SFM Calculator
Calculate RPM from surface speed (SFM or m/min) and tool diameter, or convert RPM back to surface speed.
RPM Calculator
Spindle Speed
3,820
RPM
RPM = (SFM × 3.82) / Diameter
Recommended Surface Speeds by Material
Click any row to load its SFM values into the calculator above.
| Material | HSS (SFM) | Carbide (SFM) |
|---|---|---|
| Aluminum (6061) | 300–1000 | 800–1500 |
| Aluminum (Cast / 356) | 200–600 | 500–1000 |
| Brass | 200–400 | 400–800 |
| Bronze | 100–200 | 200–500 |
| Copper | 100–300 | 300–700 |
| Cast Iron (Gray) | 50–100 | 200–400 |
| Cast Iron (Ductile) | 40–80 | 150–350 |
| Mild Steel (1018) | 60–100 | 300–600 |
| Medium Steel (1045) | 50–80 | 200–500 |
| Alloy Steel (4140) | 40–70 | 150–400 |
| Tool Steel (A2/D2) | 30–60 | 100–250 |
| Stainless Steel (303) | 50–100 | 200–450 |
| Stainless Steel (304) | 40–80 | 150–400 |
| Stainless Steel (316) | 30–60 | 100–350 |
| Titanium (Grade 2) | 30–60 | 100–200 |
| Titanium (6Al-4V) | 20–40 | 50–150 |
| Plastics (Acrylic) | 300–800 | 500–1500 |
| Plastics (Delrin / Nylon) | 200–600 | 400–1200 |
| Wood (Hardwood) | 300–600 | 600–1200 |
| Wood (Softwood / MDF) | 400–1000 | 800–1500 |
How to Calculate RPM from SFM
The Formula
RPM and surface speed are related by the tool diameter:
RPM = (SFM × 3.82) / D
where D = tool diameter in inches, 3.82 = 12/π
For metric:
RPM = (m/min × 1000) / (π × D)
where D = tool diameter in mm
What is SFM?
SFM (Surface Feet per Minute) is the speed at which the cutting edge moves across the workpiece surface. It's determined by the material being cut and the tool material (HSS vs carbide). A harder workpiece material requires a lower SFM; a harder tool material allows a higher SFM.
Why SFM Matters
- Too low — poor surface finish, rubbing instead of cutting, work hardening (especially in stainless steel)
- Too high — rapid tool wear, overheating, risk of tool breakage
- Just right — clean chips, good surface finish, long tool life
Tips for CNC
- Start at the low end of the SFM range and increase as you gain confidence
- Coated carbide end mills can run at the high end of the carbide range
- Reduce SFM by 30–50% for deep slotting compared to side milling
- Small diameter tools hit RPM limits quickly — check your spindle's max speed
Preview Your Toolpaths
Once you've dialed in your speeds, use CutViewer to visualize your G-code program in 3D before cutting.
Launch CutViewer