CNC Milling Speeds & Feeds Calculator

How to use this calculator

1. Choose Metric or Imperial units, and the fixed operation scope, milling/routing.
2. Choose what to calculate: RPM and feed rate (from a cutting speed and chip load you already have), or cutting speed and chip load (from an RPM and feed rate you already have).
3. Enter the cutter diameter and the number of flutes (effective teeth).
4. Enter the two remaining values for your chosen mode.
5. Optionally add the axial depth and radial width of cut to see material removal rate, or add your machine's maximum RPM and feed rate to compare the calculated values against them.
6. Review the calculation summary, the formula used, and any warnings shown.

What do I need to enter?

• Unit system — Metric (mm, m/min) or Imperial (in, SFM). Switching converts the figures already entered.
• What do you want to calculate? — RPM and feed rate, from a cutting speed and chip load; or Cutting speed and chip load, from an RPM and feed rate.
• Cutter diameter — the effective cutting diameter at the depth of cut, above zero and up to 1,000 mm (about 39.4 in). A ball-nose or tapered tool can cut at less than its full diameter.
• Flutes / effective teeth — a whole number from 1 to 100.
• Cutting speed and Chip load (feed per tooth) — used to find RPM and feed rate. Take both from the tool manufacturer's application data; the calculator does not recommend them.
• Spindle speed and Feed rate — used to find the cutting speed and chip load instead. Spindle speed is above zero and up to 1,000,000 RPM; a feed rate of zero is accepted as a no-feed check.
• Axial depth of cut (ap) and Radial width of cut (ae) — optional, and only for material removal rate. Enter both or leave both blank.
• Maximum spindle RPM and Maximum feed rate — optional machine limits the calculated values are compared against.

What do the results show?

• Spindle speed — the main figure when you calculate RPM and feed rate, with the feed rate beside it. When you work backwards, Cutting speed leads instead, with the chip load.
• Calculation summary — diameter, cutting speed, spindle speed, teeth, chip load, feed per revolution and feed rate, each marked as entered or calculated.
• Material removal rate — the volume of material removed per minute, shown when you enter both the axial depth and radial width of cut.
• Machine limits — whether the calculated RPM or feed rate is above or within the limit you entered. It compares only and changes no figure.
• Formula used — the equations behind the result for your chosen mode.

How are milling speeds and feeds calculated?

Spindle speed comes from the cutting speed and cutter diameter, and feed rate comes from the chip load, the number of teeth and the spindle speed. This calculator always uses the direct formula for the values entered, on full-precision numbers, rather than chaining through a rounded intermediate result.

Worked example, metric: a 10 mm diameter cutter at a cutting speed of 150 m/min needs RPM = (150 × 1,000) ÷ (π × 10) ≈ 4,774.6 RPM. With 4 flutes and a chip load of 0.05 mm/tooth, feed rate = 0.05 × 4 × 4,774.6 ≈ 954.9 mm/min.

Worked example, imperial: a 0.5 in diameter cutter at 500 SFM needs RPM = (500 × 12) ÷ (π × 0.5) ≈ 3,819.7 RPM. With 4 flutes and a chip load of 0.002 IPT, feed rate = 0.002 × 4 × 3,819.7 ≈ 30.6 IPM.

In reverse, entering a spindle speed and feed rate gives the cutting speed and chip load those values imply — useful for checking a program or a machine's actual running parameters against the tool manufacturer's application data.

Metric and imperial units used

Every calculation is normalised to millimetres, metres per minute and mm³/min internally, so switching units converts the figures already entered rather than reinterpreting the same number under a different unit.

What does this calculator assume, and what does it not do?

This calculator covers milling and routing with a single rotating cutter, at a fixed diameter, RPM, feed rate, tooth count and engagement. It does not model radial chip thinning at low radial engagement, tool runout, tool deflection, machine dynamics, coolant strategy, tool wear or material variability, and the optional material removal rate is a geometric volume-per-time figure only — it does not estimate cutting force, spindle power, torque or tool life.

It does not cover turning, tapping, thread milling, grinding, sawing, laser or EDM processes, and it does not simulate a machine, generate G-code, or draw on a tool or material database. For a rotating drill instead of a milling cutter, see the CNC Drilling Speeds and Feeds Calculator, which uses feed per revolution rather than chip load per tooth. Any warning it shows is a neutral, calculated callout — never a claim that a result is safe, optimal, suitable, manufacturer-approved or guaranteed. Always check the tool manufacturer's application data, your machine's actual limits, holder and tool condition, workholding, coolant strategy and your shop's procedures before machining.

Frequently asked questions