You can easily convert 21 centimeters into millimeters using each unit definition:
With this information, you can calculate the quantity of millimeters 21 centimeters is equal to.
In 21 cm there are 210 mm.
Which is the same to say that 21 centimeters is 210 millimeters.
Twenty-one centimeters equals to two hundred ten millimeters. *Approximation
Performing the inverse calculation of the relationship between units, we obtain that 1 millimeter is 0.0047619048 times 21 centimeters.
A millimeter is zero times twenty-one centimeters. *Approximation