1/2” wherever the router accepts it. A thicker shank is roughly sixteen times stiffer in bending, so it chatters less and runs cooler — 1/4” is for trim routers and small profiles.
Both sizes are held the same way, in a collet that grips the shank around its whole circumference. The difference is stiffness.
Why 1/2” is not twice as good but more
Bending stiffness rises with the fourth power of diameter. A 1/2” shank is around sixteen times as resistant to bending as a 1/4” one, and that shows as less chatter, a cleaner surface and a cutter that stays where you put it in a deep pass.
It also carries heat away from the cutter better, and there is more shank inside the collet.
Where 1/4” is right
Trim routers and small machines that take nothing else. Small profiles, light work, and anywhere the bit is short and the cut is shallow.
Where 1/2” is not optional
Anything large in diameter, anything run in a table, and any bit long enough to have real leverage. A large cutter on a 1/4” shank is where shanks snap, and a cutter leaving a spinning collet is a serious event.
The practical rule
If the router takes 1/2”, buy 1/2”. Keep 1/4” for the trim router and buy those bits knowingly.
Not the same as 8 mm (0.31 in)
8 mm (0.31 in) and 1/4” are close and not interchangeable. Check which collets came with the machine.
Last reviewed 2026-08-12