CNC Router Feed Rate Calculator (Chip Load, RPM and Feed)

On a CNC router the spindle speed and the feed rate have to agree. Each flute of the bit should take a chip of a certain thickness, the chip load: too thin and the bit rubs, heats up and dulls; too thick and it chatters or snaps. Bit makers publish chip loads by bit diameter and material. This calculator turns that chip load into a feed rate for your spindle speed and number of flutes, or works the other way and tells you what chip load your current settings give, so you can check them against the chart.

CNC router feed rate calculator

How it works

The relationship is the standard milling one:

feed rate = RPM × number of flutes × chip load
chip load = feed rate ÷ (RPM × number of flutes)
cutting edge speed = π × bit diameter × RPM ÷ 12 (surface feet per minute), or π × D × RPM ÷ 1,000 in m/min

Chip load is in inches (or millimetres) per tooth. The formula is linear, so if a machine cannot reach the feed rate it gives, lower the RPM in the same proportion and the chip load stays the same. The cutting edge speed row is shown because heat in wood and plastic depends on it: a large bit at full spindle speed moves its edge very fast.

Worked example

A 1/4 in two-flute bit at 18,000 RPM, where the bit maker’s chart lists a chip load of 0.004 in for the material:

Units: Inches
Work out: Feed rate from chip load
Spindle speed: 18000 RPM
Number of flutes: 2
Chip load (feed per tooth): 0.004 in
Bit diameter: 0.25 in

The calculator gives:

Feed rate: 144 in/min
Chip load: 0.004 in per tooth
Feed rate in millimetres: 3,658 mm/min
Feed per revolution: 0.008 in
Cutting edge speed: 1,178 surface ft/min

The feed works out at 144 in/min. If the machine is only happy at 100 in/min, the chip load at 18,000 RPM drops to 0.0028 in; to keep 0.004 in at that feed, slow the spindle to 12,500 RPM (100 ÷ (2 × 0.004)). The cutting edge moves at about 1,180 ft/min.

Feed rate chart for a two-flute bit (in/min)

Chip load (in)	10,000 RPM	12,000 RPM	14,000 RPM	16,000 RPM	18,000 RPM	20,000 RPM	24,000 RPM
0.001	20	24	28	32	36	40	48
0.002	40	48	56	64	72	80	96
0.003	60	72	84	96	108	120	144
0.004	80	96	112	128	144	160	192
0.005	100	120	140	160	180	200	240
0.006	120	144	168	192	216	240	288
0.008	160	192	224	256	288	320	384

Feed = RPM × 2 flutes × chip load. For a single-flute bit halve the figures; for three flutes multiply by 1.5. Multiply in/min by 25.4 for mm/min.

Notes and limits

Start from the chip load chart of the bit you are using; it depends on bit diameter, flute type and material. Smaller bits take smaller chips.
Depth of cut changes things the formula does not see. For deep slots or a long stick-out, cut shallower passes rather than cutting the feed so low that the bit rubs.
Burn marks and fine dust instead of chips often mean the chip load is too small: feed faster or slow the spindle.
Spindle speed for drilling, rather than routing, is in the drill speed calculator; lathe speeds in the wood lathe speed calculator.
Wear eye and hearing protection and keep hands clear; a broken router bit leaves at high speed.

Frequently asked questions

How do you calculate feed rate for a CNC router?

Multiply the spindle RPM by the number of flutes and by the chip load per tooth. 18,000 RPM × 2 flutes × 0.004 in = 144 in/min.

What is chip load?

The thickness of material each cutting edge removes per revolution, in inches or millimetres per tooth. It is the number the bit maker publishes, and the one that controls heat and tool life.

How do I calculate chip load from my feed and speed?

Divide the feed rate by RPM × number of flutes. 100 in/min at 18,000 RPM with 2 flutes is 0.0028 in per tooth.

Should I lower the RPM or raise the feed?

Either changes the chip load. If the machine already runs at its maximum comfortable feed, lowering the RPM is the way to get a thicker chip and less heat.

Sources

Wikipedia, "Speeds and feeds": spindle speed = cutting speed x 12 / (pi x diameter) and feed rate = RPM x number of teeth x chip load

Last reviewed: 25 October 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.