefixes is a shift of the decimal point rather than an arbitrary factor like 12, 14 or 5,280.

The decimal-point mistake

Because the operation is so simple, the errors are almost always about which way the point moves.

The check that never fails: a centimeter is bigger than a millimeter, so the number of centimeters must be smaller. If your answer grew, you moved the point the wrong way.

45 mm → 4.5 cm ✓ (number got smaller)
45 mm → 450 cm ✗ (that is 4.5 m — a factor of 100 out)
4.5 mm → 0.45 cm ✓
4.5 mm → 45 cm ✗

The second kind of error is the expensive one in practice, because 450 cm is a plausible-looking measurement in its own right and nothing about it flags as wrong.

The full metric ladder

Once you can move between mm and cm, the rest of the ladder works identically:

Unit	In millimeters	In meters
1 mm	1 mm	0.001 m
1 cm	10 mm	0.01 m
1 dm (decimeter)	100 mm	0.1 m
1 m	1,000 mm	1 m
1 km	1,000,000 mm	1,000 m

Where each unit is actually used

Millimeters dominate engineering and manufacturing. Technical drawings across most of the world are dimensioned in mm with no unit marked at all, precisely so there is no ambiguity — if a drawing says 45, it means 45 mm.
Centimeters are for everyday human-scale measurement: height, clothing, furniture, paper.
Rainfall is reported in mm; a "25 mm of rain" day is an inch.
Screws, bolts and drill bits are sized in mm. An M6 bolt has a 6 mm thread diameter.
Camera lenses are always mm — a 50 mm lens, never a 5 cm lens.
Screen bezels, phone thickness and PCB layouts are mm, because the numbers land in a convenient range.

The general rule is that mm is preferred wherever precision matters, because it avoids decimals: 8.5 cm and 85 mm are the same length, but the second is easier to read on a drawing and harder to misplace a decimal point in.

Common values with inches alongside

mm	cm	inches
1 mm	0.1 cm	0.039 in
2.5 mm	0.25 cm	0.098 in
4.5 mm	0.45 cm	0.177 in
5 mm	0.5 cm	0.197 in
10 mm	1 cm	0.394 in
25.4 mm	2.54 cm	1 in exactly
50 mm	5 cm	1.969 in
100 mm	10 cm	3.937 in
300 mm	30 cm	11.811 in

The row worth memorising is 25.4 mm = 1 inch exactly. Every other metric-to-imperial length conversion follows from it, because the inch is defined that way rather than measured.

A useful physical reference

If you need to estimate without a ruler: a standard credit card is 85.6 mm × 53.98 mm, so its short edge is almost exactly 54 mm or 5.4 cm. A UK 1p or US cent is close to 20 mm across. Most pencils are about 7 mm wide, and a standard door is 35–45 mm thick.

Both sit at the small end of the metric scale: our length conversion guide covers the full range, and millimetres to inches crosses into imperial.

Frequently asked questions

Is the conversion exact?

Yes — the factor follows from the legal definition of the units (1 in = 25.4 mm and 1 ft = 0.3048 m), so the only rounding is in the last decimal place shown.

How do I convert a different value?

Use the mm to cm converter for any number, or the length conversion guide for every length unit and formula in one place.

How many mm are in a cm?

Exactly 10. Both units are defined against the meter by powers of ten, so converting is just moving the decimal point one place.

Which way does the decimal point move?

Left when going from mm to cm, because centimeters are the larger unit and the number must get smaller. 45 mm becomes 4.5 cm. If your answer got bigger, you moved it the wrong way.

Why do engineering drawings use mm instead of cm?

Because mm avoids decimals for most part dimensions, and a whole number is much harder to misread than a decimal. On most technical drawings an unmarked number means millimeters by convention.