acalculator

What is the SFM, rpm and feed rate?

Type the tool diameter and the surface speed (SFM, or m/min in metric) to get the spindle speed in rpm. Add the chip load and the number of flutes for the feed rate. Or switch to find the surface speed from the rpm you run. Works in inch and metric units.

Your numbers

Units
Find
Spindle speed (rpm)
2,292

Run the spindle at 2,292 rpm for a surface speed of 300.

Surface speed
300
Feed rate
18.3346
Feed per revolution
0.008
Surface speed in the other unit
91.44

Spindle speed (rpm): 2,292. Run the spindle at 2,292 rpm for a surface speed of 300.

How to calculate

Works out the spindle speed in rpm from the surface speed (SFM or m/min) and the tool diameter, or the SFM from the rpm, and the feed rate from the chip load and the number of flutes.

Example with the default inputs (Units Inch, Find RPM, Tool diameter 0.5, Surface speed 300, Chip load 0.002, Flutes 4): Run the spindle at 2,292 rpm for a surface speed of 300.

Method: Inch: rpm = 12 × SFM ÷ (π × D), SFM = π × D × rpm ÷ 12. Metric: rpm = 1,000 × Vc ÷ (π × D), Vc = π × D × rpm ÷ 1,000. Feed = rpm × chip load × flutes.

  • The surface speed and chip load come from your tool maker or a machining handbook for the tool and material; the calculator does not pick them.
  • For a drill, type the feed per revolution as the chip load with 1 flute.
  • Machines often run slower than the formula on manual setups; UF’s example uses 60% of the computed values.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Units Inch, Find RPM, Tool diameter 0.25, Surface speed 100, Chip load 0.004, Flutes 1 gives Spindle speed (rpm) 1,527.887454, Feed rate 6.11155.Source: University of Florida, Mechanical and Aerospace Engineering design lab, Speeds and Feeds (N [rpm] = 12 V [sfm] ÷ (π D [in]); milling feed f [in/min] = N × feed per tooth × number of teeth; a ¼ in drill at 100 sfm runs at about 1,500 rpm and 6 in/min at 0.004 in/rev), https://web.mae.ufl.edu/designlab/Advanced%20Manufacturing/Speeds%20and%20Feeds/Speeds%20and%20Feeds.htm (retrieved 2026-10-02): N = 12 × 100 ÷ (π × 0.25) ≈ 1,500 rpm and f ≈ 6 in/min
  2. Units Inch, Find RPM, Tool diameter 0.5, Surface speed 300, Chip load 0.002, Flutes 4 gives Spindle speed (rpm) 2,291.831181, Feed rate 18.334649, Feed per revolution 0.008, Surface speed in the other unit 91.44.Source: University of Florida, Mechanical and Aerospace Engineering design lab, Speeds and Feeds (N [rpm] = 12 V [sfm] ÷ (π D [in]); milling feed f [in/min] = N × feed per tooth × number of teeth; a ¼ in drill at 100 sfm runs at about 1,500 rpm and 6 in/min at 0.004 in/rev), https://web.mae.ufl.edu/designlab/Advanced%20Manufacturing/Speeds%20and%20Feeds/Speeds%20and%20Feeds.htm (retrieved 2026-10-02)
  3. Units Inch, Find Surface speed, Tool diameter 0.5, Spindle speed (rpm) 2,000, Flutes 2 gives Surface speed 261.799388.Source: University of Florida, Mechanical and Aerospace Engineering design lab, Speeds and Feeds (N [rpm] = 12 V [sfm] ÷ (π D [in]); milling feed f [in/min] = N × feed per tooth × number of teeth; a ¼ in drill at 100 sfm runs at about 1,500 rpm and 6 in/min at 0.004 in/rev), https://web.mae.ufl.edu/designlab/Advanced%20Manufacturing/Speeds%20and%20Feeds/Speeds%20and%20Feeds.htm (retrieved 2026-10-02)
  4. Units Metric, Find RPM, Tool diameter 10, Surface speed 120, Chip load 0.05, Flutes 3 gives Spindle speed (rpm) 3,819.718634, Feed rate 572.957795.Source: Virasak, Manufacturing Processes 4-5, Unit 2: Speed and Feed (Workforce LibreTexts; cutting speed in ft/min or m/min; the shop rule RPM = cutting speed × 4 ÷ diameter, a 3/8 in drill at 100 sfm runs at about 1,066 rpm), https://workforce.libretexts.org/Bookshelves/Manufacturing/Manufacturing_Processes_4-5_(Virasak)/02%3A_Lathe_Machines/02.2%3A_Unit_2%3A_Speed_and_Feed (retrieved 2026-10-02)

