How to use the lumber calculator
Enter the length of one piece and how many you need. Pick the lumber size and the board length you’ll buy, and adjust the saw kerf if your blade is thinner or thicker than 1/8 in. Add the price per board to see the cost. If your project has pieces of different lengths, run the calculator once per length.
Need board feet or the price of hardwood sold by the board foot? Use the board foot calculator.
The formula
Pieces per board = (board length + kerf) ÷ (cut length + kerf), rounded down
Boards = pieces needed ÷ pieces per board, rounded up
The extra kerf on the board length covers the last piece, which needs no cut after it.
Worked example: 20 shelf pieces of 30 in from 8 ft 2×4s
- 8 ft = 96 in; (96 + 1/8) ÷ (30 + 1/8) = 3.19 → 3 pieces per board
- 20 ÷ 3 = 6.67 → 7 boards
- Offcut from each full board: 96 − 3 × 30 1/8 = 5 5/8 in
- 7 × 8 ft = 56 linear ft; 7 × 2 × 4 × 8 ÷ 12 = 37.33 board feet
- At $4 a board: $28.00
Pieces per board by cut length
With a 1/8 in kerf. Pick the length that wastes least for your cut.
| Cut length | 8 ft board | 12 ft board | 16 ft board |
|---|---|---|---|
| 16 in | 5 | 8 | 11 |
| 24 in | 3 | 5 | 7 |
| 30 in | 3 | 4 | 6 |
| 36 in | 2 | 3 | 5 |
| 48 in | 1 | 2 | 3 |
A 24 in cut gets only 3 from an 8 ft board and 7 from a 16 ft board because of the kerf: 4 × 24 in pieces need 96 3/8 in.
Nominal and actual lumber sizes
| Nominal | Actual size |
|---|---|
| 1×4, 1×6, 1×8 | 3/4 in thick × 3 1/2, 5 1/2, 7 1/4 in |
| 2×4, 2×6, 2×8 | 1 1/2 in thick × 3 1/2, 5 1/2, 7 1/4 in |
| 2×10, 2×12 | 1 1/2 in thick × 9 1/4, 11 1/4 in |
| 4×4, 6×6 | 3 1/2 × 3 1/2 in, 5 1/2 × 5 1/2 in |
Common mistakes
- Forgetting the kerf. Pieces that divide a board exactly (four 24 in pieces from 8 ft) come out one short.
- Measuring with nominal sizes. A wall of stacked 2×4s on edge is 3 1/2 in per board, not 4 in.
- Mixing up board feet and linear feet. Yards price framing lumber per board or per linear foot; hardwood dealers price per board foot.
- Buying the exact count. One warped board leaves you a piece short; buy a spare.
Frequently asked questions
How many 2×4s do I need?
Divide the board length plus one saw kerf by your cut length plus the kerf, round down, and that's how many pieces one board gives. Divide the pieces you need by that and round up. For 20 pieces of 30 in, an 8 ft 2×4 gives 3, so you need 7 boards.
Why can't I cut two 4 ft pieces from an 8 ft board?
Each cut turns about 1/8 in of wood into sawdust. Two 48 in pieces plus one cut need 96 1/8 in, and an 8 ft board is 96 in, so the second piece comes out 1/8 in short.
What is the actual size of a 2×4?
1 1/2 × 3 1/2 in. Lumber is named by its rough-sawn size before drying and planing. A 2×6 is 1 1/2 × 5 1/2 in, a 2×8 is 1 1/2 × 7 1/4 in and a 4×4 is 3 1/2 × 3 1/2 in.
How many board feet is a 2×4×8?
5.33 board feet (2 × 4 × 8 ÷ 12). Board feet use the nominal size, not the actual 1 1/2 × 3 1/2 in. A 2×6×16 is 16 board feet and a 2×12×16 is 32.
Which board length wastes the least?
The one whose length is closest to a whole number of your pieces plus kerf. For 30 in pieces, an 8 ft board leaves 5 5/8 in, a 16 ft board leaves 11 1/4 in and a 10 ft board leaves 29 5/8 in.
Should I buy extra boards?
Buy one or two more than the count to cover a warped, split or badly knotted board, more if you're picking from a picked-over pile. Return what you don't use.
Sources
Formulas and defaults last reviewed October 30, 2026. How we check our math.