How to Calculate Feed Rate for G01 (Beginner Guide)

Q: What units is the F value in?

The F value uses the active unit and feed mode. Under G21 it is millimeters per minute and under G20 it is inches per minute, as long as G94 (feed per minute) is active. On a lathe with G95, F is feed per revolution instead.

Q: Does feed rate carry over to the next line?

Yes. F is modal, so once set on a G01 line it stays active for following feed moves, including G02 and G03 arcs, until you change it. You do not need to repeat F on every line.

Q: What is the best way to learn G-code feed and speed codes?

Drill them with active recall instead of re-reading a chart. A free app like G-Code Sprint quizzes the F, S, and motion codes as quick timed questions and repeats whichever ones you miss, so the relationships between feed, speed, and the G-codes stick.

A G01 line moves in a straight line at a feed rate you set with the F word, the way G00 vs G01 explains. The question beginners get stuck on is where the F number comes from. It is not a guess: it is a short calculation from spindle speed, the cutter’s chip load, and how many flutes it has.

The feed rate formula

For milling, the feed rate is built from three numbers:

feed rate = RPM  x  feed per tooth  x  number of flutes

That is the standard speeds and feeds relationship. Each piece has a clear meaning:

Term	What it is	Where it comes from
RPM	Spindle speed	Calculated from cutting speed and tool diameter
Feed per tooth	Chip load each edge takes	Tooling chart for the material
Number of flutes	Cutting edges on the tool	The tool itself (2, 3, 4, …)
Feed rate (`F`)	Result, in/min or mm/min	The number you put on the `G01` line

First find the RPM

The spindle speed comes from the material’s recommended cutting speed and the tool diameter:

RPM = (1000 x cutting speed) / (pi x diameter)     [metric]

Here cutting speed is in meters per minute and diameter is in millimeters. The feed and speed calculation references give the same formula in imperial form using surface feet per minute.

A worked example

Say you are milling aluminum with a 10 mm four-flute end mill, a cutting speed of 200 m/min, and a feed per tooth of 0.05 mm:

Step	Calculation	Result
RPM	(1000 x 200) / (3.14 x 10)	about 6366 RPM
Feed rate	6366 x 0.05 x 4	about 1273 mm/min

So the spindle line is S6366 M03 and the cut is G01 X50. F1273. The F is not magic; it falls straight out of the numbers. The S value pairs with the spindle codes in M03, M04, and M05.

Units and feed mode

The F value means different things depending on two modal settings. The unit follows G20 vs G21: inches per minute or millimeters per minute. The feed mode is usually G94 (feed per minute) on a mill, but a lathe often uses G95 (feed per revolution), which changes F entirely. The F word itself is modal, so it carries to the next feed line until you change it, as noted in the LinuxCNC F-word reference.

Typical feed per tooth values

The chip load you plug in depends mostly on the material and the tool. These are rough starting points for a small end mill; always check the tool maker’s chart for the exact tool and depth of cut:

Material	Feed per tooth (mm)
Aluminum	0.05 to 0.20
Mild steel	0.03 to 0.10
Stainless steel	0.02 to 0.08
Hard plastics	0.05 to 0.25

Notice the harder the material, the lower the feed per tooth, which is why the same RPM gives a slower F in steel than in aluminum. The same arithmetic scales to extremes: the eye-watering F values on high-speed machines like Datron mills are ordinary chip-load math at very high RPM. Depth of cut and stepover also matter: a heavier cut usually means backing the feed down from these numbers.

Why the number matters

A feed rate that is too high overloads the cutter and a feed that is too low rubs instead of cuts, both of which shorten tool life or break the tool. That is the link between this calculation and why end mills break on G01. Getting the F right the first time is one of the things a practice routine on the G-code practice hub helps you internalize.

Bottom line

The F on a G01 line is feed rate, calculated as RPM times feed per tooth times the number of flutes. Find the RPM from the cutting speed and tool diameter first, then multiply through. Watch the unit (G20/G21) and feed mode (G94/G95), because they change what the F number means.

Sources

Frequently asked questions

How do you calculate feed rate for G01?

For milling, feed rate equals spindle RPM times feed per tooth times the number of flutes. Find the RPM from the material’s cutting speed and the tool diameter, then multiply by the chip load and the flute count to get the F value.

What units is the F value in?

It uses the active unit and feed mode: millimeters per minute under G21 or inches per minute under G20, as long as G94 is active. A lathe with G95 uses feed per revolution.

Does feed rate carry over to the next line?

Yes. F is modal, so once set it stays active for following feed moves, including G02 and G03 arcs, until you change it.

What is the best way to learn G-code feed and speed codes?

Drill them with active recall. A free app like G-Code Sprint quizzes the F, S, and motion codes as quick timed questions and repeats whichever ones you miss.

G-Code Sprint is a study and practice tool only. Always follow your instructor, employer, machine manual, and shop safety procedures.