How it works

In one turn the cutting edge travels π × D. So the surface speed is V = π × D × N, with N the spindle speed in revolutions per minute. With the units of each system:

  • Inch: D in inches, V in surface feet per minute (SFM), 12 inches in a foot: rpm = 12 × SFM ÷ (π × D) and SFM = π × D × rpm ÷ 12
  • Metric: D in millimeters, V in meters per minute, 1,000 mm in a meter: rpm = 1,000 × V ÷ (π × D) and V = π × D × rpm ÷ 1,000
  • feed rate = rpm × chip load × flutes, in inches per minute (or mm/min); feed per revolution = chip load × flutes
  • the surface speed in the other unit: m/min = SFM × 0.3048, SFM = m/min ÷ 0.3048 (1 ft = 0.3048 m exactly)

π is the double-precision value of π; the maths runs in double precision.

Rules

  • The diameter is from 0.0001 to 10,000; the surface speed from 0.001 to 100,000; the rpm from 0.001 to 10,000,000; the chip load from 0 to 100; flutes a whole number from 1 to 100.
  • Leave the chip load empty to skip the feed rate.

Output format. Rpm to the whole number; surface speed, feed rate, feed per revolution and the other unit to 6 significant figures.

Worked examples by hand

¼ in drill at 100 SFM, 0.004 in per revolution (UF Example 1A). rpm = 1,200 ÷ (π × 0.25) = 1,527.9, feed = 1,527.9 × 0.004 = 6.11 in/min.

½ in, 4 flute end mill at 300 SFM, 0.002 in chip load. rpm = 3,600 ÷ (π × 0.5) = 2,291.8; feed per revolution = 0.002 × 4 = 0.008 in; feed = 2,291.8 × 0.008 = 18.33 in/min; 300 SFM × 0.3048 = 91.44 m/min.

½ in tool at 2,000 rpm. SFM = π × 0.5 × 2,000 ÷ 12 = 261.80 SFM.

10 mm, 3 flute cutter at 120 m/min, 0.05 mm chip load. rpm = 120,000 ÷ (π × 10) = 3,819.7; feed = 3,819.7 × 0.15 = 572.96 mm/min.

Other questions people ask

What is SFM?

SFM is surface feet per minute: how fast the cutting edge moves past the work. It depends on the tool and work material, not the tool size, so tool makers and handbooks give it per material. The metric version is meters per minute (m/min).

How do I convert SFM to rpm?

rpm = 12 × SFM ÷ (π × D), with D the tool diameter in inches. The 12 turns feet into inches and π × D is the distance the edge travels in one turn. A ½ in end mill at 300 SFM runs at 3,600 ÷ (π × 0.5) ≈ 2,292 rpm.

What is the "SFM × 4 ÷ D" shop rule?

12 ÷ π is 3.82, which shops round to 4 for quick mental maths: rpm ≈ 4 × SFM ÷ D. A 3/8 in drill at 100 SFM gives 4 × 100 ÷ 0.375 ≈ 1,066 rpm by the rule, against 1,019 rpm by the exact formula. The calculator uses the exact π.

How do I calculate the feed rate?

Feed rate = rpm × chip load × number of flutes. The chip load is how thick a chip each tooth takes. At 2,292 rpm with 4 flutes and a 0.002 in chip load, the feed is 2,292 × 0.002 × 4 ≈ 18.3 in/min.

How do I use it for drilling?

A drill is fed per revolution. Type the feed per revolution as the chip load and set flutes to 1. UF’s example ¼ in drill at 100 SFM runs at about 1,528 rpm, and at 0.004 in per revolution feeds about 6.1 in/min.

How do I work in metric?

Pick metric: the diameter is in millimeters, the surface speed in m/min and the chip load in mm per tooth. Then rpm = 1,000 × Vc ÷ (π × D). A 10 mm cutter at 120 m/min runs at 120,000 ÷ (π × 10) ≈ 3,820 rpm.