Planer cuts per inch calculator
How many knife cuts a planer or jointer makes in each inch of board, and how far apart the knife marks are. Pick a planer to use the speed, knives and feed from its manual, or enter your own.
On a jointer you feed by hand: enter your best estimate of the feed in ft/min.
Cuts per inch
178.6
Knife marks 0.0056 in apart (0.142 mm)
Arithmetic: exact by definition
Why: 10,000 RPM × 3 knives is 30,000 cuts a minute, spread over 168 in of feed: 178.6 cuts per inch.
Why these numbers?
The situation
10,000 RPM, 3 knives, fed at 14 ft/min.
From your input: speed, knives and feed
What that means
Each knife cuts once per turn: 10,000 × 3 = 30,000 cuts a minute.
Arithmetic: exact by definition
The number
30,000 ÷ (14 × 12) = 178.6 cuts per inch, knife marks 0.0056 in apart.
Arithmetic: exact by definition
The formula
Each knife takes one cut every time the cutterhead turns, so a cutterhead makes RPM × knives cuts a minute. The board moves the feed rate in feet per minute, which is 12 times as many inches. Divide one by the other: cuts per inch = RPM × knives ÷ (feed × 12). More cuts per inch means smaller scallops between knife marks, which is why a slower feed gives a smoother surface. It applies to straight knives; a helical head with many small inserts does not cut one full-width scallop per insert, so this formula does not describe it.
Makers' stated figures, checked
Planer makers often print cuts per inch beside the speed, knives and feed. The figure should follow from the other three. Where it doesn't by more than 5%, this table says so, and the calculator uses the specs rather than the stated figure.
| Planer | Feed | From its specs | Maker states | Check |
|---|---|---|---|---|
| DeWALT DW735 (13 in) | 14 ft/min | 178.6 | 179 | Agrees |
| DeWALT DW735 (13 in) | 26 ft/min | 96.2 | 96 | Agrees |
| Grizzly G0453 (15 in) | 16 ft/min | 75 | 63 | Disagrees |
| Grizzly G0453 (15 in) | 30 ft/min | 40 | 42 | Agrees |
| Grizzly G0454 (20 in) | 16 ft/min | 100 | 104 | Agrees |
| Grizzly G0454 (20 in) | 20 ft/min | 80 | 83 | Agrees |
Snipe, cutterheads and the planer's safety rules are in the thickness planer guide.