Lumber Calculator
Two numbers on every lumber price tag confuse almost everyone who buys wood for the first time: the "2×4" on the sign and the "1.5 × 3.5" on the measuring tape — and the "board feet" figure that decides what you actually pay. Here is the short version: a 2×4 is called that because it STARTED as a 2×4 before planing; the planed board you take home is 1.5 × 3.5 inches. And a board foot is just a unit of volume — one square foot of board, one inch thick, however you slice it. This lumber calculator handles both sides of the confusion: it converts nominal sizes to actual dimensions automatically, computes board feet the way lumberyards charge (nominal sizes, not actual), adds a waste factor for your cuts, and estimates the cost from your price per board foot.
How to use this calculator
- Pick the nominal board size from the list (2×4, 2×6, 1×8, and so on) or choose "Custom size" and enter thickness and width yourself.
- Enter the length of each board in feet or metres, and how many boards you need.
- Leave "Board feet from" on nominal — that is how lumberyards charge. The results also show what the actual planed dimensions are.
- Add 10% waste for framing or 15% for decking and angled cuts, so offcuts don't send you back to the store.
- Enter your local price per board foot if you know it, for a cost estimate. Otherwise leave it blank.
- Read the results: board feet per board, total board feet, the buy quantity with waste, and the cost.
Formula
Worked example
You need 10 boards of 2×4×8 for a project. Step one — board feet per board, using the nominal size like lumberyards do: (2 × 4 × 8) ÷ 12 = 5.33 board feet. Step two — total: 5.33 × 10 = 53.33 board feet. Step three — waste: 53.33 × 1.10 = 58.67 board feet to buy. Step four — cost at $1.50 per board foot: 58.67 × 1.50 = $88.00. Now the trap most people fall into: if you measured the actual board (1.5 × 3.5 in) and used those numbers, you would get (1.5 × 3.5 × 8) ÷ 12 = 3.5 board feet per board, 35 total — 34% less. That is the real wood volume, but the yard charges the nominal figure, so price your project on 58.67, not 35.
Metric version: 20 pieces of 50 × 100 mm × 2.4 m. Convert first: 50 mm = 1.97 in, 100 mm = 3.94 in, 2.4 m = 7.87 ft. Per board: (1.97 × 3.94 × 7.87) ÷ 12 = 5.09 board feet. Total: 5.09 × 20 = 101.7 board feet; with 10% waste = 111.9 board feet (0.264 m³). At $2.00 per board foot: $223.75.
Measurement tips
- Price on nominal, build on actual. Board feet use the name size (2×4); your cut lengths and spans use the real size (1.5 × 3.5 in). Mixing them up is the classic beginner error.
- A 1×12×12 ft board is exactly 12 board feet — use it as a mental anchor. A 2×4×8 is 5.33, roughly half of that anchor.
- Buy 10–15% extra, not 0%. Offcuts, bad knots and one miscut are normal; a second trip to the yard costs more time than the spare boards cost money.
- Sort boards at the yard. Sight down each board for crowns, twists and wane — board feet are identical but a banana-shaped 2×4 is firewood.
- Pressure-treated lumber shrinks as it dries. A "wet" treated 2×4 can measure fuller than its dry size — buy for the project you are building now, not the size on the tag.
- For decking, count fasteners and joists too — the lumber calculator totals board feet, but a deck project also needs screws, hangers and concrete for posts.
Common mistakes
- Using actual dimensions for board feet. (1.5 × 3.5 × 8) ÷ 12 = 3.5 BF looks right but the yard charges 5.33 BF — you would under-budget by a third.
- Forgetting the ÷ 12. Thickness × width × length alone gives "inch-feet," not board feet — always divide by 12.
- Mixing units. Millimetres straight into the formula without converting to inches, or metres treated as feet, gives nonsense — the calculator converts for you, but hand math needs the conversion.
- Ordering exactly the calculated amount. One bad board or one wrong cut and the project stalls; waste allowance exists for a reason.
- Assuming all 2×4s are equal. Species (SPF vs Douglas fir vs southern pine), grade (#1, #2, stud) and moisture change strength and price — board feet only measure volume.
- Confusing board feet with linear feet. Ten linear feet of 2×4 is 6.67 board feet; of 2×12 it is 20 — width matters for board feet, not for linear feet.
