There are 20.32 centimeters in 8 inches. There are 2.54 centimeters in an inch, so determining how many centimeters are in 8 inches requires multiplying 2.54 by 8.
The system used in the United States is different from that used in many other places and in the scientific world. Converting within the system and to the metric system often involves working with irregular numbers. The metric system is based on tens, so conversions within the metric system are easy, often just requiring moving a decimal point. For instance, one meter is equivalent to 10 decimeters, 100 centimeters and 1000 millimeters.