CNC Feed Rate Calculator — RPM, Flutes & Chip Load
Calculate CNC milling feed rate from spindle RPM, flute count, and chip load, with the formula shown step by step.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Feed Rate (IPM) = RPM x Number of Flutes x Chip Load per Tooth
The feed rate is the product of spindle speed (RPM), the number of cutting edges (flutes), and the chip load per tooth (inches). SFM is calculated as pi times diameter times RPM divided by 12. MRR equals feed rate times depth of cut times width of cut.
Worked Examples
Example 1: Aluminum Roughing with 1/2 Inch End Mill
Problem:Calculate feed rate for a 0.5 inch, 3-flute carbide end mill in 6061 aluminum at 10,000 RPM with 0.005 inch chip load.
Solution:Feed Rate = RPM x Flutes x Chip Load\nFeed Rate = 10,000 x 3 x 0.005 = 150 IPM\nSFM = pi x 0.5 x 10,000 / 12 = 1,309 SFM\nWith 0.25 inch depth and 0.25 inch width of cut:\nMRR = 150 x 0.25 x 0.25 = 9.375 cubic inches/min
Result:Feed Rate: 150 IPM | SFM: 1,309 | MRR: 9.375 in3/min
Example 2: Steel Finishing with Chip Thinning
Problem:A 0.5 inch, 4-flute end mill finishes mild steel at 4,000 RPM, 0.003 inch chip load, with 0.05 inch radial engagement (10% of diameter).
Solution:Basic Feed Rate = 4,000 x 4 x 0.003 = 48 IPM\nRadial engagement = 0.05 / 0.5 = 10%\nChip Thin Factor = 1 / (2 x sqrt(0.1 x 0.9)) = 1.667\nAdjusted Chip Load = 0.003 x 1.667 = 0.005\nAdjusted Feed Rate = 4,000 x 4 x 0.005 = 80 IPM
Result:Adjusted Feed Rate: 80 IPM (67% faster than nominal to maintain proper chip thickness)
Frequently Asked Questions
What is CNC feed rate and why does it matter?
CNC feed rate is the speed at which the cutting tool moves through the workpiece material, measured in inches per minute (IPM) or millimeters per minute. It directly affects surface finish quality, tool life, and machining time. A feed rate that is too slow causes rubbing instead of cutting, which generates excessive heat and accelerates tool wear. A feed rate that is too fast can overload the tool, cause chipping, or break the cutter entirely. Finding the optimal feed rate balances productivity with tool longevity and part quality.
How do I calculate CNC feed rate from RPM, flutes, and chip load?
The basic feed rate formula is Feed Rate equals RPM multiplied by the number of flutes multiplied by the chip load per tooth. For example, with 8,000 RPM, 4 flutes, and a chip load of 0.004 inches per tooth, the feed rate is 8,000 times 4 times 0.004 which equals 128 inches per minute. RPM determines how fast the tool spins, flutes determine how many cutting edges engage per revolution, and chip load determines how much material each cutting edge removes per pass. All three variables must be balanced for optimal cutting performance.
What is chip load and how do I choose the right value?
Chip load is the thickness of material removed by each cutting edge (flute) during one revolution of the tool, measured in inches per tooth or millimeters per tooth. The correct chip load depends on the tool diameter, workpiece material, and tool material. For carbide end mills in aluminum, typical chip loads range from 0.003 to 0.006 inches per tooth. For steel, values are lower at 0.001 to 0.004 inches per tooth. Tool manufacturers provide recommended chip load charts for their specific products. Using the manufacturer-recommended value is always the best starting point.
What is surface feet per minute and how does it relate to RPM?
Surface feet per minute, or SFM, is the speed at which the outer edge of the cutting tool moves across the workpiece surface. It is calculated as SFM equals pi times tool diameter times RPM divided by 12. SFM is a material-dependent constant, meaning each material has an optimal cutting speed range. For example, aluminum typically cuts best at 500 to 1,000 SFM while mild steel prefers 80 to 120 SFM. To find the correct RPM for a given SFM, rearrange the formula to RPM equals SFM times 12 divided by pi times diameter.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy