Centimeters to millimeters converter

How to convert centimeters to millimeters

Multiply centimeters by 10 to get millimeters, which moves the decimal point one place to the right. 1 cm = 10 mm, 4.2 cm = 42 mm and 11 cm = 110 mm. Use it when a part, screw or drawing is sized in millimeters but you measured in centimeters.

Reading centimeters and millimeters on a ruler

Centi means a hundredth and milli a thousandth of a meter (NIST), so each centimeter holds ten millimeter marks. A reading of 4.2 cm is four full centimeters plus two marks: 42 mm.

Count from the zero line, which on many rulers sits a little in from the end. If an object runs from the 1.3 cm mark to the 5.5 cm mark, its length is 5.5 − 1.3 = 4.2 cm, or 42 mm.

Decimal places to watch

• 1.05 cm → 10.5 mm, while 1.5 cm → 15 mm.
• 4.05 cm → 40.5 mm: four centimeters and half a millimeter.
• 0.5 cm → 5 mm, half a centimeter.

Splitting 1 cm into three equal gaps gives 1/3 cm each, which is 3.33333333333 mm, so mark about 3.3 mm.

Centimeters to millimeters chart

Square and cubic centimeters

Area and volume need the factor squared or cubed: 1 cm² = 100 mm² and 1 cm³ = 1,000 mm³.

• A 4.2 cm × 2 cm rectangle is 8.4 cm². Its sides are 42 mm and 20 mm, so the area is 840 mm².
• 2.5 cm³, the same as 2.5 mL, is 2,500 mm³.

Common mistakes

• Appending a zero instead of moving the point. 1.05 cm is 10.5 mm; writing 1.050 changes nothing.
• Multiplying an area by 10. 8.4 cm² is 840 mm², not 84 mm².
• Dropping the tolerance. ±0.05 cm is ±0.5 mm. A converted reading is only as precise as the ruler.
• Mixing up mm and cm on a drawing. Metric engineering drawings are normally dimensioned in millimeters, so a bare "42" there means 42 mm, not 42 cm.

Related conversions

For the reverse direction, use mm to cm. Cm to meters divides by 100 and cm to inches covers inch rulers. The length converter handles other lengths, and the area converter and volume converter handle squared and cubed units.