# CNC Cutting Speed Calculator

CNC cutting speed calculator returns SFM and spindle RPM for any material and cutter diameter. Covers CNC mill, CNC router, and CNC lathe using the Machinery's Handbook SFM formula.

## What this calculates

Cutting speed is the velocity of the cutting edge across the workpiece, measured in surface feet per minute (SFM). On a CNC machine, the control programs spindle speed as RPM, so you need a CNC cutting speed calculator to translate between the SFM in a tooling catalog and the RPM in your G-code. This tool handles CNC milling, CNC routing, and CNC turning with one equation and a library of material-specific SFM values.

## Inputs

- **Operation** — options: CNC mill / VMC, CNC router, CNC lathe (D = workpiece) — On a CNC lathe, diameter is the workpiece OD, not the tool. On a mill or router, it is the cutter diameter.
- **Diameter** (in) — min 0.001 — Tool diameter for milling/routing, workpiece diameter for lathe turning.
- **Material** — options: Aluminum (metal), Mild steel / 1018, Alloy steel / 4140, Stainless steel, Cast iron, Brass, Bronze, Titanium, Tool steel, Hardwood (router), MDF (router), Plywood (router), Plastic (Delrin, acrylic) — Material being cut.
- **Tool Material** — options: HSS, Carbide — Carbide is standard on modern CNC; HSS for small-shop hobby machines.
- **SFM Override** (SFM) — min 0 — Optional. Pull an SFM value from a tool vendor data sheet to override the material table.

## Outputs

- **Cutting Speed** (SFM) — Surface feet per minute for the material + tool combination.
- **Cutting Speed (metric)** (m/min) — Vc = SFM x 0.3048.
- **Spindle RPM** (RPM) — RPM = (SFM x 12) / (pi x D).
- **Cutting Speed (ft/s)** (ft/s) — SFM / 60. Useful as a sanity check.

## Details

CNC cutting speed formula

  - SFM comes from a material table. Aluminum = 1500 SFM carbide, mild steel = 400, stainless = 220, titanium = 120.

  - RPM = (SFM x 12) / (pi x D) where D is the diameter of whatever is spinning (tool on mill/router, workpiece on lathe).

  - Metric: Vc (m/min) = SFM x 0.3048.

Example (CNC mill): 1/2 inch carbide end mill in 6061 aluminum at 1500 SFM. RPM = (1500 x 12) / (pi x 0.5) = 11,459 RPM. Vc = 457 m/min.

Example (CNC lathe): 2 inch bar of 1018 mild steel, carbide insert at 400 SFM. RPM = (400 x 12) / (pi x 2) = 764 RPM. Vc = 121.9 m/min.

Example (CNC router): 1/4 inch compression bit in hardwood at 2200 SFM. RPM = (2200 x 12) / (pi x 0.25) = 33,614 RPM. Most CNC routers top out at 24,000 RPM, so cap there and accept the lower actual SFM.

CNC cutting speed by material

  
    MaterialHSS SFMCarbide SFM
  
  
    Aluminum 60616001500
    Mild steel (1018)100400
    Alloy steel (4140)70280
    Stainless 304/31660220
    Cast iron80260
    Brass250500
    Bronze150350
    Titanium Ti-6Al-4V40120
    Tool steel (A2, D2)50180
    Hardwood12002200
    MDF15002500
    Plywood13002300
    Plastic (Delrin, acrylic)8002000
  

Why CNC cutting speed is set in SFM, not RPM

SFM stays constant across tool sizes; RPM does not. A 1/4 inch carbide end mill in aluminum wants 1500 SFM, same as a 1 inch end mill. Only RPM changes (22,918 vs 5,730 RPM). By specifying cutting speed in SFM, the Machinery's Handbook and tool vendor tables give you one value per material, and the CNC cutting speed calculator handles the per-tool RPM translation.

CNC cutting speed for turning (lathe)

On a CNC lathe, D is the workpiece diameter, not the tool diameter. G96 (constant surface speed) in the control tells the machine to hold SFM constant as the workpiece OD shrinks during the cut by raising RPM. This CNC cutting speed calculator returns the SFM for your material and the RPM at the current workpiece diameter. Program G96 S400 to hold 400 SFM throughout the job.

CNC cutting speed on a router

CNC routers spin 10,000 to 24,000 RPM, which caps the maximum SFM you can hit on small bits. For a 1/8 inch bit in hardwood at 2200 SFM target, the required RPM is 67,229, well out of reach; the actual SFM at 24,000 RPM is 785 SFM. Compensate by raising chip load (feed per tooth) to keep the feed rate productive. Wood forgives lower SFM more than metal does.

## Frequently Asked Questions

**Q: How do I calculate CNC cutting speed?**

A: SFM comes from a material table (1500 SFM for aluminum with carbide, 400 for mild steel, 220 for stainless). Then RPM = (SFM x 12) / (pi x D), where D is the diameter of whatever is spinning. For a 1/2 inch carbide end mill in aluminum, RPM = (1500 x 12) / (pi x 0.5) = 11,459 RPM.

**Q: What is the difference between cutting speed and feed rate on a CNC?**

A: Cutting speed (SFM) sets how fast the cutting edge moves across the work, which becomes spindle RPM after converting by diameter. Feed rate (IPM or mm/min) sets how fast the tool moves through the work, which depends on chip load and flutes. Both come from separate equations and show up in G-code as S (spindle) and F (feed).

**Q: Do CNC routers use the same cutting speed formula as CNC mills?**

A: Yes. SFM = (pi x D x RPM) / 12 is the same formula. CNC routers just spin higher RPM (10,000-24,000) to hit wood SFM targets (1200-2800). For small bits in hardwood the required RPM exceeds most router max, so you cap at the machine max and accept slightly lower SFM.

**Q: What SFM should I use for stainless steel on a CNC mill?**

A: 220 SFM carbide for 304/316 stainless, 60 SFM HSS. For a 1/2 inch carbide end mill at 220 SFM, RPM = (220 x 12) / (pi x 0.5) = 1,681 RPM. Stainless work-hardens fast, so keep feed up and RPM conservative. Use the sfmOverride field if your tool vendor specifies a different SFM.

**Q: Does this CNC cutting speed calculator support metric?**

A: Yes. Tool or workpiece diameter accepts mm via the unit toggle. Cutting speed is returned in SFM and m/min (Vc = SFM x 0.3048). RPM is unit-independent. A European CNC running Siemens 840D or Heidenhain iTNC can program either via G96 for constant surface speed or M3 Sxxxx for fixed RPM.

**Q: What happens on a lathe when workpiece diameter changes?**

A: Surface speed drops proportionally to diameter at constant RPM. A 2 inch bar at 764 RPM cuts at 400 SFM; at 1 inch diameter the same 764 RPM only makes 200 SFM. CNC lathes solve this with G96 (constant surface speed), which commands the control to raise RPM as D drops. Without G96 you step RPM up manually after each pass.

---

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