Running your CNC router at the right feed rate does more than save time. It protects the spindle, extends the life of every bit you own, and is usually the difference between a clean edge and a scrapped sheet. Getting there starts with one calculation.
When we say feed rate, we mean the rate the cutter travels through the material, in inches per minute (IPM). It is how fast the tool is making chips. When we say speed, we mean how fast the spindle turns, in revolutions per minute (RPM). They are two different numbers and they have to agree with each other.
Want the number without the maths? The CNC Feed Rate Calculator holds published chip loads for 1,118 bits. Pick your bit and material, set your spindle RPM, and it returns feed rate and plunge rate in IPM or mm/min. Updated August 2026.
The three values you need
To calculate a feed rate you need to know:
- RPM — how fast your spindle is turning.
- Number of cutting edges — the flute count of the bit.
- Chip load — how thick a chip each cutting edge should take, in inches. This is a property of the specific bit and the material you are cutting.
Chip load is the one people guess at, and it is the one that breaks tools. It is published per tool series, and it changes with both the cutting diameter and the material. There is a chart further down, and every bit on this site lists its own figures on the product page.
The formula
Feed Rate (IPM) = RPM × number of cutting edges × chip load
A worked example
Take the LMT Onsrud 63-600 series, a single-flute upcut spiral built for high-speed cutting of aluminum. In a 1/8" cutting diameter, LMT Onsrud publishes a chip load range of 0.002" to 0.004".
Most CNC router spindles will run to 24,000 RPM, and some reach 30,000 or even 50,000. In practice most operators work in the 18,000–20,000 RPM band. Running at 20,000 RPM with a single cutting edge:
| Chip load | Calculation | Feed rate |
|---|---|---|
| 0.002" | 20,000 × 1 × 0.002 | 40 IPM |
| 0.003" | 20,000 × 1 × 0.003 | 60 IPM |
| 0.004" | 20,000 × 1 × 0.004 | 80 IPM |
So cutting aluminum with a 1/8" single-flute tool at 20,000 RPM, your feed rate should sit between 40 and 80 IPM. Start at 60 IPM and adjust from there.
Skip the arithmetic — use the free Feed Rate Calculator. It is an RPM and feed rate calculator: set your spindle RPM, pick your bit and material, and it returns IPM and plunge rate from published tooling data.
Chip load chart by material
The figures below are LMT Onsrud's published chip load ranges (in inches) for the 65-000 series single-flute upcut spiral, taken from the LMT Onsrud 2023 catalogue. They are a solid starting point for any single-flute O-flute geometry.
| Material | 1/16" | 1/8" | 3/16" | 1/4" | 3/8" |
|---|---|---|---|---|---|
| Softwood | .001–.003 | .002–.004 | .003–.006 | .004–.006 | .005–.007 |
| Hardwood | .001–.003 | .002–.004 | .003–.005 | .004–.006 | .005–.007 |
| MDF | .001–.003 | .002–.004 | .003–.005 | .004–.006 | .005–.007 |
| Plywood | .001–.003 | .002–.004 | .003–.005 | .004–.006 | .005–.007 |
| Soft plastic | .002–.004 | .004–.006 | .006–.008 | .008–.012 | .008–.012 |
| Acrylic | .002–.004 | .006–.008 | .008–.010 | .010–.012 | .010–.012 |
| Aluminum | .002–.004 | .002–.004 | .003–.006 | .003–.006 | .004–.008 |
Chip load varies by tool geometry, not just by diameter and material. Compression, downcut and multi-flute tools carry different figures — check the feeds and speeds listed on the individual product page before cutting.
What happens when the number is wrong
Every variable in the formula pushes the feed rate up or down, and each failure mode looks different at the machine.
Too fast and you chip or blow out the material, or you snap the bit outright. Too slow and the cutter rubs instead of cutting. The chips come off too thin to carry heat away, so the heat goes into the tool and the workpiece instead — that is what dulls an edge prematurely and what melts acrylic back together behind the cutter.
Raising spindle speed is one way to raise feed rate, but past a point the extra RPM just generates heat that softens and dulls the edge. This is why experienced operators run below the spindle's maximum rather than at it. The other lever is the tool: a bit with a higher chip load rating, or a second flute, will let you move faster. But flute count and geometry also determine your edge finish, so that choice is not free.
Tools that do this for you
- Feed Rate Calculator — chip load, RPM and IPM for a specific bit and material.
- CNC Bit Finder — tell it your material and machine, get a bit recommendation.
- Cut Doctor — describe or photograph a bad cut and get a diagnosis with the fix.
- Free CNC Tool Library — feeds and speeds ready to import into Vectric and Fusion. No login required.
