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MachiningCalc / Lathe Speed & Feed Calculator

Turning · lathe speeds and feeds

Lathe Speed & Feed Calculator

Use turning diameter, SFM and feed per revolution to calculate lathe RPM and linear feed. The page also explains why diameter, stability and interrupted cuts matter.

Instant resultNo signupFormula shownReviewed Aug 2026
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Calculator

Enter the known values. Results update automatically.

Spindle speed—
Linear feed—
Estimated MRR—
Surface speed—

Use tooling manufacturer data and stay within machine, tool and workholding limits. Calculated values are not a substitute for validated process parameters.

Immediate resultUpdates as inputs change
US machining unitsSFM, RPM, IPR, IPM
Formula visibleNo black-box output
Focused intentOne machining job per URL

Calculation method

Calculation

RPM = SFM × 12 ÷ (π × Turning Diameter)IPM = RPM × Feed per RevolutionMRR ≈ SFM × 12 × Radial DOC × Feed per Revolution

Use values in the units shown by the calculator. Inputs must be positive and physically meaningful.

Turning & lathe speeds and feeds

In turning, the workpiece is the rotating diameter. Enter the diameter at the cut—not simply the stock's original size—when translating SFM into RPM.

Turning diameter and constant surface speed

As diameter changes, the RPM needed for a constant SFM changes too. CNC lathes can use constant surface speed modes, but a maximum RPM should be set so the spindle does not accelerate beyond safe machine and workholding limits.

Raw material hardness, stability and interrupted cuts

Hardness and material condition affect the starting cutting data. Interrupted cuts and unstable workholding often require more conservative parameters than a continuous, rigid cut. Long slender parts can be limited by deflection before theoretical cutting data is reached.

How to interpret feed and MRR

IPM is simply RPM × IPR. The MRR output is an approximate turning-volume relationship based on cutting speed, radial depth of cut and feed per revolution. It is useful for comparison but is not a spindle-power model.

Methodology, standards and validation

MachiningCalc separates deterministic math from cutting-data recommendations. The calculator performs the geometry and unit relationships; the correct starting cutting data still depends on the tool, work material, coating, machine and setup.

Sources and methodological references

Frequently asked questions

Lathe Speed & Feed Calculator: common questions

How do you calculate feed and speed for a lathe?

Use workpiece diameter and SFM to calculate RPM, then multiply RPM by feed per revolution to calculate linear feed.

What formula is used for lathe RPM?

RPM = SFM × 12 ÷ (π × turning diameter in inches).

Can this be used as a lathe RPM calculator?

Yes. Enter target SFM and the diameter currently being cut to get the corresponding spindle RPM.

Why does RPM change during facing?

The cutting diameter decreases as the tool approaches center, so constant surface speed control increases RPM to maintain approximately the same SFM, subject to the machine's RPM limit.

What affects practical turning speed?

Material hardness, insert grade and geometry, approach angle, interrupted cuts, rigidity, depth of cut, coolant and machine limits can all change the usable setting.