CNC Feed Speed Calculator
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.
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
| Material | Carbide SFM | Chip load (IPT) |
|---|---|---|
| Aluminum 6061 | 1500 | 0.004 |
| Mild steel (1018) | 400 | 0.002 |
| Alloy steel (4140) | 280 | 0.0015 |
| Stainless 304/316 | 220 | 0.0015 |
| Titanium Ti-6Al-4V | 120 | 0.001 |
| Hardwood | 2200 | 0.008 |
| MDF | 2500 | 0.010 |
| Plywood | 2300 | 0.009 |
| Plastic (Delrin) | 2000 | 0.005 |
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.