Spade bits are usually available in metric sizes ranging from 6mm to 36mm and imperial sizes ranging from 1/4″ to 1 1/2″.
The size of the bit is usually engraved onto the body of the spade itself so that each bit can be easily identified.
If the number on the bit contains a fraction, or is below 2, it is in inches (imperial). If it is above 2, it is in millimetres (metric).
Length of shank
Standard spade bit shanks are usually 150mm (6″) long, although they are available up to 400mm (16″) long for drilling deeper holes in timber frames or walls. The length of the bit can be increased even further with an extension accessory.
Stubby spade bits, with a shank length of around 100mm (4″) are also available. The reduced shank length allows the bits to be used in tight spaces, for example drilling holes in rafters, or inside a pantry or airing cupboard.