Drill Bit Speed (RPM) Calculator

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 the cutting speed is quoted in surface metres per minute (SMM) rather than surface feet, and one SFM is 0.3048 SMM:

RPM = (SMM x 1000) / (pi x Bit Diameter in mm)

If you have an SFM figure from an imperial chart and a bit measured in millimetres, fold the conversion in:

RPM = (SFM x 304.8) / (pi x Bit Diameter in mm)

Leaving the 0.3048 out is an easy slip and it inflates the answer by a factor of 3.28. A 6.35 mm bit in mild steel at 90 SFM wants 1,375 RPM; drop the conversion and you get 4,511, which will ruin the bit in seconds.

Recommended Surface Speed (SFM) by Material:

Practical RPM Examples (HSS bit in mild steel):

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.