Drill speed (RPM) calculator
The right drill speed depends on the bit diameter and the material: small bits and soft metals run fast, large bits and hard metals run slow. The calculator uses the standard formula RPM = cutting speed (SFM) × 3.82 ÷ bit diameter in inches, with typical starting speeds for high-speed steel (HSS) bits. Start at the low end of the range and adjust: if the chips turn blue or the bit squeals, slow down.
Drill speed chart (RPM, HSS bits)
| Bit (in) | Aluminum | Brass and bronze | Cast iron (soft, gray) | Mild steel | Alloy or tool steel (annealed) | Stainless steel | Acrylic and other hard plastics |
|---|---|---|---|---|---|---|---|
| 1/16 | 12,224–18,336 | 9,168–12,224 | 6,112–9,168 | 4,890–6,112 | 2,445–4,278 | 1,834–3,056 | 3,056–6,112 |
| 1/8 | 6,112–9,168 | 4,584–6,112 | 3,056–4,584 | 2,445–3,056 | 1,222–2,139 | 917–1,528 | 1,528–3,056 |
| 3/16 | 4,075–6,112 | 3,056–4,075 | 2,037–3,056 | 1,630–2,037 | 815–1,426 | 611–1,019 | 1,019–2,037 |
| 1/4 | 3,056–4,584 | 2,292–3,056 | 1,528–2,292 | 1,222–1,528 | 611–1,070 | 458–764 | 764–1,528 |
| 5/16 | 2,445–3,667 | 1,834–2,445 | 1,222–1,834 | 978–1,222 | 489–856 | 367–611 | 611–1,222 |
| 3/8 | 2,037–3,056 | 1,528–2,037 | 1,019–1,528 | 815–1,019 | 407–713 | 306–509 | 509–1,019 |
| 1/2 | 1,528–2,292 | 1,146–1,528 | 764–1,146 | 611–764 | 306–535 | 229–382 | 382–764 |
| 5/8 | 1,222–1,834 | 917–1,222 | 611–917 | 489–611 | 244–428 | 183–306 | 306–611 |
| 3/4 | 1,019–1,528 | 764–1,019 | 509–764 | 407–509 | 204–357 | 153–255 | 255–509 |
| 1 | 764–1,146 | 573–764 | 382–573 | 306–382 | 153–267 | 115–191 | 191–382 |
Cutting speeds used (SFM): Aluminum 200–300; Brass and bronze 150–200; Cast iron (soft, gray) 100–150; Mild steel 80–100; Alloy or tool steel (annealed) 40–70; Stainless steel 30–50; Acrylic and other hard plastics 50–100.
How to use the numbers
- These are starting points. Harder grades, deep holes and worn bits need slower speeds; cutting oil lets you stay near the top of the range in steel.
- Cobalt bits handle heat better than HSS, but use the same speeds in stainless and hard steel and let the extra heat resistance give you a longer life.
- Hand drills rarely show RPM. Use the low gear and a light trigger for large bits and hard metals, and the high gear for small bits.
- Wood and plastics are less sensitive to speed, but acrylic melts if the bit runs too fast; see how to drill plexiglass without cracking it.
For which bit to use in each metal, see best drill bits for metal, drill bits for stainless steel and drilling cast iron. For bit sizes, see the drill bit size chart.