---
title: "SFM to RPM: The Formula, a Chart and Worked Examples"
description: "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."
image: "https://nestingcalc.com/og-default.png"
url: "https://nestingcalc.com/guides/sfm-to-rpm/"
locale: "en"
---

# 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](https://nestingcalc.com/rpm-calculator/) 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 |

Router bits live in the top rows, big face mills in the bottom one — same table, because the geometry is the same.

## 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](https://nestingcalc.com/cutting-speed-calculator/) 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](https://nestingcalc.com/guides/feeds-and-speeds-basics/) covers that pairing, [RPM Formula: How to Calculate Spindle Speed](https://nestingcalc.com/guides/rpm-formula/) the formula in depth — and the [RPM](https://nestingcalc.com/rpm-calculator/) plus [Feeds & speeds](https://nestingcalc.com/feeds-speeds-calculator/) tools do the arithmetic.

[RPM](https://nestingcalc.com/rpm-calculator/) [Open the linear cutting calculator](https://nestingcalc.com/linear-cutting-calculator/)

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