How to Calculate Grinding Wheel Speed
The complete formula for SFM, RPM, and wheel diameter — with worked examples for both Imperial and Metric units.
Imperial (Inch) Formulas
SFM = RPM × D × π ÷ 12
RPM = SFM × 12 ÷ (D × π)
Diameter = SFM × 12 ÷ (RPM × π)
SFM = Surface Feet per Minute
D = Wheel diameter in inches
RPM = Spindle revolutions per minute
π ≈ 3.1416
Metric Formulas
MPS = RPM × D(mm) × π ÷ 60,000
RPM = MPS × 60,000 ÷ (D × π)
Diameter = MPS × 60,000 ÷ (RPM × π)
MPS = Meters per Second
D = Wheel diameter in millimeters
RPM = Spindle revolutions per minute
Worked Example 1: Find RPM from SFM
You want 4,000 SFM on a 7-inch grinding wheel. What RPM?
RPM = SFM × 12 ÷ (D × π)
RPM = 4,000 × 12 ÷ (7 × 3.1416)
RPM = 48,000 ÷ 21.99
RPM ≈ 2,182
Worked Example 2: Find SFM from RPM
Your surface grinder is running at 3,000 RPM with a 5-inch wheel. What SFM?
SFM = RPM × D × π ÷ 12
SFM = 3,000 × 5 × 3.1416 ÷ 12
SFM = 47,124 ÷ 12
SFM ≈ 3,927
Worked Example 3: Find Diameter from RPM and SFM
You need 3,500 SFM and your machine maxes out at 2,500 RPM. What wheel diameter?
Diameter = SFM × 12 ÷ (RPM × π)
Diameter = 3,500 × 12 ÷ (2,500 × 3.1416)
Diameter = 42,000 ÷ 7,854
Diameter ≈ 5.35 inches
Use the Calculator Instead
Save time and avoid math errors. Enter any two values — wheel diameter, SFM/MPS, or RPM — and our calculator finds the third instantly.
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