Feeds and Speeds Calculator — RPM & Feed Rate (SFM Formula)
Who this is for: For machinists and hobby CNC owners dialing in a new material or tool — who want the conservative starting point, not a paid app's 40-field interrogation.
Not the right tool for: Rigid tapping, thread milling and helical interpolation — start from the tool maker's chart · Guaranteed parameters — treat results as starting points and adjust from sound, finish and spindle load
Material and tool in — get RPM, feed rate, chipload and MRR from the classic SFM formula, with carbure notes.
Quick answer: Feeds & speeds: RPM = SFM × 3.82 ÷ tool diameter; feed = RPM × flutes × chipload (chipload ≈ 1% of diameter per flute, ×1.5 aluminum). A 1/4" 2-flute in aluminum: 250 SFM → 3,820 RPM → 28.7 IPM (chipload 0.0038-inch/flute). Carbide runs 3–4× the SFM — start conservative and dial up.
2–4 typical
| Diameter | RPM | Feed (IPM) |
|---|---|---|
| 8/64" (0.125) | 7,640 | 29.03 |
| 12/64" (0.1875) | 5,093 | 28.52 |
| 16/64" (0.25) | 3,820 | 29.03 |
| 20/64" (0.3125) | 3,056 | 28.73 |
| 24/64" (0.375) | 2,547 | 28.53 |
| 32/64" (0.5) | 1,910 | 28.65 |
| 40/64" (0.625) | 1,528 | 28.73 |
| 48/64" (0.75) | 1,273 | 28.77 |
| 1" | 955 | 28.65 |
| RPM | (SFM × 3.82) ÷ diameter |
|---|---|
| Feed | RPM × flutes × chipload |
| SFM (HSS) | Al 250 · brass 200 · steel 90 · SS 60 · Ti 40 |
| Chipload | ≈0.01 × D per flute (steel); ×1.5 Al |
| Example | 1/4" 2fl Al: 3,820 RPM · 28.7 IPM |
| Status | Quotes use standard shop math: machine-hour rate × cycle time + materials + setup amortization. SFM/chipload figures are conservative HSS starting points (carbide runs 3–4× SFM). Compiled September 2026 — estimates for budgeting; real quotes come from the shop that runs the job. |
How are feeds and speeds calculated?
Two formulas run the shop. Spindle speed: RPM = (SFM × 3.82) ÷ tool diameter in inches, where SFM is surface feet per minute for the material–tool pair. Feed: IPM = RPM × flute count × chipload, where chipload is the per-flute bite (≈ 1% of diameter per flute for steels, ×1.5 for aluminum at these diameters). This calculator uses conservative HSS SFM starting points — aluminum 250, brass 200, mild steel 90, stainless 60, cast iron 70, titanium 40 — and outputs the RPM, feed, chipload and an approximate MRR. Carbide tools run 3–4× these SFM values, so the output is deliberately the safe start: dial up from here while chips stay golden and the spindle stays quiet.
Common uses
- First cuts in a new material on a hobby CNC or manual mill
- Sanity-checking a CAM post's guessed speeds before hitting cycle start
- Comparing HSS vs carbide starting parameters
- Teaching the RPM/IPM relationship without the paid-app wall of fields
Why the calculator stays conservative
Paid feeds-and-speeds apps model tool coating, stickout, coolant and machine rigidity — worth it in production. This calculator gives the HSS-baseline starting point that won't break a hobby machine on day one: cut air first, then take light passes, then push. The gap between 'safe' and 'optimal' is where shops make money — and where hobby machines make scrap. Respect the gap until your spindle tells you otherwise.