21 millimeters in decimeters

Conversion formula
How to convert 21 millimeters to decimeters?

We know (by definition) that: $1\mathrm{mm}=0.01\mathrm{dm}$

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

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

Conclusion: 21 ⁢ mm = 0.21 ⁢ dm

Conversion in the opposite direction

The inverse of the conversion factor is that 1 decimeter is equal to 4.76190476190476 times 21 millimeters.

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

Approximation

An approximate numerical result would be: twenty-one millimeters is about zero decimeters, or alternatively, a decimeter is about four point seven five times twenty-one millimeters.