In computer hardware terminology, word size (word length) is the number of bits that a CPU can process at one time (the word). Processors with many different word sizes have existed, though powers of two (8, 16, 32, 64) have predominated for many years. See 8-bit, 16-bit, 32-bit, 64-bit, 128-bit.

A processor's word size is often equal to the width of its external data bus though sometimes the bus is made narrower than the CPU (often half as many bits) to economise on packaging and circuit board costs.