liminfoCNC Feeds & Speeds Calculator
Free web tool: CNC Feeds & Speeds Calculator
RPM = SFM × 12 / (π × D), Feed = RPM × Flutes × Chip Load
Spindle Speed
6,112 RPM
Feed Rate
91.7 in/min
SFM
800
Chip Load
0.005 in/tooth
About CNC Feeds & Speeds Calculator
The CNC Feeds & Speeds Calculator computes the two most critical parameters for any CNC milling operation: spindle speed in RPM and feed rate in inches per minute. Enter the workpiece material, end mill diameter in inches, number of flutes, and optionally override the surface footage (SFM) and chip load to get recommended cutting parameters tailored to your tooling and material combination.
The calculator uses the industry-standard formulas: RPM = SFM × 12 / (π × D), where D is the tool diameter in inches, and Feed Rate = RPM × Flutes × Chip Load. The tool ships with pre-configured SFM and chip load values for 12 common materials: Aluminum 6061 (800 SFM), Aluminum 7075 (600 SFM), Mild Steel (100 SFM), Stainless 304 (65 SFM), Stainless 316 (55 SFM), Tool Steel (50 SFM), Titanium 6Al-4V (50 SFM), Brass (400 SFM), Copper (300 SFM), Cast Iron (80 SFM), Acetal/Delrin (500 SFM), and Nylon (400 SFM). Custom SFM and chip load fields let you override these defaults for specific tooling brands or coatings.
In addition to RPM and feed rate, the calculator also displays the material removal rate (MRR) — a measure of how efficiently the cutter is removing material. All calculations run entirely in your browser. The tool is designed for machinists, CNC programmers, students, and hobbyists who need fast, accurate starting parameters for new jobs.
Key Features
- Computes spindle speed: RPM = SFM × 12 / (π × D) for any end mill diameter
- Computes feed rate: Feed Rate = RPM × Number of Flutes × Chip Load (in/min)
- Pre-loaded SFM and chip load data for 12 materials: aluminum alloys, steels, stainless, titanium, brass, copper, cast iron, and engineering plastics
- Optional custom SFM and chip load fields to override material defaults for coated tools or specialty tooling
- Displays material removal rate (MRR) as an additional productivity metric
- Supports end mills with 1 to 12 flutes
- 100% client-side — no data is ever sent to a server
- No sign-up, no download, and completely free to use
Frequently Asked Questions
What is SFM (Surface Feet per Minute)?
SFM is the cutting speed at the tool's outer edge — the speed at which the work material moves past the cutting edge in feet per minute. Higher SFM values are used for soft materials like aluminum; lower SFM values protect cutting edges when machining hard materials like titanium or tool steel. Cutting tool manufacturers specify recommended SFM ranges for each material and tool coating.
What is chip load?
Chip load (also called feed per tooth) is the thickness of the material removed by each cutting edge per revolution, measured in inches per tooth. It determines how aggressively each flute cuts. Too low a chip load causes rubbing (heat and tool wear); too high causes chatter or tool breakage. Typical values range from 0.002 in/tooth for titanium to 0.008 in/tooth for Delrin.
How do I use the RPM formula?
The formula RPM = SFM × 12 / (π × D) converts surface speed to rotational speed. For example, machining Aluminum 6061 (SFM = 800) with a 0.5-inch end mill: RPM = 800 × 12 / (3.1416 × 0.5) ≈ 6,111 RPM. Larger tools require lower RPM for the same surface speed; smaller tools require higher RPM.
What is the feed rate formula?
Feed Rate (in/min) = RPM × Number of Flutes × Chip Load. With 3 flutes and a chip load of 0.005 in/tooth at 6,111 RPM: Feed Rate = 6,111 × 3 × 0.005 ≈ 91.7 in/min. More flutes allow a higher feed rate for the same chip load, which is why 4-flute end mills are common in production environments.
Can I use custom SFM and chip load values?
Yes. The calculator provides optional Custom SFM and Custom Chip Load fields. If you leave these blank, the material presets are used. If you enter a value, it overrides the preset. This is useful when using coated end mills (TiN, TiAlN, etc.) that allow 20–50% higher SFM than uncoated tools, or when following a specific tooling manufacturer's recommendations.
Why are stainless steel and titanium SFM values so much lower than aluminum?
Hard and work-hardening materials like stainless steel 304 (65 SFM) and titanium 6Al-4V (50 SFM) generate much more heat during cutting and are much harder on cutting edges. Running these materials at high SFM would cause rapid tool wear or catastrophic tool failure. Aluminum 6061 (800 SFM) is soft, highly thermally conductive, and easy on tools, enabling much faster cutting speeds.
What is the material removal rate (MRR)?
Material removal rate (MRR) is a measure of machining productivity, expressed in cubic inches per minute. It is calculated as: MRR = Tool Diameter × Chip Load × Number of Flutes × Feed Rate. Higher MRR means you are removing more material per unit of time, which translates to shorter cycle times. MRR must be balanced against tool life and surface finish requirements.
Are these values for roughing or finishing passes?
The preset values represent moderate starting points suitable for roughing passes with full-depth slotting or profiling. For finishing passes, you typically increase SFM by 10–20% and reduce chip load by 30–50% to improve surface finish. For high-efficiency machining (HEM/trochoidal milling) with small radial engagement, you can use significantly higher chip loads than these defaults.