Common Router Collet Sizes

• 1/8" (3.175mm): Hobby machines, Dremels, engraving.
• 1/4" (6.35mm): Standard for mid-size routers and CNCs.
• 1/2" (12.7mm): Industrial routers; much stiffer, less vibration.
• 6mm / 8mm: Common European metric standards.

Warning: Never put a 6mm bit in a 1/4" collet. It may seem to fit but will vibrate loose under load.