CNC Feeds & Speeds Calculator
Enter your cutter size, flute count, and the material you are cutting to get safe starting RPM, feed rate, and depth of cut - with a quick deflection risk check. Everything runs in your browser, nothing uploaded.
Cutter Diameter
Flute Count
Material
Cutting Type
Machine Max RPM
Recommended Settings
Full cutting details
RPM Usage
Learn more: chip load, RPM caps, and why slow feeds break bits
Every other number here comes from chip load
Chip load is how much material one cutting edge removes per revolution. Feed rate is RPM x flute count x chip load, so the feed you get back is just the speed that holds chip load where the material wants it.
Each material carries a published chip load range, and the calculator takes the midpoint. Hardwood and plywood sit at 0.004 to 0.008 inch per tooth, which is 0.10 to 0.20 mm. Aluminium 6061 is far finer at 0.001 to 0.003 inch, or 0.025 to 0.076 mm.
Cutting type then scales that midpoint: slotting 0.8, pocketing 1.0, profiling 1.2. A full-width slot buries the cutter on both sides, so it gets the reduced number; an edge pass only loads one side, so it can take more per tooth.
Going too slow is its own way to break a bit
Backing the feed off feels like the cautious move, but the cutting edge needs a chip of a certain thickness to bite. Below that it skids along the surface instead of slicing, and the friction goes into the tool as heat. Plenty of bits die that way, at feeds their owner picked because they felt safe.
A 3 mm two-flute cutter slotting MDF on a 24,000 RPM router gets 7,315 mm/min, which works out to 0.1524 mm per tooth.
Now turn the feed override down to 50 percent without touching the spindle. Feed drops to about 3,658 mm/min and chip load halves to 0.0762 mm, well under what the material was rated for. If you need to slow a cut down, drop RPM by the same proportion so chip load stays put.
What the RPM cap and the deflection flag actually mean
Ideal RPM comes from the material's surface speed range: RPM equals SFM x 12 divided by pi x diameter in inches. Small cutters want very high RPM, and hobby spindles often cannot get there. That same 3 mm cutter in MDF asks for 32,340 RPM, so a 24,000 RPM machine caps it and the feed rate falls with it, chip load unchanged.
Deflection risk works differently from what the name suggests. Depth of cut is a fixed fraction of cutter diameter set by the material, so the depth-to-diameter ratio never changes when you resize the bit.
Aluminium and carbon fibre take a shallow 0.25 and 0.2 of diameter, and read Low with three flutes or fewer. Hardwood, plywood, MDF and HDPE at 0.5, plus acrylic at 0.4, read Medium. Softwood at 0.6 and foam at a full diameter read High, which is your cue to split that depth across two or more passes rather than taking it in one.
FAQ
Why does the calculator cap RPM at my machine's maximum?
On small cutters the surface-speed formula often asks for more RPM than a hobby router or spindle can deliver. A 3 mm cutter in MDF wants 32,340 RPM, so a 24,000 RPM machine gets capped there and the feed rate is recalculated from the RPM you can actually reach, which keeps chip load on target.
Why does deflection risk change with material but not with bit diameter?
Depth of cut is calculated as a fixed fraction of cutter diameter, and that fraction is set by the material. The ratio of depth to diameter is therefore the same whether you run a 3 mm or a 12 mm cutter, so only the material moves the flag, plus flute count at the shallow end.
Does slotting really need a slower feed than profiling?
Yes, and the gap is large. A 6 mm two-flute cutter in hardwood runs at 12,128 RPM either way, but slotting gets 2,957 mm/min at 0.1219 mm per tooth while profiling gets 4,436 mm/min at 0.1829 mm per tooth.