How many boards do you need?

How do I figure out how many boards I need?

List every piece you need to cut, then fit them onto boards of the length you will buy, longest pieces first, leaving room for each saw cut. Two 50 inch pieces and four 30 inch pieces from 8 ft (96 inch) boards take 3 boards. Adding up the lengths — 220 inches ÷ 96 = 2.3 boards — would say 3 as well here, but it often comes out short because pieces cannot be split across boards.

What is kerf and why does it matter?

Kerf is the width of the saw cut, the wood that turns into sawdust. A common circular saw blade cuts about ⅛ inch. Two 48 inch pieces do not quite fit in one 96 inch board once you account for the cut between them — the calculator counts it for you.

What lengths does lumber come in?

Dimensional lumber such as 2×4s and 2×6s is usually sold in 8, 10, 12, 14 and 16 ft lengths, and longer pieces in even feet. Try different stock lengths here: a longer board sometimes wastes less and costs less per foot.

How much extra lumber should I buy?

The layout already includes the offcuts. Add a board or two for knots, splits, warped boards and mistakes — more if you are buying from a bin you cannot pick through.

How is this different from board feet?

Board feet measure the volume of wood, which is how hardwood is priced. This calculator is for cutting: it tells you how many boards of a given length to buy for your pieces. The board foot calculator turns a list of boards into board feet and cost.

Is the layout the cheapest possible?

Not always. The tool places the longest pieces first, each on the first board with room. It is quick and reliable, but a hand-made layout can sometimes save a board. Trying a different board length is a good second check.

Why does it say a piece is too long?

A piece longer than the board you chose cannot be cut from it. Pick a longer stock length or join two shorter boards.

How the cut layout works

Pieces are sorted from longest to shortest. Each piece goes on the first board that still has room for it, and every cut uses up one kerf of the board's length. When no board has room, a new one is opened. The waste figure is the share of the bought length that does not end up in a piece, including the kerf and the offcuts. Enter the finished lengths you need; the stock length is the length sold, and the cross-section of the board does not enter into this calculation.

Worked example: 13 pieces

The list is 4 pieces of 42 inches, 6 of 30 inches and 3 of 18 inches, with a ⅛ inch kerf. The pieces add up to 402 inches, which is 4.19 boards of 8 ft, so a simple division says 5. The layout needs 5 boards of 8 ft: 42 + 42; 42 + 42; 30 + 30 + 30; 30 + 30 + 30; 18 + 18 + 18. Waste is 16.2%. With 10 ft boards it takes 4 boards and the waste is 16.2%. Comparing both counts with the price per board tells you which length is cheaper.

Mistakes to avoid

Dividing the total length by the board length undercounts, because a piece cannot be split between boards. Forgetting the kerf is the second one: with 20 cuts of ⅛ inch you lose 2½ inches, and a piece that was just fitting no longer does. Third, lumber is sold in nominal sizes, so a 2×4 is not 2 by 4 inches, and boards have defects; the calculator does not allow for knots, splits or ends that must be trimmed off, so keep a spare.

Sources: the layout is a first-fit-decreasing cutting method; ⅛ inch is the width of a common circular saw blade; stock lengths follow common U.S. retail sizes. Related tools: board foot calculator, linear feet calculator, deck calculator, square footage calculator.