There are two common sizes of router bit shanks: 1/4" and 1/2". Many routers come with interchangeable 1/4" and 1/2" collets so that either size bit can be used, but some accept only 1/4"-dia. shank bits.

Moving from 1/4" shanks to 8mm shanks multiplies the shank area by 4.04169 times. Everything else being equal, this should translate to 8mm shanks being ~4 times stronger than 1/4" shanks.

An essential component of any router, the collet is what holds the router bit in place as it rotates at over 20,000 RPM. The collet is a piece of precisely machined steel bored to match the shank of a router bit and tapered on the outside to slide into the hollow spindle of the router motor.

The shank is the solid shaft part of a router bit. It is part of the bit that goes into the router's collet and is secured with the collet nut. At the bottom end of the shank is the bit body, the part that contains the cutter or cutters that shape or cut the wood.