CNC Feeds & Speeds Calculator
Spindle RPM and feed rate (IPM) for end-mill milling operations, from material cutting speed, tool diameter, flute count, and chip load per tooth.
How this is calculated
RPM = (SFM × 3.82) ÷ tool diameter (in)
Feed rate (IPM) = RPM × flutes × chip load per tooth
3.82 is 12/π, converting the tool's surface speed (feet per minute) into spindle RPM based on its circumference — a standard machining relationship documented in Machinery's Handbook. Chip load per tooth is pre-filled from a common shop starting rule (tool diameter ÷ 200) when you change tool diameter, but it's editable — real chip load should be tuned to your specific tool, coating, and machine rigidity.
Reference: typical carbide end mill cutting speeds
| Material | SFM range |
|---|---|
| Aluminum 6061 | 600-1000 |
| Brass 360 | 200-400 |
| Mild steel 1018 | 400-550 |
| Stainless 304 (annealed) | 300-400 |
| Titanium 6Al-4V | 100-200 |
In-depth guide: Spindle speed and feed rate formulas, explained
How this calculator works
RPM = SFM × 3.82 ÷ diameter (Machinery's Handbook) and feed = RPM × flutes × chip load; carbide end-mill SFM ranges by material. Metric mode: n = vc × 1000 ÷ (π × d), vf = n × z × fz (milling) or n × f (turning, drilling); the same SFM ranges converted to m/min.
Worked example
A 1/2" 3-flute carbide end mill in aluminum 6061 at 800 SFM.
RPM = 800 × 3.82 ÷ 0.5 = 6,112 RPM. With the default chip load of 0.0025"/tooth (diameter ÷ 200), feed = 6,112 × 3 × 0.0025 = 45.8 in/min.
In 1018 mild steel, a 1/4" 4-flute at 450 SFM gives 6,876 RPM and 34.4 in/min at 0.00125"/tooth.
Frequently asked questions
How do I calculate RPM from SFM?
RPM = SFM × 3.82 ÷ tool diameter in inches. 3.82 is 12 ÷ π, which converts surface feet per minute into spindle revolutions.
What is chip load?
Chip load is the thickness of material each cutting edge removes per revolution. Too low rubs and dulls the tool; too high can break it.
What if my spindle cannot reach the RPM?
Run at your maximum RPM and keep the same chip load, which lowers the feed rate proportionally. The cut will be slower but the tool will be loaded correctly.
Are these values for HSS tools?
No, the SFM ranges assume carbide. Uncoated HSS tooling usually runs about 2–3 times slower.
Common mistakes
- Using the radius instead of the diameter. RPM = 3.82 × SFM ÷ D uses the tool diameter. Using the radius doubles the speed.
- Keeping the feed after capping the RPM. If the spindle cannot reach the calculated RPM, recalculate the feed at the lower speed with the same chip load.
- Feeding too lightly. A chip load far below the tool maker's value makes the edge rub and heat up; in stainless steel it also work-hardens the surface.
- Ignoring chip thinning. At a radial engagement below half the diameter the real chip is thinner than the programmed feed per tooth; see the speeds and feeds guide for the correction.
What it does not cover
Cutting speeds for turning and drilling (enter the tool maker's value); tapping; tool-specific chip loads (the default is editable).
Sources
- Harvey Tool, General machining guidelines (RPM and IPM formulas)
- Sandvik Coromant, milling formulas and definitions
Sources, checks and limits for this calculator →
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