Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Spindle speed
2,674
Feed rate
42.8
Calculated RPM (uncapped)
2,674
Limited by machine max RPM (1=yes)
0
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter cutting speed (sfm) (ft/min).
- 2Enter tool diameter (in).
- 3Enter number of flutes.
- 4Enter chip load per tooth (in).
- 5Enter machine max spindle rpm.
- 6Read your spindle speed on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
Feeds and speeds are the two settings that determine tool life, surface finish, and cycle time on every milling or drilling operation. Cutting speed (SFM or surface meters per minute) comes from the material/tool combination — a carbide end mill in aluminum runs far faster than the same tool in stainless. This calculator converts that surface speed into spindle RPM based on tool diameter, then computes the table feed rate from chip load per tooth and flute count. Running too slow wastes cycle time and can cause rubbing instead of cutting, which actually shortens tool life; running too fast burns tools and can shatter carbide inserts. Chip load numbers here are a good general starting point but always check your tool manufacturer's chart for the exact insert grade and coating, since modern coated carbide can often run faster than older uncoated data suggests.
Worked example
Using the values the calculator loads with:
Inputs
- Cutting speed (SFM): 350 ft/min
- Tool diameter: 0.5 in
- Number of flutes: 4
- Chip load per tooth: 0.004 in
- Machine max spindle RPM: 12000
Results
- Spindle speed: 2,674
- Feed rate: 42.8
- Calculated RPM (uncapped): 2,674
- Limited by machine max RPM (1=yes): 0
What each field means
Inputs
- Cutting speed (SFM) (ft/min)
- The cutting speed (sfm) used in the calculation, measured in ft/min. Starts at 350 ft/min so you have a working example on load.
- Tool diameter (in)
- The tool diameter used in the calculation, measured in in. Starts at 0.5 in so you have a working example on load.
- Number of flutes
- The number of flutes used in the calculation. Starts at 4 so you have a working example on load. Accepted range: 1–12.
- Chip load per tooth (in)
- The chip load per tooth used in the calculation, measured in in. Starts at 0.004 in so you have a working example on load.
- Machine max spindle RPM
- The machine max spindle rpm used in the calculation. Starts at 12000 so you have a working example on load.
Results
- Spindle speed
- Returned as a whole number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Feed rate
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Calculated RPM (uncapped)
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Limited by machine max RPM (1=yes)
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Where do I find SFM and chip load values for my material?
Tool manufacturers (Harvey, Helical, Kennametal, Sandvik) publish free feeds-and-speeds charts by material and tool coating. As rough starting points: aluminum runs 300-1000 SFM, mild steel 100-350 SFM, stainless 100-250 SFM, and titanium 40-120 SFM with carbide tooling.
What happens if my calculated RPM exceeds machine max?
The calculator caps it at your machine's max spindle speed and flags it. You're now running below optimal SFM, which is fine for finish passes but means you'll get shorter tool life for roughing at that reduced surface speed relative to a machine with a higher-RPM spindle.
Should I trust the calculated feed rate exactly?
Use it as a starting point, then adjust by ear and by chip color. Bring feed down 20-30% for interrupted cuts or thin-wall parts prone to chatter, and increase it if chips come off blue (overheating) rather than golden/silver for steel.
Does chip load change with depth of cut?
Not directly in this formula, but heavier radial or axial depth of cut generates more heat and tool deflection, so shops commonly reduce chip load 10-20% when running full-diameter slotting versus light finishing passes.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). CNC Feeds and Speeds Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/cnc-feeds-and-speeds
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/cnc-feeds-and-speeds" target="_blank" rel="noopener">CNC Feeds and Speeds Calculator — RevenueLab</a> (2026).</p>
Source: [CNC Feeds and Speeds Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/cnc-feeds-and-speeds) (2026).