How to use this calculator
- Choose the unknown. Select whether you want spindle RPM or cutting speed.
- Enter the diameter. Use cutter diameter for milling and drilling, or work diameter for turning.
- Enter speed or RPM. Enter the known value. Use the reference table as a starting point for cutting speed.
How it works
Cutting speed is the tangential speed at the tool/work contact:
Vc = n × π × D
where n is spindle speed and D is the diameter at the cut.
In metric, Vc = n × π × D / 1000 with D in mm to get m/min. In imperial,
SFM = n × π × D / 12 with D in inches. Inverting the same formula gives
RPM = Vc / (π × D).
Worked example
Verified against the live calculator
For a 1-inch tool at 100 SFM:
RPM = 12 × 100 ÷ (π × 1) = 382 RPM. Running that same 1-inch tool at
382 RPM gives SFM = 382 × π × 1 ÷ 12 = 100 SFM. The calculator defaults
to that round-trip example.
Reference data
Starting cutting speeds for single-point turning, in surface feet per minute, for HSS and uncoated carbide. Use these as baseline values, then adjust for toolmaker recommendations, coolant, tool engagement and machine rigidity.
| Material | HSS (SFM) | Carbide (SFM) | Notes |
|---|---|---|---|
| Aluminum | 300 | 800 | Free-machining; high speeds, watch built-up edge. |
| Brass | 200 | 600 | Free-cutting brass; very machinable. |
| Bronze | 120 | 350 | Harder than brass. |
| Cast iron (gray) | 60 | 200 | Abrasive; carbide preferred. |
| Mild steel (1018) | 90 | 350 | General low-carbon steel. |
| Alloy steel (4140) | 60 | 250 | Heat-treatable; lower when hardened. |
| Stainless (304) | 50 | 200 | Work-hardens; keep feed up, avoid dwelling. |
| Tool steel | 45 | 175 | Hard; reduce speed as hardness rises. |
| Titanium | 35 | 150 | Low speed; heat builds fast. |
| Plastic | 400 | 800 | Sharp tools; clear chips. |
| Aluminum (cast A356) | 250 | 700 | Silicon content is abrasive; use flood coolant. |
| Magnesium | 400 | 900 | Excellent machinability; fine chips are a fire risk, never use water-based coolant. |
| Copper | 150 | 400 | Gummy; sharp tools and positive rake reduce built-up edge. |
| Free-machining steel (12L14) | 130 | 450 | Leaded/resulfurized; best machinability of the steels. |
| Medium-carbon steel (1045) | 70 | 300 | Tougher than 1018; common shaft steel. |
| Alloy steel (4340) | 50 | 220 | High-strength; reduce speed in the heat-treated condition. |
| Hardened steel (45-55 HRC) | 20 | 100 | Hard turning; carbide or CBN only, light finishing cuts (HSS not suitable). |
| Stainless (303) | 70 | 250 | Free-machining austenitic; the easiest stainless. |
| Stainless (316) | 45 | 180 | Gummier than 304 and work-hardens; keep cuts positive. |
| Stainless (416) | 90 | 280 | Free-machining martensitic stainless. |
| Stainless (17-4 PH) | 40 | 150 | Precipitation-hardening; speed depends on condition (H900 lower). |
| Stainless (410) | 60 | 220 | Martensitic; machine in the annealed condition where possible. |
| Ductile iron (65-45-12) | 50 | 180 | Nodular iron; tougher and less abrasive than gray. |
| Malleable iron | 80 | 250 | Machines more like steel than gray iron. |
| Inconel / nickel superalloy | 15 | 60 | Very tough and work-hardens; low speed, positive rake, rigid setup. |
| Monel (Ni-Cu) | 40 | 120 | Work-hardens; keep the feed up and never dwell. |
| Hastelloy | 15 | 60 | Nickel superalloy; treat like Inconel. |
| Fiberglass / GFRP | 150 | 500 | Abrasive; carbide or diamond tooling and dust extraction. |
Source: Standard machining references (Machinery's Handbook turning-speed tables; common shop practice). Verify against your tooling maker's data.
Frequently asked questions
What is cutting speed?
Cutting speed is the surface speed at the cutting edge, usually given as SFM in imperial shops or m/min in metric shops. It is set by the work material, tool material and cutting conditions.
How do you calculate cutting speed from RPM?
Use Vc = RPM × π × D. With D in millimetres, divide by 1000 to get m/min. With D in inches, Vc in SFM = RPM × π × D / 12.
How do you calculate RPM from cutting speed?
Use RPM = Vc ÷ (π × D). In imperial, RPM = 12 × SFM ÷ (π × diameter in inches), or about 3.82 × SFM ÷ D.
Is the diameter the cutter or the part?
For milling and drilling, use the cutter or drill diameter. For turning and boring, use the workpiece diameter at the cutting edge.
Should I use HSS or carbide cutting speeds?
Use the tool material you are actually running. Carbide can usually run several times faster than HSS, but the best value still depends on coating, coolant, rigidity and depth of cut.
Method & assumptions
- The formula uses the diameter at the cutting edge. For facing or taper cuts, cutting speed changes as the diameter changes.
- The reference table is a starting point, not a process guarantee; coatings, coolant, stickout, interrupted cuts and machine stiffness change the usable speed.
- For a fuller feeds-and-speeds workflow, pair this with the chip load calculator or CNC speeds and feeds calculator.