21 decimeters in kilometers
Result
The conversion result is: 21 decimeters is equivalent to 0.0021 kilometers.[1]
Conversion formula
How to convert 21 decimeters to kilometers?
We know (by definition) that: 1 dm = 0.0001 km.
We can set up a proportion to solve for the number of kilometers.
1 dm / 21 dm = 0.0001 km / x km
Now, we cross multiply to solve for our unknown x:
x km = (21 dm / 1 dm) * 0.0001 km → x km = 0.0021000000000000003 km
Conclusion:
21 dm = 0.0021000000000000003 km

Conversion in the opposite direction
The inverse of the conversion factor is that 1 kilometer is equal to 476.190476190476 times 21 decimeters.
It can also be expressed as: 21 decimeters is equal to 1 / 476.190476190476 kilometers.
Approximation
An approximate numerical result would be: twenty-one decimeters is about zero kilometers, or alternatively, a kilometer is about four hundred and seventy-six point one eight times twenty-one decimeters.
Footnotes
[1] The precision is 15 significant digits (fourteen digits to the right of the decimal point).
Results may contain small errors due to the use of floating point arithmetic.