21 cm to mm conversion result above is displayed in three different forms: as a decimal (which could be rounded), in scientific notation (scientific form, standard index form or standard form in the United Kingdom) and as a fraction (exact result). Every display form has its own advantages and in different situations particular form is more convenient than another. For example usage of scientific notation when working with big numbers is recommended due to easier reading and comprehension. Usage of fractions is recommended when more precision is needed.
If we want to calculate how many Millimeters are 21 Centimeters we have to multiply 21 by 10 and divide the product by 1. So for 21 we have: (21 × 10) ÷ 1 = 210 ÷ 1 = 210 Millimeters
So finally 21 cm = 210 mm