# SFPM Calculator

SFPM calculator converts spindle RPM and tool diameter to surface feet per minute, or SFPM back to RPM. Uses the standard SFPM = (pi x D x RPM) / 12 Machinery's Handbook formula.

## What this calculates

SFPM stands for surface feet per minute, the cutting speed at the edge of a spinning tool or workpiece. Every CNC speed chart and Machinery's Handbook entry spec's cutting speed in SFPM because the number stays the same across tool sizes, so tool life stays predictable. This SFPM calculator runs the standard SFPM = (pi x D x RPM) / 12 formula in either direction.

## Inputs

- **Calculation Mode** — options: RPM + diameter -> SFPM, SFPM + diameter -> RPM — Pick the input you know.
- **Tool or Workpiece Diameter** (in) — min 0.001 — Tool diameter for milling/drilling, workpiece diameter for turning.
- **Spindle RPM** (RPM) — min 0 — Used in RPM -> SFPM mode.
- **Target SFPM** (SFPM) — min 0 — Used in SFPM -> RPM mode.
- **Reference Material** — options: Aluminum (600/1400 SFPM), Mild steel / 1018 (100/400 SFPM), Alloy steel / 4140 (70/250 SFPM), Stainless (60/200 SFPM), Cast iron (80/260 SFPM), Brass (250/500 SFPM), Titanium (40/120 SFPM), Plastic (800/2000 SFPM) — Used to show a recommended SFPM from the Machinery's Handbook.
- **Tool Material** — options: HSS, Carbide — For the SFPM reference lookup only.

## Outputs

- **SFPM** (SFPM) — Surface feet per minute = (pi x D x RPM) / 12.
- **Vc (metric)** (m/min) — SFPM x 0.3048.
- **Required Spindle RPM** (RPM) — RPM for the target SFPM.
- **Recommended SFPM (Reference)** (SFPM) — Machinery's Handbook target for the chosen material + tool.
- **Match vs Recommended** — formatted as text — Quick check against the Machinery's Handbook SFPM.

## Details

SFPM formula

  - SFPM = (pi x D x RPM) / 12 with D in inches.

  - Solved for RPM: RPM = (SFPM x 12) / (pi x D).

  - Metric: Vc (m/min) = (pi x D_mm x RPM) / 1000.

For a 1/2 inch carbide end mill at 3000 RPM: SFPM = (pi x 0.5 x 3000) / 12 = 392.7 SFPM. To hit 400 SFPM on the same tool, RPM = (400 x 12) / (pi x 0.5) = 3,056 RPM. The SFPM calculator runs either direction.

Recommended SFPM by material

  
    MaterialHSS SFPMCarbide SFPM
  
  
    Aluminum 60616001400
    Mild steel (1018)100400
    Alloy steel (4140)70250
    Stainless steel60200
    Cast iron80260
    Brass250500
    Titanium Ti-6Al-4V40120
    Plastic (Delrin)8002000
  

Why SFPM and not RPM directly

Cutting speed is a property of the material-tool pair, not the tool size. Mild steel with carbide wants 400 SFPM whether the tool is a 1/8 end mill, a 1 inch drill, or a 3 inch bar on a lathe. Only the RPM changes. SFPM lets you keep one cutting speed table and apply it to any tool diameter: just plug into RPM = (SFPM x 12) / (pi x D). An SFPM calculator is the translator.

Which diameter to use

The D in the SFPM formula is whichever surface is spinning:

  - Milling: end mill or face mill cutting diameter.

  - Drilling: drill diameter.

  - Turning: workpiece OD currently being cut (not the tool, because the tool is stationary).

This is why a lathe SFPM calculation uses workpiece diameter, but a mill SFPM calculation uses tool diameter. Same formula, different D.

Calculating RPM from SFPM

Start with the SFPM target from a Machinery's Handbook table or a tool vendor data sheet. Plug into RPM = (SFPM x 12) / (pi x D). For a 3/8 inch carbide drill in mild steel at 250 SFPM: RPM = (250 x 12) / (pi x 0.375) = 2,546 RPM. If your machine tops out below that, cap at the machine max and accept the lower SFPM.

SFPM vs SFM

SFPM and SFM mean the same thing: surface feet per minute. Older texts and some vendors spell it out as SFPM; most modern tables shorten it to SFM. The number and the formula are identical. This SFPM calculator accepts either name and returns the same value.

## Frequently Asked Questions

**Q: How do I calculate SFPM?**

A: SFPM = (pi x D x RPM) / 12, where D is the diameter of the spinning part (tool for milling and drilling, workpiece for turning) in inches, and RPM is the spindle speed. For a 1/2 inch tool at 3000 RPM, SFPM = (pi x 0.5 x 3000) / 12 = 392.7 SFPM. This SFPM calculator does the conversion in either direction.

**Q: What is SFPM in machining?**

A: SFPM (surface feet per minute) is the linear velocity of a cutting edge across the workpiece, measured in feet per minute. It is the standard cutting speed unit in US machining. Cutting tools wear based on temperature at the edge, and temperature correlates with SFPM, so programming by SFPM gives predictable tool life across different tool sizes.

**Q: What SFPM should I use for carbide milling steel?**

A: For mild steel (1018), 400 SFPM is a solid starting point. For 4140 alloy steel, drop to 250. For stainless, 200. For a 1/2 inch carbide end mill in 1018 at 400 SFPM, RPM = (400 x 12) / (pi x 0.5) = 3,056 RPM. Chip color should be light straw to light blue if you are in the right zone.

**Q: How do I convert SFPM to RPM?**

A: RPM = (SFPM x 12) / (pi x D). For a target 400 SFPM on a 1/4 inch drill: RPM = (400 x 12) / (pi x 0.25) = 6,112 RPM. Plug any target SFPM and tool diameter into this SFPM calculator to get the spindle RPM to program.

**Q: Is SFPM the same as SFM?**

A: Yes. SFPM (surface feet per minute) and SFM (surface feet per minute) are identical. The 'P' is sometimes dropped for brevity but the formula and the units are exactly the same. Any SFM calculator doubles as an SFPM calculator.

**Q: What is the metric version of SFPM?**

A: Cutting velocity Vc in m/min. Formula: Vc = (pi x D_mm x RPM) / 1000 with D in millimeters. SFPM x 0.3048 = m/min. 400 SFPM = 121.9 m/min. European CNC controls (Siemens, Heidenhain) program cutting speed in m/min; US controls (Haas, Mazak) use SFPM. Both come from the same circumference x RPM equation.

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Source: https://vastcalc.com/calculators/physics/sfpm
Category: Physics
Last updated: 2026-04-08