Calculate the correct cutting speed (RPM) and feed rate (IPM) for your CNC or manual machine tool. Enter your tool diameter, surface feet per minute (SFM), number of teeth/flutes, and chip load — and get back the ideal spindle RPM and feed rate in inches per minute. Switch between inch and metric units to match your setup.
Spindle Speed (RPM)
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Feed Rate
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Surface Speed (SFM)
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Chip Load per Tooth
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RPM vs Machine Max
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Material Removal Rate (MRR)
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RPM is calculated using the formula: RPM = (SFM × 3.82) / Tool Diameter (inches). The constant 3.82 is derived from 12 / π. For metric, use RPM = (SMM × 318.31) / Tool Diameter (mm), where SMM is surface meters per minute.
Surface Feet Per Minute (SFM) is the speed at which the cutting edge of a tool moves across the workpiece surface, measured in feet per minute. It is a material-dependent recommended value — harder materials require lower SFM to protect the tool edge and prevent heat buildup.
Feed rate (IPM) = RPM × Number of Teeth × Chip Load per Tooth. Chip load is the thickness of material removed by each cutting edge per revolution, and it varies by tool diameter, material, and tool type. For example, a 4-flute end mill at 5,000 RPM with a 0.002" chip load yields 40 IPM.
For mild steel with a coated carbide end mill (0.5" diameter), a typical SFM is around 300–500, giving an RPM of roughly 2,300–3,800. Harder alloy steels and stainless require lower SFM (100–300), so RPM drops accordingly. Always start conservatively and increase as you observe chip color and surface finish.
SFM and RPM are linked by the tool's circumference. A larger diameter tool traveling at the same RPM sweeps more surface per minute — so to maintain the same SFM, a larger tool must run at a lower RPM. The relationship is: SFM = (RPM × Diameter × π) / 12.
In turning, feed rate is typically expressed as inches per revolution (IPR) rather than IPM. Feed Rate (IPM) = RPM × Feed per Revolution (IPR). The chip load concept still applies — consult the insert manufacturer's recommended feed per revolution for the workpiece material and depth of cut.
Coated carbide inserts and end mills are much harder and more heat-resistant than high-speed steel. The coating (e.g. TiAlN, TiN) further reduces friction and heat transfer into the tool. This allows carbide tools to run at 3–5× higher surface speeds than HSS, significantly increasing productivity.
A general rule of thumb: chip load ≈ 1% of tool diameter for most end mills in steel, and 1–2% for aluminum. For example, a 0.5" end mill in aluminum might use 0.005"–0.010" per tooth, while the same tool in stainless steel may use 0.001"–0.002" per tooth. Always check your tool manufacturer's recommendations.