What is 21 inches to millimeters? The answer is that 21 inches equals 533.4 millimeters.
How to convert 21 inches to mm? We know that one inch is equal to 25.4 millimeters, so to convert 21 inches to millimeters, we must multiply the amount of centimeters by 25.4, which means that 21 inches is equal to 21 inches × 25.4 = 533.4 mm.
See also: How to convert 21 mm to inches.