SFM to RPM: The Formula, a Chart and Worked Examples
Convert surface feet per minute to spindle RPM in one step — the 3.82 shortcut, a chart from 1/4 to 4 inches, and where the metric formula differs.
What SFM actually measures
SFM — surface feet per minute — is the speed of the tool edge through the material, expressed in feet. A 4-inch cutter rim travels π × 4 ≈ 12.57 inches per revolution, so at 500 rpm its edge covers about 524 feet per minute: 524 SFM. SFM is the imperial twin of metric cutting speed Vc, and 1 SFM = 0.3048 m/min exactly.
The formula — and the 3.82 shortcut
rpm = (SFM × 12) / (π × D), with D in inches. Since 12/π = 3.8197, machinists shorten it to rpm ≈ 3.82 × SFM / D. Worked example: mild steel at 100 SFM with a 1/2-inch end mill → 3.82 × 100 / 0.5 = 764 rpm. The RPM calculator runs this in both unit systems and shows revs per second as well.
SFM to RPM chart
Read across from the diameter, down from the surface speed:
| Diameter | 50 SFM | 100 SFM | 200 SFM | 300 SFM |
|---|---|---|---|---|
| 1/4 in | 764 | 1,528 | 3,056 | 4,584 |
| 1/2 in | 382 | 764 | 1,528 | 2,292 |
| 3/4 in | 255 | 509 | 1,019 | 1,528 |
| 1 in | 191 | 382 | 764 | 1,146 |
| 2 in | 95 | 191 | 382 | 573 |
| 4 in | 48 | 95 | 191 | 286 |
RPM back to SFM (and to metric)
Reverse the formula to check what a given spindle speed really delivers: SFM = π × D × rpm / 12, or SFM ≈ 0.262 × D × rpm. Metric equivalents: Vc = π × D × n / 1000, and 100 m/min ≈ 328 SFM. The nav.cs calculator shows m/min and SFM side by side so a metric drawing and an imperial datasheet can be compared directly.
Two classic mistakes
First, unit mixing: the 3.82 shortcut only works with the diameter in inches. A 12 mm bit entered as “12” comes out about 25× too slow (12 mm = 0.472 in). Second, treating RPM as the whole setup: surface speed decides edge temperature, but chip load per tooth decides whether the bit cuts or rubs. Feeds and Speeds Basics: RPM and Feed Rate Formulas covers that pairing, RPM Formula: How to Calculate Spindle Speed the formula in depth — and the RPM plus Feeds & speeds tools do the arithmetic.