Find how fast the cutting edge of a router bit is moving, or the dial speed that keeps a big bit in a safe range. The calculator works in surface feet per minute (SFPM), the number that actually decides burn marks, chatter and bit stress.
A router’s speed dial sets spindle RPM, but the wood only feels the speed at the edge of the cutter. A ¼ in straight bit at 22,000 RPM moves its edge at about 1,440 ft/min. A 3½ in panel-raising bit at the same dial setting moves its edge at more than 20,000 ft/min — fast enough to overheat the carbide, burn the stock and put real stress on the brazed joints.
Tip speed is the number to hold steady as bits get bigger. The calculator converts in both directions so you can check a setting or find one.
Every revolution, the cutting edge travels one circumference: π × D inches. Multiply by RPM for inches per minute and divide by 12 for feet per minute.
SFPM = π × D × RPM ÷ 12 · RPM = 12 × SFPM ÷ (π × D)
Use the cutting diameter — the largest circle the carbide sweeps — not the shank size.
Router and bit makers widely print a speed chart that steps the RPM down as diameter goes up: about 24,000 RPM for bits up to 1 in, 18,000 for 1¼–2 in, 16,000 for 2¼–2½ in, and 12,000 for 3–3½ in. Run those through the calculator and a pattern appears: the chart is keeping the tip speed in a band, not picking random numbers.
| Bit diameter | Chart RPM | Tip speed (SFPM) |
|---|---|---|
| 1 in | 24,000 | 6,283 |
| 2 in | 18,000 | 9,425 |
| 2.5 in | 16,000 | 10,472 |
| 3.5 in | 12,000 | 10,996 |
Small bits run well below that band because most routers top out around 22,000–24,000 RPM. Large bits are the ones you have to slow down deliberately. The stamp on the bit, if it has one, overrides any chart.
| Bit diameter | 10,000 RPM | 16,000 RPM | 22,000 RPM |
|---|---|---|---|
| 0.25 in | 654 | 1,047 | 1,440 |
| 0.5 in | 1,309 | 2,094 | 2,880 |
| 1 in | 2,618 | 4,189 | 5,760 |
| 1.5 in | 3,927 | 6,283 | 8,639 |
| 2 in | 5,236 | 8,378 | 11,519 |
| 2.5 in | 6,545 | 10,472 | 14,399 |
| 3 in | 7,854 | 12,566 | 17,279 |
| 3.5 in | 9,163 | 14,661 | 20,159 |
Checking a table-router setup. A 1½ in cove bit at 18,000 RPM: π × 1.5 × 18,000 ÷ 12 = 7,069 SFPM. Comfortably inside the band the chart uses, so the setting is reasonable.
Choosing a speed for a panel raiser. You want a 2½ in raised-panel bit to run near 9,000 SFPM. Switch the mode to “Solve RPM”, enter 2.5 in and 9,000. The answer is 13,751 RPM. Set the router to its nearest dial mark below that, then take the profile in several shallow passes.
Spotting a problem. A 3½ in bit left at 22,000 RPM from the last job gives 20,159 SFPM — about twice the chart band. Turn the dial down before you switch on.
Use bits over about 1½ in in diameter only in a router table, never handheld. Insert the shank at least three-quarters of its length, then pull it back about 1/16 in so the collet is not gripping the fillet at the end of the shank. Inspect carbide for chips and cracks before each use, and never exceed a maximum RPM stamped on the bit, whatever this page or a chart says. Feed against the rotation of the bit; climb cuts on a router table can pull the work out of your hands.
Look up the RPM for each dial position in the manual or on the router’s label. The relationship is rarely linear.
No. Wood species, bit design, depth of cut and feed rate all matter. Use the tip speed to compare settings and to keep big bits out of the danger zone.
Yes. The cutting diameter and RPM give the edge speed. Spiral bits are usually small, so they tend to run near the router’s top speed.
Convert millimetres to inches first (divide by 25.4). A 50 mm bit is 1.97 in.