Router Bit Speed Calculator

A router bit is safe at full speed only up to a point. The bigger the bit, the faster its outer edge travels for the same RPM, and past a limit the bit is overrun, which is how carbide tips let go. This gives you the top RPM to run a bit of a given diameter, with the rim speed, so a large panel-raising or round-over bit gets slowed down the way it should be.

Why big bits must run slower

RPM is how many times the bit spins a minute, but what stresses a bit is rim speed, how fast the cutting edge itself moves through the air and the wood. Rim speed is the circumference times the RPM, so doubling the diameter doubles the rim speed at the same RPM. A small straight bit barely notices full router speed; a wide panel bit at the same RPM is flinging its edge dangerously fast, which is why its safe RPM is far lower.

The speed steps

The long-standing guidance steps the maximum RPM down as the diameter climbs: up to about an inch runs at full speed, then it drops through roughly 18,000, 16,000 and 12,000 as the bit passes two, two and a half, and three and a half inches, with the biggest raised-panel bits down around 10,000. This page returns the step for your bit, then caps it at your router's own top speed, because the router cannot spin faster than it can.

The bit's own rating comes first

These steps are a sensible default, but the maker's printed maximum always wins. Many bits carry a stamped or boxed max RPM, and it is the number the tooling engineer stands behind for that exact bit. If the packaging says a lower figure than this calculator, follow the packaging. If it says higher, there is no need to exceed what you need for a clean cut anyway.

Slow down, then feed steady

A variable-speed router is what makes big bits safe, so set the speed before the bit ever touches wood. With a large bit, take the cut in passes rather than hogging the full profile at once, keep the feed moving so the edge is cutting and not rubbing, and use a router table with the bit buried in a fence. Speed, pass depth and a steady feed together are what give a clean profile without scaring anyone.

More MakerKiln tools

• feedsCNC Feeds and SpeedsRPM, feed, plunge and depth of cut from real chip-load math.→
• cutting speedCutting Speed to RPM CalculatorSpindle RPM from a target cutting speed and a diameter, for drilling, turning or milling.→
• stepoverStepover and Scallop HeightThe ridge a ball-nose bit leaves between passes, and the stepover to hit a finish.→

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