About the Speeds and Feeds Calculator
This speeds and feeds calculator converts a recommended cutting speed and chip load into the numbers you actually program: spindle RPM and table feed rate. It handles milling (feed per tooth × number of flutes), drilling (feed per revolution) and turning (feed per revolution, using the workpiece diameter), in either inch units (SFM, IPT, IPM) or metric units (m/min, mm/tooth, mm/min). It also estimates the metal removal rate from your depth and width of cut.
Machinists, CNC programmers, hobby mill and lathe owners and machining students can use it to start from a tooling catalogue’s recommendation and check that the spindle can actually reach the required speed. If the RPM is higher than your machine’s maximum, the calculator caps it and recalculates the feed and the real surface speed.
Cutting speed and chip load depend on the workpiece material, the tool material and coating, rigidity and coolant, so always take them from your tool manufacturer’s data where possible. As a rough guide, uncoated carbide tools typically run two to four times faster than high-speed steel, and aluminum can be cut several times faster than steel.
With the default inputs, the spindle speed is 3,820 RPM. Change any value above to recalculate instantly.
How to use the speeds and feeds calculator
- 1Choose milling, drilling or turning and your preferred units.
- 2Enter the cutting speed from your tool manufacturer for the workpiece material.
- 3Enter the tool diameter (workpiece diameter for turning) and, for milling, the flute count.
- 4Enter the chip load per tooth (milling) or feed per revolution (drilling and turning).
- 5Add depth and width of cut and your spindle limit, then program the RPM and feed shown.
Formula and method
Spindle speed comes from the cutting speed — how fast the cutting edge moves across the material — divided by the circumference of the tool (or of the workpiece on a lathe). In inch units, surface feet per minute are multiplied by 12 to convert to inches; in metric, metres per minute are multiplied by 1,000 to convert to millimetres.
For milling, the feed rate is RPM × number of teeth × chip load per tooth. For drilling and turning it is RPM × feed per revolution. Metal removal rate is width × depth × feed for milling, the drill’s cross-section area × feed for drilling, and cutting speed × feed per rev × depth of cut for turning. If the RPM exceeds the machine maximum, the capped RPM is used for feed and removal rate.
- SFM / Vc
- Cutting speed (surface ft/min or m/min)
- D
- Tool diameter, or workpiece diameter for turning
- z
- Number of teeth (flutes)
- f_z
- Chip load (feed per tooth)
- f_n
- Feed per revolution
- ae, ap
- Width (radial) and depth (axial) of cut
- MRR
- Metal removal rate
Worked examples
1/2" 3-flute HSS end mill in aluminum at 500 SFM
RPM = 500 × 12 ÷ (π × 0.5) ≈ 3,820. Feed = 3,820 × 3 flutes × 0.002 in ≈ 22.9 in/min. With a 0.25 in deep, 0.25 in wide cut, the removal rate is 0.25 × 0.25 × 22.9 ≈ 1.43 in³/min.
8 mm drill in steel at 25 m/min
RPM = 25 × 1,000 ÷ (π × 8) ≈ 995. Feed = 995 × 0.15 mm/rev ≈ 149 mm/min. The drill removes its 50.3 mm² cross-section at that feed: about 7.5 cm³/min.
Small 1/8" end mill capped by a 10,000 RPM spindle
The ideal speed is 600 × 12 ÷ (π × 0.125) ≈ 18,335 RPM, but the spindle tops out at 10,000. At 10,000 RPM the feed is 10,000 × 2 × 0.001 = 20 in/min and the real cutting speed is only π × 0.125 × 10,000 ÷ 12 ≈ 327 SFM.
Frequently asked questions
How do you calculate spindle RPM from SFM?+
RPM = SFM × 12 ÷ (π × diameter in inches), often shortened to RPM ≈ 3.82 × SFM ÷ D. In metric, RPM = cutting speed (m/min) × 1,000 ÷ (π × diameter in mm).
How do you calculate feed rate for milling?+
Feed rate (in/min or mm/min) = RPM × number of flutes × chip load per tooth. For example, a 3-flute cutter at 4,000 RPM with a 0.002 in chip load feeds at 4,000 × 3 × 0.002 = 24 in/min.
What is chip load?+
Chip load (feed per tooth) is the thickness of material each cutting edge removes per revolution. Too small a chip load causes rubbing and heat; too large risks breaking the tool. Tool makers publish chip loads by tool diameter and workpiece material.
What cutting speed should I use?+
It depends on the workpiece and tool material. As a rough guide, high-speed steel in mild steel runs around 80–100 SFM, carbide in mild steel several hundred SFM, and carbide in aluminum often 800 SFM or more. Always start from the tool manufacturer’s recommendation.
Why is my small end mill speed limited?+
Small tools need very high RPM to reach their recommended surface speed. When the spindle cannot go that fast, keep the chip load and accept a lower feed rate; the calculator shows the reduced actual surface speed when the RPM is capped.