Calculate recommended drill RPM and surface speed from drill diameter, material, and tool type, or find SFM from diameter and spindle speed.
- All Construction Calculators
- Surface Speed (SFM) Calculator
- Spindle Speed Calculator
- Countersink Depth Calculator
- Drill Tip Length Calculator
Drill SFM Formula
The calculator uses one of two equations depending on the tab.
Surface speed from RPM:
SFM = pi * D * RPM / 12
RPM from a target surface speed (used by the Recommended RPM and RPM from SFM tabs):
RPM = 12 * SFM / (pi * D)
- SFM = surface feet per minute at the drill’s outer edge
- D = drill diameter in inches
- RPM = spindle speed in revolutions per minute
If you enter diameter in mm, it is converted to inches (divide by 25.4). If you enter target speed in m/min, it is converted to ft/min (multiply by 3.281). The 12 in the formula converts inches per minute to feet per minute. The Recommended RPM tab multiplies a base SFM for the work material by a tool factor (1.0 HSS, 1.15 cobalt, 2.5 carbide) before solving for RPM.
Typical SFM Values
Use these as a starting point. Drop 30 to 50 percent for deep holes, no coolant, or hard spots.
| Work Material | HSS (SFM) | Cobalt (SFM) | Carbide (SFM) |
|---|---|---|---|
| Aluminum | 300 | 345 | 750 |
| Brass / bronze | 200 | 230 | 500 |
| Mild steel | 100 | 115 | 250 |
| Alloy steel | 70 | 80 | 175 |
| Stainless steel | 50 | 58 | 125 |
| Cast iron | 80 | 92 | 200 |
| Titanium | 30 | 35 | 75 |
| Plastic | 250 | 288 | 625 |
Common HSS RPM at 100 SFM by drill size:
| Diameter | RPM at 100 SFM |
|---|---|
| 1/8 in (3.18 mm) | 3056 |
| 1/4 in (6.35 mm) | 1528 |
| 3/8 in (9.53 mm) | 1019 |
| 1/2 in (12.7 mm) | 764 |
| 3/4 in (19.05 mm) | 509 |
| 1 in (25.4 mm) | 382 |
Example and FAQ
Example: A 1/2 in HSS twist drill in mild steel. Target SFM is 100. RPM = (12 × 100) ÷ (π × 0.5) = 764 RPM. Set the drill press to the closest available step, usually 700 to 800 RPM.
What is SFM? Surface feet per minute. It is the linear speed of the cutting edge at the outer diameter of the drill.
Why does SFM matter more than RPM? SFM is set by the material and the tool. A small drill needs high RPM to hit the same SFM that a large drill reaches at low RPM.
Should I run faster with coolant? Flood coolant lets you stay near the top of the recommended SFM range. Dry drilling or pecking deep holes calls for the lower end.
What if my machine cannot reach the calculated RPM? Use the highest speed available and reduce feed slightly. Running below the target SFM is safer than running above it.
