# CNC Feed Speed Calculator

CNC feed speed calculator for CNC mills, routers, and lathes. One entry returns spindle RPM, SFM, feed rate in IPM and mm/min, and caps RPM at your machine max.

## What this calculates

A CNC feed speed calculator combines the two programming values a CNC control needs: spindle speed (S word, RPM) and feed rate (F word, IPM or mm/min). Enter cutter diameter, material, tool material, and flute count, and this calculator returns both. An optional machine RPM cap reduces the calculated RPM to what your spindle can actually reach, then recomputes feed automatically. Imperial and metric outputs for any CNC control.

## Inputs

- **Cutter Diameter** (in) — min 0.001 — Tool diameter for milling or routing, workpiece diameter for turning on a CNC lathe.
- **Material** — options: Aluminum, Mild steel / 1018, Alloy steel / 4140, Stainless steel, Cast iron, Brass, Titanium, Hardwood, Softwood, MDF, Plywood, Plastic — Material being cut.
- **Tool Material** — options: HSS, Carbide — Carbide runs 2-5x the SFM of HSS in most materials.
- **Flutes** — min 1, max 8 — Cutting edges. 2 for aluminum and router bits, 3-4 for steel, 4-5 for stainless.
- **Machine RPM Cap** (RPM) — min 0 — Optional. If the calculator wants a higher RPM than your machine can reach, it caps here and recomputes feed.
- **Chip Load Override** (in/tooth) — min 0 — Optional. Enter a specific chip load per tooth from a vendor data sheet.

## Outputs

- **Spindle RPM (Speed)** (RPM) — RPM = (SFM x 12) / (pi x D), capped to the machine maximum if set.
- **Cutting Speed** (SFM) — SFM used for the RPM calculation.
- **Feed Rate (IPM)** (in/min) — Feed rate = RPM x chip load x flutes.
- **Feed Rate (metric)** (mm/min) — Feed rate in mm/min for metric CNCs.
- **Chip Load** (IPT) — Chip load per tooth used in the feed equation.
- **Capped at Machine Max?** — formatted as text — Whether the calculated RPM exceeded the machine cap.

## Details

CNC feed speed equations

  - Spindle RPM (speed) = (SFM x 12) / (pi x D) with D in inches.

  - Feed rate (IPM) = RPM x chip load per tooth x flutes.

  - Metric: feed rate (mm/min) = IPM x 25.4.

Example: 1/4 inch 2-flute carbide end mill in 6061 aluminum. SFM = 1500, chip load = 0.003 IPT (scaled for 1/4 inch tool). RPM = (1500 x 12) / (pi x 0.25) = 22,918 RPM. Feed = 22,918 x 0.003 x 2 = 137.5 IPM = 3,492 mm/min. If machine max is 10,000 RPM, the cap kicks in: RPM = 10,000, feed = 10,000 x 0.003 x 2 = 60 IPM, and effective SFM drops to 654.

CNC feed speed chart

  
    MaterialCarbide SFMChip load (IPT)
  
  
    Aluminum 606115000.004
    Mild steel (1018)4000.002
    Alloy steel (4140)2800.0015
    Stainless 304/3162200.0015
    Titanium Ti-6Al-4V1200.001
    Hardwood22000.008
    MDF25000.010
    Plywood23000.009
    Plastic (Delrin)20000.005
  

CNC feed speed calculator metric mode

A CNC feed speed calculator metric workflow is a single unit toggle. Tool diameter accepts mm; the RPM equation does not care about units because the diameter cancels out in the (pi x D) term when you use consistent units. Feed rate in mm/min equals IPM x 25.4. 137.5 IPM = 3,492 mm/min. For European CNC controls running Siemens 840D or Heidenhain TNC7, paste the mm/min output straight into the F word under G94.

Machine RPM cap

Most small CNCs cannot reach the RPM a full SFM calculation wants. A 24,000 RPM router tops out below the 33,614 RPM a 1/4 inch bit in hardwood at 2200 SFM calls for. A 10,000 RPM benchtop VMC cannot hit the 22,918 RPM needed for a 1/4 inch end mill in aluminum. The machine RPM cap field solves this: enter your spindle max, and the calculator substitutes the cap for any higher result and recomputes feed. Effective SFM drops to whatever the cap allows.

Feed and speed on a CNC lathe

On a CNC lathe, D in the RPM formula is the workpiece OD, not the tool. G96 (constant surface speed) in the lathe control holds SFM constant as the diameter drops during the cut. Feed rate on a lathe is typically programmed in IPR (inches per revolution) under G95 rather than IPM: feed in IPR equals chip load for a single-point tool. This CNC feed speed calculator focuses on milling and routing; use the feed rate calculator lathe for turning-specific feed in IPR.

When to override chip load

Vendor data sheets (Harvey Tool, Helical, Onsrud) sometimes specify chip loads higher or lower than the generic table in this calculator. Override when: you have a high-performance end mill that ships with a spec sheet; you are slotting full width and need to drop IPT 25 percent; you are pocketing with less than 50 percent radial engagement and can push IPT up with chip thinning. Leave default for everything else.

## Frequently Asked Questions

**Q: How do I calculate CNC feed and speed?**

A: Two equations: RPM = (SFM x 12) / (pi x D) for speed, and feed rate (IPM) = RPM x chip load x flutes for feed. For a 1/4 inch 2-flute carbide end mill in aluminum at 1500 SFM and 0.003 IPT: RPM = 22,918, feed = 137.5 IPM. This CNC feed speed calculator does both.

**Q: Does this CNC feed speed calculator work in metric?**

A: Yes. Tool diameter accepts mm via the unit toggle. The calculator returns feed rate in both IPM and mm/min, so a CNC feed speed calculator metric workflow works without any unit math. mm/min = IPM x 25.4. 137.5 IPM = 3,492 mm/min.

**Q: What does the machine RPM cap field do?**

A: If the raw SFM calculation wants higher RPM than your spindle can reach (say 22,918 RPM on a 10,000 RPM machine), the cap substitutes your machine max and recomputes feed at the reduced RPM. Effective SFM drops proportionally. Leave at 0 to skip the cap.

**Q: Why does chip load per tooth scale with tool diameter?**

A: Small tools have smaller flute volume and less tool stiffness, so they cannot take the full chip load a large tool can. This calculator scales chip load down to 50 percent for tools under 1/8 inch, 75 percent for 1/8-1/4 inch, and up to 125 percent for tools over 3/4 inch. Override the auto-scaling with the chip load override if you have vendor-specific data.

**Q: How is CNC feed speed different from manual feed speed?**

A: The math is identical. CNC machines accept higher feeds and speeds because they have better rigidity, coolant, and repeatability. A manual Bridgeport runs 10-20 percent below the CNC value from this calculator; a benchtop mill runs 30-50 percent below. Back off the feed output accordingly for manual use.

**Q: Is CNC feed speed calculator output good for routers too?**

A: Yes, with the machine cap set to your router max (10,000-24,000 RPM). At router RPMs, metal-mode SFM is unreachable for small bits, so use the wood or plastic materials instead. A 1/4 inch compression bit in hardwood at 18,000 RPM and 0.008 IPT with 2 flutes feeds at 288 IPM (24 ft/min), a realistic CNC router feed speed.

---

Source: https://vastcalc.com/calculators/physics/cnc-feed-speed
Category: Physics
Last updated: 2026-04-08