Lumber Calculator

How to calculate how much lumber you need

Lumber is bought by the board, in set lengths, and a piece can only be cut from a board of its own size. So the question is how many boards of each size and length you need to cut all your pieces. Work it out in four steps:

1. List the pieces. Write each size, length and quantity, like six 2x4s that are 3 ft 6 in long.
2. Choose the board lengths. Use the lengths your supplier stocks, because the best plan depends on which lengths you can buy.
3. Fit the pieces into boards. Put the longest pieces first and fill each board with the next piece that still fits. Every cut uses up the width of the saw blade, so allow for it if the fit is tight.
4. Add spares if you want them. Boards can be split, crooked or full of knots, so many people buy a few extra. The calculator rounds your percentage up to whole boards.

The calculator does steps 2 to 4. It also adds up the linear feet and the board feet. For the board feet of a single board or a mixed order by thickness, width and length, use the board foot calculator. To count the studs, plates and headers of a wall first, use the framing calculator.

Example: a small framing order

You need four 2x4 pieces that are 7 ft long, two 2x4 pieces that are 5 ft long and six 2x6 pieces that are 3 ft 6 in long. The board lengths are 8, 10, 12, 14 and 16 ft, with no kerf and no spares. These are the three lines already in the calculator.

The pieces add up to 38 + 21 = 59 ft and the boards to 38 + 22 = 60 ft, so 1 ft of the 60 is left over, which is 1.7%. By the nominal size the order is 25.33 board feet of 2x4 and 22 board feet of 2x6, 47.33 in all. By the actual size it is 16.63 and 15.13, or 31.75 in all. Check which of the two a supplier uses to quote.

Nominal and actual lumber sizes

A 2x4 is named by its rough size before drying and planing, so it is smaller in real life. The American Softwood Lumber Standard, PS 20-25 (January 2025, table 3, checked October 2026), gives the minimum dressed sizes of dry lumber. The calculator uses them for the actual board feet. The last two columns show the board feet in one linear foot of each size.

These are minimums, and green lumber or a particular piece can be a little bigger. If your lumber is another size, such as a hardwood or a rough-sawn board, use the board foot calculator, which accepts any thickness and width.

Lumber lengths, kerf and spares

• Lengths. PS 20-25 says standard lengths are multiples of 1 ft or 2 ft, as set by the grading rules. The lengths a yard keeps are its own choice, so the box starts with an example list that you should replace.
• Longer than listed. For lumber 2 in or less in nominal thickness, the standard trims a board to at least its nominal length and at most 3 in over, or 12 in over for 2 in dimension lumber wider than 12 in or longer than 20 ft, unless the purchase contract says otherwise. The calculator uses the nominal length and ignores that extra.
• Kerf. It is the wood the saw turns into sawdust. The box starts empty because the width depends on your blade. A board with n pieces has n − 1 cuts between them.
• Spares. The calculator rounds the percentage up to whole boards and adds them in the length you buy most of. It rounds each lumber size on its own. It assumes no percentage unless you type one.

Lumber for a wall, a deck or a fence

Work out the pieces first, then type them here. For a deck, the deck calculator counts the deck boards, joists and screws. For a fence, the fence calculator counts the posts, rails and pickets. If you only know the area of a surface and the thickness of the lumber, multiply the two to get board feet, since a square foot of 1 inch lumber is 1 board foot.

Frequently asked questions