SFM Calculator
Calculate RPM from surface speed (SFM or m/min) and tool diameter, or convert RPM back to surface speed.
RPM Calculator
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 CutViewerFrequently Asked Questions
How do you calculate RPM from SFM?
Multiply the surface speed in SFM by 3.82, then divide by the tool diameter in inches. For example, a 1/2-inch carbide end mill in 6061 aluminum at 1,000 SFM runs at 1,000 × 3.82 / 0.5 = 7,640 RPM.
How do you calculate RPM from cutting speed in m/min?
Multiply the cutting speed in m/min by 1,000, then divide by π times the tool diameter in mm. A 6 mm end mill at 200 m/min runs at 200,000 / (3.1416 × 6) ≈ 10,610 RPM.
What does SFM stand for?
SFM stands for surface feet per minute: how fast the cutting edge travels across the workpiece. It depends on the workpiece material and the tool material, and it is the starting point for choosing spindle speed.
What if the calculated RPM is higher than my spindle can run?
Run at your spindle's maximum RPM and keep the chipload the same by lowering the feed rate to match. This is common with small end mills in aluminum and plastics on hobby CNC routers.
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