Spindle Speed Calculator — RPM from Cutting Speed & Diameter
Find the right spindle speed (RPM) for a lathe, mill, drill, or CNC machine. Enter the recommended cutting speed for your material and tool diameter — in metric (m/min, mm) or imperial (SFM, inches) — and get the spindle RPM to dial in.
Unit system
m/min
mm
Revolutions per minute to achieve the target cutting speed at this diameter
- 1
Circumference factor (π × D)
π × 25 = 78.5398Tool circumference in mm; K = 1000 converts m/min → mm/min. - 2
Spindle speed
1,000 × 200 ÷ 78.5398 = 2,546
How does this calculator work?
RPM = 1000 × Vc / (π × D) in metric (Vc in m/min, D in mm), or RPM = 12 × SFM / (π × D_in) in imperial. For a 25 mm end mill cutting aluminium at 200 m/min: RPM = 200 000 / (π × 25) ≈ 2546 RPM. Look up Vc for your material from a machinability chart.
Formula
How this is calculated
Cutting speed (Vc) is the linear speed at which the cutting edge moves across the workpiece surface — a property of the material and tool pair. Harder or tougher materials need lower cutting speeds to avoid overheating. Machinability charts list recommended values by material: aluminium typically 200–600 m/min (660–2000 SFM), mild steel 60–100 m/min (200–330 SFM), stainless steel 50–80 m/min, titanium 30–60 m/min.
The spindle must rotate fast enough that its rim moves at exactly Vc. For a tool of diameter D mm, each revolution covers πD mm of surface. To cover 1000 × Vc mm per minute (converting m to mm), the spindle makes (1000 × Vc) / (πD) revolutions per minute — the standard formula. In imperial, since diameter is in inches and speed in surface feet per minute, the constant is 12 (converting feet to inches): N = 12 × SFM / (π × D_in).
In practice, round to the nearest speed your machine supports. For roughing, start 10–20% below the calculated value; for finishing you may approach the maximum. Exceeding the recommended speed shortens tool life exponentially. The RPM vs diameter curve shows how speed must increase as cutter diameter decreases for the same cutting speed.
Frequently asked questions
Machinability tables in Machinery's Handbook or the tool manufacturer's catalogue list recommended Vc for combinations of workpiece material (aluminium, steel, titanium…) and tool material (HSS, carbide, cermet). Carbide tools typically run 3–5× faster than HSS.
Excessive speed generates too much heat at the cutting edge, rapidly accelerating tool wear, causing built-up edge on the tool, and in extreme cases burning the workpiece or shattering carbide inserts. Stay at or below the recommended RPM and use appropriate cutting fluid.
1 m/min = 3.281 SFM. For example, 100 m/min ≈ 328 SFM. Use the unit selector — the calculator adjusts the constant automatically.
Also known as
TG we-Calculate Editorial Team. (2026). Spindle Speed Calculator — RPM from Cutting Speed & Diameter [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/spindle-speed-calculator
TG we-Calculate Editorial Team. "Spindle Speed Calculator — RPM from Cutting Speed & Diameter." TG we-Calculate. 2026. https://we-calculate.com/calculator/spindle-speed-calculator.
TG we-Calculate Editorial Team, "Spindle Speed Calculator — RPM from Cutting Speed & Diameter," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/spindle-speed-calculator
@misc{wecalculate_spindle_speed_calculator, title = {Spindle Speed Calculator — RPM from Cutting Speed & Diameter}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/spindle-speed-calculator}}, year = {2026}, note = {TG we-Calculate} }
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