21 centimeters in millimeters

Conversion formula
How to convert 21 centimeters to millimeters?

We know (by definition) that: $1\mathrm{cm}=10\mathrm{mm}$

We can set up a proportion to solve for the number of millimeters.

Now, we cross multiply to solve for our unknown $x$:

Conclusion: 21 ⁢ cm = 210 ⁢ mm

Conversion in the opposite direction

The inverse of the conversion factor is that 1 millimeter is equal to 0.00476190476190476 times 21 centimeters.

It can also be expressed as: 21 centimeters is equal to $\frac{1}{\mathrm{0.00476190476190476}}$ millimeters.

Approximation

An approximate numerical result would be: twenty-one centimeters is about two hundred and ten millimeters, or alternatively, a millimeter is about zero times twenty-one centimeters.