Drill Speed (RPM) Calculator
Correct drill press or hand-drill rpm from bit diameter and material — twist drills, brad-point, Forstner, hole saws and spade bits in wood, aluminium, mild steel, stainless and plastics — from recommended surface speeds.
How it works
rpm = SFM × 12 ÷ (π × diameter in inches) = 3.82 × SFM ÷ D(in); metric: rpm = 318 × cutting speed (m/min) ÷ D(mm); multiply by the bit-type factor
A drill bit’s cutting edge should move through the material at a speed the material tolerates — expressed as surface feet per minute — and because the edge travels around the circumference, the right rpm falls as the bit gets bigger. Wood forgives a wide range, but metals are unforgiving: run high-speed steel too fast in steel and the tip blues and dulls in seconds, run stainless at wood speeds and it work-hardens. Large-diameter cutters that remove a lot of material — Forstner bits, hole saws, countersinks — are slowed further to control heat and chatter. Drill presses offer a handful of belt speeds, so pick the nearest one below the calculated figure. Cutting fluid, sharp bits and steady feed matter as much as the number.
Worked example
A 10 mm HSS twist drill in mild steel at 100 SFM: 3.82 × 100 ÷ 0.394 ≈ 970 rpm — set the press to 1,000. The same bit in softwood could run 2,900 rpm; a 35 mm Forstner in hardwood only about 350.
Frequently asked questions
Why does my bit smoke in steel?
Too fast, too little feed, or no lubricant — friction rather than cutting. Halve the speed, add cutting oil and push firmly enough to make continuous chips.
Do cobalt or carbide bits change the speed?
They tolerate 20–50% more speed and far more heat than HSS, which is what makes them worth the price for stainless and hardened steel.
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This calculator is for educational purposes. Double-check important results before acting on them.