You can easily convert 400 millimeters into centimeters using each unit definition:
With this information, you can calculate the quantity of centimeters 400 millimeters is equal to.
In 400 mm there are 40 cm.
Which is the same to say that 400 millimeters is 40 centimeters.
Four hundred millimeters equals to forty centimeters. *Approximation
Performing the inverse calculation of the relationship between units, we obtain that 1 centimeter is 0.025 times 400 millimeters.
A centimeter is zero times four hundred millimeters. *Approximation