Machining · feeds speeds
Surface Speed Calculator
Solve surface speed, spindle RPM or effective diameter with exact π in metric or inch units.
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Solve surface speed, RPM or diameter
Uses exact π and converts between metric and inch displays.
Example geometry values are preloaded to demonstrate the reversible equation. They are not a cutting-speed recommendation.
Show calculation steps
| Step | Expression | Raw value |
|---|---|---|
| Surface-speed relationship | vc = π × Dmm × RPM ÷ 1000 | 188.495559 m/min |
How to use the surface speed calculator
Choose surface speed, RPM or diameter as the unknown. Select metric or inch display units and enter the other two values. The tool converts to a canonical metric calculation and uses exact π.
Metric: vc = π × Dmm × RPM ÷ 1000
Imperial: SFM = π × Din × RPM ÷ 12
The familiar RPM ≈ SFM × 3.82 ÷ diameter is a rounded shortcut because 12/π is approximately 3.82. This calculator retains exact π internally.
Effective diameter matters
For a milling cutter, use the effective cutting diameter relevant to the operation. For a lathe, use the workpiece diameter at the cut. A ball nose, tapered tool, lead angle or changing workpiece diameter may require a more specific model. Shank diameter is not automatically cutting diameter.
Worked examples
A 10 mm effective diameter at 6,000 RPM has π × 10 × 6000 ÷ 1000 = 188.496 m/min. A 1 inch diameter at 1,200 RPM has π × 1 × 1200 ÷ 12 = 314.159 SFM.
Source-value boundary
This page relates diameter, speed and RPM; it does not recommend a surface speed. Obtain the starting value for the exact tool, work material, hardness, operation and setup from current manufacturer information. Then check spindle, workholding, balance, power, tool and machine limits.
Common mistakes
- Mixing radius and diameter.
- Mixing SFM and metres/minute.
- Using a rounded 3.82 shortcut as though it were exact.
- Treating shank diameter as effective diameter without checking geometry.
- Treating a calculated RPM as an approved machine setting.
FAQ
Is cutting speed the same as spindle speed?
No. Cutting or surface speed is linear speed at the circumference; spindle speed is revolutions per minute.
Can this choose a speed for steel, aluminum or wood?
No recommendation table is bundled. Use the current source for the exact tool, material and operation.
Why do two calculators differ slightly?
One may use exact π and another the rounded 3.82 conversion shortcut. Units and diameter conventions can also differ.
References and further reading
These sources define or explain the calculation. Source editions and model limits are part of the result—not decorative citations.
- Formulas and definitions for milling — metric, Sandvik Coromant; accessed 2026-08-22. CHIP-C01
- Mill Speeds and Feeds Formulas — Inch, Haas Automation; accessed 2026-08-22. CHIP-C02
- NIST Guide to the SI, National Institute of Standards and Technology; accessed 2026-08-22. CHIP-C04