21 millimeters in kilometers

Conversion formula
How to convert 21 millimeters to kilometers?

We know (by definition) that: $1\mathrm{mm}=1e-06\mathrm{km}$

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

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

Conclusion: 21 ⁢ mm = 2.1e-05 ⁢ km

Conversion in the opposite direction

The inverse of the conversion factor is that 1 kilometer is equal to 47619.0476190476 times 21 millimeters.

It can also be expressed as: 21 millimeters is equal to $\frac{1}{\mathrm{47619.0476190476}}$ kilometers.

Approximation

An approximate numerical result would be: twenty-one millimeters is about zero kilometers, or alternatively, a kilometer is about forty-seven thousand, six hundred and nineteen point zero five times twenty-one millimeters.