Metric
RPM = (1000 × Vc) / (π × D)
Vc = (π × D × RPM) / 1000
D: mm · Vc: m/min · fn: mm/rev
Free CNC turning tool
CNC Turning Speed & Feed Calculator
Convert workpiece diameter and known cutting speed or RPM into theoretical turning values. Use metric or imperial units.
Theoretical conversion
Calculation results
Enter your known values and calculate. No cutting parameters are preselected.
Theoretical values only. Verify manufacturer guidance and machine limits before machining.
Calculation reference
Turning formulas and units
D is the workpiece diameter at the cutting position. It is not the insert size or a milling cutter diameter.
Imperial
RPM = (12 × Vc) / (π × D)
Vc = (π × D × RPM) / 12
D: inch · Vc: SFM (ft/min) · fn: in/rev
Vf = RPM × fn in both unit systems. Linear feed Vf is mm/min or in/min.
Unit conversion uses 1 inch = 25.4 mm and 1 foot = 0.3048 m. Switching units converts existing valid inputs, keeping their physical meaning unchanged.
Constant surface speed and machine limits
With constant surface speed turning, required RPM increases as the cutting diameter decreases. Set and verify a maximum spindle speed using the machine manufacturer's instructions and the limits of the workholding and setup. This tool does not apply a machine RPM limit.
Results are theoretical conversions based on user inputs, not machining parameter recommendations for a specific workpiece, insert, or machine. Refer to manufacturer technical documentation and machine limits for actual machining. RPM alone cannot determine insert selection, grade, chipbreaker, or tool life.
Demonstration only
Example calculations
These examples demonstrate arithmetic. They do not recommend speeds or feeds for any material.
- Metric RPM: D = 50 mm, Vc = 150 m/min, fn = 0.2 mm/rev → RPM ≈ 954.93 and Vf ≈ 190.99 mm/min.
- Reverse calculation: D = 50 mm, RPM = 1000 → Vc ≈ 157.08 m/min.
- Imperial RPM: D = 2 inch, Vc = 500 SFM → RPM ≈ 954.93.
Official documentation
Technical References
Manufacturer formulas support the conversions. No manufacturer cutting-parameter recommendations are reproduced.
- Sandvik Coromant: Turning formulas, metric (PDF)
Spindle speed, cutting speed, machined diameter, and feed per revolution.
- Sandvik Coromant: Turning formulas, inch (PDF)
Corresponding formulas with cutting speed in ft/min and diameter in inches.
- Kennametal: Speeds and Feeds Calculator
Independent speed, RPM, feed-per-revolution and linear-feed conversion reference. For this turning tool, diameter always means the workpiece cutting diameter.