Surface Feet Per Minute Calculator
Surface feet per minute, SFM, is the cutting speed at the edge of a rotating tool or workpiece, and it is the speed that tooling makers specify for each material. It equals the circumference at the cutting diameter multiplied by the spindle speed. Because diameters are usually measured in inches and surface speed in feet, the working formula is pi times the diameter in inches times the RPM divided by 12. This calculator converts in both directions: enter a diameter and RPM to get SFM, or enter a diameter and a target SFM to get the spindle speed.
Surface speed formula
SFM = pi * diameter (in) * RPM / 12
RPM = target SFM * 12 / (pi * diameter (in))
Circumference (in) = pi * diameter (in)
Surface speed (m/min) = SFM * 0.3048
Dividing the inch-based circumference by 12 converts to feet, since one foot is exactly 12 inches. One foot is exactly 0.3048 meters, used to report metric surface speed.
Cutting speed context
- SFM is the speed of the cutting edge across the work, the figure tooling makers specify.
- Recommended SFM depends on material, tool, coolant, and operation; use your tooling data.
- For turning, use the workpiece diameter; for milling and drilling, the cutter diameter.
- The 12 and 0.3048 factors are exact by the definition of the foot.
- Pair the RPM result with a feed-per-tooth figure to set the feed rate.
Surface feet per minute: frequently asked questions
What is surface feet per minute?
Surface feet per minute, SFM, is the speed at which the cutting edge passes the workpiece, measured in feet of surface per minute. It equals pi times the diameter in feet times the spindle speed in revolutions per minute. Because tool and stock diameters are usually in inches, the working formula is pi times diameter in inches times RPM divided by 12.
How do I find the RPM for a target SFM?
Rearrange the formula: RPM equals SFM times 12 divided by pi times the diameter in inches. Pick the recommended cutting speed for your material and tool, then compute the spindle speed that delivers it at your diameter.
Why divide by 12?
Diameter is entered in inches but surface speed is in feet per minute. One foot is 12 inches, so dividing the inch-based circumference by 12 converts to feet. The factor is exact by the definition of the foot.
What cutting speed should I use?
Recommended SFM depends on the workpiece material, the tool material, coolant, and the operation, so this calculator does not assume a value. Use the speed from your tooling manufacturer's data or a machining handbook for your material and tool combination.
Does the formula work for turning and milling?
Yes. In turning, the diameter is the workpiece diameter; in milling and drilling, it is the cutter diameter. The relationship between surface speed, diameter, and RPM is the same geometry in every case.
Official sources
- National Institute of Standards and Technology: Office of Weights and Measures (foot and inch definitions).
- National Institute of Standards and Technology: NIST unit reference.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.