Speeds and Feeds Calculator — CNC Machining RPM & Feed Rate
Enter the recommended cutting speed for your workpiece–cutter combination, the cutter diameter, chip load per tooth and number of flutes to get spindle RPM and feed rate instantly — in imperial (SFM / inches) or metric (m/min / mm) units.
Unit system
SFM
in
in/tooth
Revolutions per minute for the target cutting speed at this diameter
- 1
Numerator (SFM × 12)
200 × 12 = 2,400 - 2
Denominator (π × D)
π × 0.5 = 1.570796 - 3
Spindle speed
2,400 ÷ 1.570796 = 1,528
How does this calculator work?
Spindle RPM = (SFM × 12) / (π × D) for imperial, or (1000 × Vc) / (π × D) for metric; Feed Rate = RPM × chip load × flutes. Enter the cutting speed from your material chart, cutter diameter, chip load per tooth, and flute count to get correct RPM and feed rate for your CNC milling operation.
Formula
How this is calculated
Machining speeds and feeds link three key variables: cutting speed (how fast the tool edge moves through the material surface), spindle RPM (how fast the cutter rotates), and feed rate (how fast the tool advances into the work). The cutting speed is set by the workpiece material and tool material — carbide cuts aluminium at 300–600 SFM; HSS on mild steel runs 70–100 SFM. Given the recommended cutting speed and the cutter diameter, the spindle RPM is back-calculated from the circumferential-speed relationship: RPM = (Vc × k) / (π × D), where k is 12 for imperial units (converting feet to inches) and 1000 for metric (converting metres to millimetres).
Feed rate is then the product of RPM, chip load per tooth (the material thickness each cutting edge removes per revolution), and the number of flutes. Chip loads are found in tooling manufacturer tables and vary by cutter diameter, workpiece material, and depth of cut. Setting chip load too low causes rubbing and premature wear; too high risks tool breakage. The curve on the right shows how RPM changes as diameter varies at your chosen cutting speed — useful when swapping tool sizes.
These formulas assume sharp tooling, sufficient chip evacuation, and steady-state cutting. They do not account for dynamic effects such as vibration, runout, or thermal softening. Always start conservatively and adjust based on actual chip colour, surface finish, and tool life.
Frequently asked questions
Tooling manufacturers (Kennametal, Sandvik, OSG, Helical) publish material-specific speed charts for their end mills, drills, and inserts. Typical starting points: HSS on mild steel 70–100 SFM; carbide on aluminium 300–600 SFM; carbide on stainless 100–200 SFM. Always start at the low end and increase if finish and tool life allow.
Cutting speed is a peripheral (surface) speed. A larger cutter travels a longer path per revolution, so fewer revolutions are needed to keep the cutting edge at the same SFM — hence lower RPM for bigger tools at the same cutting speed.
Chip load (feed per tooth) is the material thickness each cutting edge removes per revolution. Too low causes rubbing and heat buildup; too high risks tool breakage. Manufacturers specify safe ranges per tool diameter and material, and these vary by axial and radial depth of cut.
TG we-Calculate Editorial Team. (2026). Speeds and Feeds Calculator — CNC Machining RPM & Feed Rate [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/speeds-and-feeds-calculator
TG we-Calculate Editorial Team. "Speeds and Feeds Calculator — CNC Machining RPM & Feed Rate." TG we-Calculate. 2026. https://we-calculate.com/calculator/speeds-and-feeds-calculator.
TG we-Calculate Editorial Team, "Speeds and Feeds Calculator — CNC Machining RPM & Feed Rate," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/speeds-and-feeds-calculator
@misc{wecalculate_speeds_and_feeds_calculator, title = {Speeds and Feeds Calculator — CNC Machining RPM & Feed Rate}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/speeds-and-feeds-calculator}}, year = {2026}, note = {TG we-Calculate} }
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