What is a kerf in woodworking?

A kerf is the narrow slot a saw blade cuts through material as it passes. The blade removes a thin sliver of wood (or metal, plastic, etc.) equal to the blade's tooth-set width. Because that material is turned to dust and shavings, it is lost forever — which is why you must account for it when planning how many pieces fit on a board.

How wide is a typical saw kerf?

Common kerf widths: standard circular saw blade 3.0–3.5 mm (about 1/8 in); table saw 3.2 mm; thin-kerf table saw blade 2.4 mm; fine handsaw 1.2–1.8 mm; bandsaw 0.9–1.2 mm; track saw 2.6–3.0 mm; laser cutter 0.1–0.3 mm. The exact value is printed on the blade packaging or in the manufacturer spec sheet.

Why does kerf loss matter for a cut list?

On a short project the loss is trivial, but it compounds quickly. Cutting twenty 400 mm pieces with a 3.2 mm kerf removes 60.8 mm from a single 2400 mm board — that is more than 2.5 % of the board gone before a single piece is measured. If you ignore kerf when ordering timber, you frequently run short on the last board.

What is the formula for pieces per board?

The standard formula is: n = floor((L + k) / (p + k)), where L is the board length, p is the piece length and k is the kerf width. The (L + k) trick works because the last piece does not need a trailing kerf, so you can add one virtual kerf to the board length to make the arithmetic symmetric.

What is an off-cut or remainder?

After all full pieces have been cut from a board, whatever material is left is the off-cut. The calculator shows this remainder in your chosen unit. A large off-cut can often be used for smaller pieces in the same project or saved for future work.

How do I reduce kerf waste?

Use the thinnest blade that suits the material and cut quality you need (bandsaw or thin-kerf blade instead of a standard circular saw). Nest identical pieces back-to-back so one board yields more pieces. Optimise piece length so the off-cut from one board is long enough to yield pieces for a different part of the project.

Kerf Calculator — Gera Tools

A kerf calculator tells you exactly how many finished pieces you can cut from a single length of board, how much material the blade will consume as dust, what the off-cut remainder will be, and how many boards you need to buy for your whole project. Whether you are building furniture, laying decking, cutting aluminium extrusions or preparing a cabinet cut list, running these numbers before you start prevents the frustration of running short mid-project.

What is kerf and why does it matter?

Every saw blade has a physical thickness. As it passes through timber, metal or sheet material it grinds away a thin strip equal to the kerf width — typically 1–4 mm for woodworking saws. That material becomes sawdust and is gone. On any single cut the loss seems negligible, but on a project with many identical pieces it adds up fast.

Consider twenty 400 mm cabinet shelves from a 2,400 mm board with a 3.2 mm blade. The 19 internal cuts consume 60.8 mm — more than a full extra shelf’s worth of waste. Miss that in your shopping list and you will be one board short on delivery day.

How the calculator works

The tool applies the standard kerf-accounting formula to find the maximum number of pieces n that fit on one board of length L, given piece length p and kerf width k:

n = floor( (L + k) / (p + k) )

The logic behind the formula: each piece “slot” on the board is (p + k) wide because the blade must pass after every cut — except the very last piece, which needs no trailing kerf. Adding one virtual kerf to the board length makes all slots symmetric, so a single floor-division gives the answer in one step.

The total kerf loss per board is then (n − 1) × k (one fewer cut than pieces), and the off-cut remainder is L − n × p − (n − 1) × k.

To find the number of boards needed for a whole project, the calculator simply divides the total quantity by pieces-per-board and rounds up.

Worked example

You are building a bookcase and need 14 shelves each 380 mm deep. Your timber comes in 2,400 mm lengths. Your circular saw blade has a kerf of 3.2 mm.

Pieces per board: floor( (2400 + 3.2) / (380 + 3.2) ) = floor( 2403.2 / 383.2 ) = floor( 6.27 ) = 6 shelves
Total kerf loss per board: (6 − 1) × 3.2 = 16 mm
Off-cut: 2400 − 6 × 380 − 5 × 3.2 = 2400 − 2280 − 16 = 104 mm
Boards needed for 14 shelves: ceil( 14 / 6 ) = 3 boards
Total kerf loss across 3 boards: 3 × 16 = 48 mm

Without accounting for kerf you might have guessed floor(2400 / 380) = 6 per board anyway, but the benefit of the tool compounds when piece or kerf dimensions are less tidy, or when you are nesting multiple different lengths from the same stock.

Board (mm)	Piece (mm)	Kerf (mm)	Pieces	Off-cut (mm)
2400	400	3.2	5	387.2
2400	380	3.2	6	104
2400	300	3.2	7	280.8
3000	500	2.4	5	490.4
96 in	18 in	0.125 in	5	5.5 in

Formula note

The formula n = floor((L + k) / (p + k)) is equivalent to asking: if the board were extended by exactly one kerf, how many (piece + kerf) slots would fit? That extension is then removed mathematically by the floor, yielding the correct count without a loop. For non-uniform cut lists (different piece lengths), the greedy first-fit algorithm is more appropriate, but for identical pieces this closed-form solution is exact and fast.