Bits come in many different types, shapes, profiles and sizes, such as slotted, Phillips, hexagonal and Torx. The specified dimension indicates the distance between two opposing surfaces of the bit. This distance is also called the width across flats:
The usual measurement for a hexagon is 1⁄4inch (6.35 millimetres). Furthermore, standardised drive sizes are given as 5⁄16 inch (≈ eight millimetres) or 7⁄16inch (≈ eleven millimetres). Otherwise, there are so-called mini bits. These have a size of four millimetres(5⁄32inch).