Cutting Speed Calculator
Enter your material's recommended surface speed and the tool diameter, and get the spindle RPM to dial in — for milling, this is your cutter diameter.
Typical cutting speeds (SFM)
| Material | HSS tool | Carbide tool |
|---|---|---|
| Aluminum (free machining) | 300–600 | 800–2000 |
| Mild steel (1018) | 80–110 | 350–550 |
| Alloy steel (4140) | 50–80 | 250–400 |
| Stainless steel (304) | 40–70 | 200–350 |
| Cast iron (gray) | 60–90 | 300–500 |
| Brass (free cutting) | 150–300 | 500–1000 |
| Titanium (Ti-6Al-4V) | 20–30 | 100–180 |
Typical starting points, not a substitute for the tool manufacturer's data sheet — coatings, insert grade, and coolant all shift these ranges.
Where the formula comes from
Cutting speed (SFM) describes how fast the tool's cutting edge moves through the material, not how fast the spindle spins — those are the same thing only for one specific diameter. RPM = (SFM × 12) ÷ (π × diameter) converts between them by working out the tool's circumference in feet and dividing the required linear speed by it. A bigger tool covers more surface per revolution, so it needs fewer revolutions per minute to hit the same SFM — that's why a 1 inch end mill and a 1/4 inch end mill in the same material call for very different RPMs even though the material hasn't changed.
The quick shop version of this formula
Most shop reference cards shorten this to RPM = (SFM × 4) ÷ diameter, since 12÷π ≈ 3.82, close enough to 4 for a fast mental estimate at the machine. This calculator uses the exact constant rather than the rounded one, so expect its answer to run a few percent lower than the "×4" shortcut — both are fine starting points, but use the exact number when you're already fighting chatter or tool life at the edge of the recommended range.
Why the reference table is a range, not a number
The right SFM for a job depends on more than material and tool material alone: insert coating, rigidity of the setup, coolant (flood, mist, or dry), and how deep a finish you need all push the workable range up or down. Start near the middle of the listed range, and move toward the low end if you're seeing excess heat, edge chipping, or chatter, or toward the high end once you've confirmed the setup is rigid and well cooled.