21 kilometers in millimeters

Conversion formula
How to convert 21 kilometers to millimeters?

We know (by definition) that: $1\mathrm{km}=1000000\mathrm{mm}$

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 ⁢ km = 21000000 ⁢ mm

Conversion in the opposite direction

The inverse of the conversion factor is that 1 millimeter is equal to 4.76190476190476e-08 times 21 kilometers.

It can also be expressed as: 21 kilometers is equal to $\frac{1}{\mathrm{4.76190476190476e-08}}$ millimeters.

Approximation

An approximate numerical result would be: twenty-one kilometers is about twenty-one million millimeters, or alternatively, a millimeter is about zero times twenty-one kilometers.