Using the speed guide takes four quick steps:

1. Measure the Cutting Diameter: Note the bit's overall cutting diameter, not the shank. A 1/2-inch straight bit sits in the "Under 1\"" band; a 2-1/2-inch roundover sits in the "2\" to 2.5\"" band.
2. Select the Diameter Band: Choose the range that fits your bit in the calculator to reveal the recommended RPM window.
3. Start at the Low End: Dial to the bottom of the range, especially for a new bit or hard stock, then raise only if cut quality suffers.
4. Tune by Sound and Finish: A steady hum with a clean edge means you are dialed in; screaming, burning, or chatter means you adjust speed or feed.

The governing rule is simply: bigger diameter, lower RPM, because rim speed rises with both.

• RPM (Revolutions Per Minute): The speed at which the router's motor and the bit are spinning.
• Bit Diameter: The overall cutting diameter of the router bit.
• Rim Speed: The actual speed of the outermost cutting edge of the bit as it moves through the wood. This increases with both RPM and bit diameter.
• Chatter: A form of vibration that occurs when a bit is run at an improper speed or with too heavy a cut, resulting in a poor surface finish.

"The sounds your router makes are a great indicator. If the router is screaming at a high pitch, the speed is likely too high for the bit. If it's bogging down and groaning, you're either taking too deep a cut or your feed rate is too fast. A happy router has a steady, powerful hum." - Fine Woodworking Magazine

"For very large bits, like a panel-raising bit, always start at the absolute lowest speed your router can manage. You can always nudge the speed up slightly if the cut quality is poor, but you can't undo the damage from a bit that shatters because it was spun too fast."

Example 1: A 1/4-inch straight bit.
Under 1 inch in diameter, so the guide calls for 18,000-24,000 RPM — near full throttle. Small bits need high speed to shear cleanly rather than rub.

Example 2: A 2-1/4-inch roundover.
That lands in the 2 to 2.5-inch band: 12,000-16,000 RPM. Drop toward the bottom of the range in oak or MDF to avoid chatter marks around the arc.

Example 3: A 3-1/2-inch panel-raising bit.
Over the 3.5-inch threshold, so 8,000-10,000 RPM, and you start at 8,000. A wide-diameter bit at rim speed already exceeds a small bit spinning much faster, which is why it gets the slowest setting.

• Reading the Shank, Not the Cutter: Banding should follow the cutting diameter. A 1/2-inch bit in a 3-inch profile head belongs in the slow band, not the fast one.
• Running a Large Bit at Full Speed: Flooring the dial on a big profile is the most common cause of burning, violent vibration, and catastrophic bit failure.
• Fixing Tear-Out with More RPM Alone: Often the real culprit is a too-fast feed or a dull edge. Adjust feed and sharpness before chasing the problem with speed.
• Ignoring the Sound: A screaming pitch means too fast; a bogging groan means too deep or too fast a feed. The motor tells you before the workpiece does.