Surface Footage Calculator
Surface footage is the linear speed of the cutting edge across the workpiece, measured in surface feet per minute (SFM). Tooling vendors and the Machinery's Handbook spec cutting speeds in surface footage because the number stays constant across tool sizes. This surface footage calculator converts RPM and diameter to SFM, or works backwards from a target SFM to spindle RPM, for milling, turning, or drilling.
The surface footage formula
- Surface footage (SFM) = (pi x D x RPM) / 12 with D in inches.
- Solved for RPM: RPM = (SFM x 12) / (pi x D).
- Metric: Vc (m/min) = (pi x D_mm x RPM) / 1000.
For a 1 inch end mill at 1500 RPM: SFM = (pi x 1 x 1500) / 12 = 392.7 SFM. That is right at the 400 SFM target for 1018 mild steel with carbide. For a 2 inch workpiece on a lathe spinning at 760 RPM: SFM = (pi x 2 x 760) / 12 = 397.9 SFM.
Surface footage targets by material
| Material | HSS SFM | Carbide SFM |
|---|---|---|
| Aluminum 6061 | 600 | 1400 |
| Mild steel (1018) | 100 | 400 |
| Medium carbon (1045) | 80 | 300 |
| Alloy steel (4140) | 70 | 250 |
| Stainless 304/316 | 60 | 200 |
| Cast iron | 80 | 260 |
| Brass | 250 | 500 |
| Titanium Ti-6Al-4V | 40 | 120 |
| Plastic (Delrin) | 800 | 2000 |
Which diameter goes in the formula
The D in the surface footage formula is whichever surface spins:
- Milling: D is the end mill or face mill cutting diameter. The tool spins.
- Drilling: D is the drill diameter. The drill spins.
- Turning: D is the workpiece diameter (OD currently being cut). The work spins.
This is why a 1 inch end mill at 1500 RPM makes the same SFM as a 1 inch workpiece at 1500 RPM on a lathe. Same circumference, same tangential velocity.
Surface footage drops as the lathe removes material
On a manual lathe at constant RPM, surface footage drops proportionally as the OD shrinks during a face or profile cut. Cut a 2 inch bar down to 1 inch and SFM halves. CNC lathes solve this automatically with G96 constant surface speed, which tells the control to raise RPM as D drops. On a manual lathe, step the RPM up at each pass. A surface footage calculator gives you the current SFM at the current diameter.
Chip color as a sanity check
The right surface footage in steel gives light straw to light blue chips; in aluminum, silver spiral chips with minimal built-up edge. Dark blue or purple chips mean SFM is too high. Silver chips that smear mean SFM is too low. Use the calculator to get a starting RPM, then fine-tune by chip color.