Feed Speed Calculator
This feed speed calculator is the fastest way to get spindle RPM and feed rate for an end mill. Pick your tool diameter, material, and flute count, and the calculator runs the two industry-standard equations (SFM to RPM, and chip load to IPM) with baseline values drawn from Machinery's Handbook. Same math a drill speed and feed calculator uses, trimmed for milling.
The two equations a feed speed calculator runs
Every feed speed calculator boils down to two formulas:
- RPM = (SFM x 12) / (pi x D) converts a target surface cutting speed (surface feet per minute) into spindle RPM for a given tool diameter D.
- Feed rate = RPM x chip load x flutes converts that RPM into a linear feed rate in inches per minute.
For a 1/4 inch carbide end mill in aluminum at 1200 SFM: RPM = (1200 x 12) / (pi x 0.25) = 18,335 RPM. At 2 flutes and 0.003 IPT chip load, feed rate = 18,335 x 0.003 x 2 = 110 IPM.
Baseline SFM by material
These are conservative midpoints safe for a general-purpose shop. Production cells with rigid machines and high-pressure coolant can run 20-30 percent faster; hobby machines should back off 20-30 percent.
- Aluminum: 600 SFM HSS, 1200 SFM carbide
- Mild steel: 100 SFM HSS, 350 SFM carbide
- Stainless: 60 SFM HSS, 200 SFM carbide
- Titanium: 40 SFM HSS, 120 SFM carbide
- Plastic: 800 SFM HSS, 1800 SFM carbide
When the feed speed calculator output needs tuning
If the chip is dust and the cut sounds like a scream, feed is too light. If the tool is chattering or flexing, RPM is too high or depth of cut is too deep. The calculator gives a starting point, not a final answer. Bump feed in 10 percent steps until the chip color turns light blue in steel (good heat carrying away in the chip) or the chip stays silvery in aluminum (low heat). Drill speed and feed calculator behavior is the same: start conservative, walk it up.