Drill Bit Speed (RPM) Calculator
Find the recommended drill speed in RPM based on bit size and material being drilled.
Covers HSS, carbide, and cobalt drill bits.
Enter the drill bit size and material to find the recommended RPM.
Understanding Drill Bit Speed Selection
The correct drilling speed (RPM) depends on two main factors: the diameter of the drill bit and the material being drilled. Using too high a speed overheats the bit, dulling it prematurely and potentially hardening the workpiece material (work hardening). Using too low a speed is inefficient and can cause the bit to grab and stall. The relationship between bit size and RPM is governed by the concept of surface feet per minute (SFM), which represents how fast the cutting edge moves through the material.
The RPM Formula:
RPM = (SFM x 12) / (pi x Bit Diameter in inches)
Or equivalently:
RPM = (SFM x 3.82) / Bit Diameter in inches
In metric:
RPM = (SFM x 1000) / (pi x Bit Diameter in mm)
Recommended Surface Speed (SFM) by Material:
| Material Being Drilled | HSS Bit (SFM) | Carbide Bit (SFM) | Cobalt Bit (SFM) |
|---|---|---|---|
| Softwood (pine, cedar) | 300 - 400 | 400 - 600 | N/A |
| Hardwood (oak, maple) | 150 - 250 | 250 - 400 | N/A |
| Aluminum | 200 - 300 | 400 - 600 | 250 - 350 |
| Mild steel | 80 - 100 | 200 - 300 | 100 - 140 |
| Stainless steel | 30 - 50 | 100 - 150 | 50 - 70 |
| Cast iron | 60 - 80 | 150 - 250 | 80 - 110 |
| Brass | 200 - 300 | 300 - 500 | 250 - 350 |
| Copper | 150 - 200 | 250 - 400 | 175 - 250 |
| Plastic / acrylic | 100 - 200 | 200 - 350 | N/A |
| Concrete (masonry bit) | N/A | 40 - 60 | N/A |
Practical RPM Examples (HSS bit in mild steel):
| Bit Diameter | Imperial | Metric | RPM |
|---|---|---|---|
| 1/16 in | 0.0625 in | 1.6 mm | 5,500 |
| 1/8 in | 0.125 in | 3.2 mm | 2,750 |
| 1/4 in | 0.25 in | 6.4 mm | 1,375 |
| 3/8 in | 0.375 in | 9.5 mm | 920 |
| 1/2 in | 0.5 in | 12.7 mm | 690 |
| 3/4 in | 0.75 in | 19 mm | 460 |
| 1 in | 1.0 in | 25.4 mm | 345 |
Feed Rate:
Feed rate is how fast the drill advances into the material. For hand drilling, this is controlled by pressure. The rule of thumb is 0.001 to 0.003 inches (0.025 to 0.076 mm) per revolution for small bits in metal, and 0.004 to 0.010 inches (0.1 to 0.25 mm) per revolution for larger bits. Too much pressure bends or breaks small bits. Too little pressure causes rubbing instead of cutting, which work-hardens the material.
Cooling and Lubrication:
When drilling metal, cutting fluid or oil dramatically extends bit life and improves hole quality. For steel, use cutting oil. For aluminum, use WD-40 or kerosene. For stainless steel, use sulfurized cutting oil and go slow. For wood and plastic, no lubrication is needed, but back the bit out frequently to clear chips and prevent overheating.
Important Safety Note:
Never exceed the maximum RPM rating of the drill bit. Most drill bits are rated for a maximum RPM based on their construction. Exceeding this limit risks bit failure, which can send fragments flying at high speed. Always wear safety glasses when drilling.