Router Bit Speed Calculator

A router bit’s safe speed falls as its diameter rises, because the cutting edge travels further per revolution. Spin a big bit too fast and the load on the carbide brazing and the steel body climbs until something lets go, at which point you have shrapnel leaving a machine at head height.

Use the published caps. Do not calculate them from a rim speed.

You will find “maximum rim speed” figures quoted for carbide, and it is tempting to divide one by pi times the diameter and call that your limit. It gives nonsense. A constant rim speed would put a 32 mm bit at well over 100,000 RPM, which no bit on earth survives. Manufacturers publish caps that fall much faster than that, because the failure mode is not rim speed alone but the bending load on an unsupported cutter far from the shank.

Manufacturer recommended maximums:

This calculator interpolates between those figures and always rounds down. If your bit’s own packaging quotes a lower number, that number wins.

The single-speed router problem. A fixed-speed router runs somewhere around 22,000 to 27,000 RPM and has no way to slow down. That is fine up to about 12 mm and unsafe above it. Anything over roughly 32 mm needs a variable-speed router, in a table, with the speed set before you switch on. A 3-inch panel-raising bit at 24,000 RPM is one of the few genuinely dangerous things a home shop can do by accident.

Feed rate matters as much as speed. Too slow and the bit burns the wood and dulls itself; too fast and you tear out grain. You are aiming for small, even chips and a clean surface. And take deep profiles in several passes, not one.