Factors that affect accuracy
- Nominal vs actual: a 2×4 is really 1.5 × 3.5 in, a 1×6 is 0.75 × 5.5 in, a 4×4 is 3.5 × 3.5 in. Board feet use the nominal figure by lumberyard convention.
- Waste percentage: 10% covers straight framing with a few cuts; decking with picture-frame borders or diagonal boards wants 15%; complex pergolas and angled work can need 20%.
- Species and grade: southern pine, Douglas fir and SPF differ in strength and price per board foot — the same volume can cost double in a harder species.
- Board length availability: common lengths are 8, 10, 12, 14, 16 ft. Planning around stocked lengths cuts waste; odd lengths mean more offcuts.
- Moisture content: green or pressure-treated lumber is heavier and slightly larger; kiln-dried (KD) is stable and the standard for indoor framing.
- Price per board foot: varies by region, species and week — check your local yard's price rather than trusting a national average.
Sources & assumptions
- Board-foot formula (T″ × W″ × L′ ÷ 12) and billing on nominal size per the National Hardwood Lumber Association grading rules — the convention US lumberyards use.
- Nominal-to-actual planed dimensions (2×4 = 1.5 × 3.5 in, 1×6 = 0.75 × 5.5 in, 4×4 = 3.5 × 3.5 in) per the American Lumber Standard (PS 20).
- 1 board foot = 144 cubic inches = 0.00236 m³; waste allowances 10–15% per standard framing and decking practice.
What this calculator doesn't do
- Board feet measure volume, not strength or grade — a knotty #3 stud and a clear #1 board can have identical board feet and very different usefulness.
- The price per board foot is only an estimate input you provide; real lumber prices vary by species, grade, region and week, and this tool doesn't track them.
- Nominal-vs-actual conversion follows the American Lumber Standard (PS 20) for standard framing sizes; rough-sawn or specialty lumber follows different rules.
- It doesn't plan your cut list — it totals the volume of the boards you enter. Optimizing cuts to minimize waste is a separate planning step.
Frequently asked questions
How do you calculate board feet?
Multiply thickness (inches) × width (inches) × length (feet), then divide by 12. A 2×4×8 is (2 × 4 × 8) ÷ 12 = 5.33 board feet. The calculator above does it for any size, including metric.
What is a board foot of lumber?
One board foot is a unit of volume: 144 cubic inches of wood — one square foot, one inch thick. A 1×12×12 ft board is exactly 12 board feet. It is how hardwood and framing lumber are priced at the yard.
Is a 2x4 actually 2x4 inches?
No — a modern 2×4 measures 1.5 × 3.5 inches. The name is the nominal size (what it was before planing); the actual planed size follows the American Lumber Standard. Board feet are still charged on the nominal 2×4 figure.
How many board feet are in a 2x4x8?
5.33 board feet using the nominal size (2 × 4 × 8 ÷ 12), which is what lumberyards charge. Using the actual 1.5 × 3.5 in dimensions gives 3.5 board feet — the real wood volume, but not the billed figure.
How much does a board foot of lumber cost?
It varies by species, grade and region — common framing lumber runs roughly $1–$3 per board foot at retail in 2026, with hardwoods much higher. Enter your yard's price above for an exact estimate.
What is the difference between board feet and linear feet?
Linear feet measure length only (a 10 ft board = 10 linear feet whatever its width). Board feet measure volume: 10 linear feet of 2×4 is 6.67 BF, but 10 linear feet of 2×12 is 20 BF — width counts.
How do I calculate board feet in metric?
Convert to inches and feet first: millimetres ÷ 25.4 = inches, metres × 3.28084 = feet, then apply (T × W × L) ÷ 12. Or just enter metric numbers above — the calculator converts automatically and also shows cubic metres.
How many 2x4s do I need for a wall?
Studs go 16 inches on center, so a wall needs roughly (length in feet × 0.75) + 1 studs, plus top and bottom plates (2 boards per wall length each). Enter your stud count in the calculator above for the board-foot total — and check local code for your spacing.
Next step
Take the board-foot total to the lumberyard and ask their price per board foot for your species and grade — then sight down every board for crowns and twists before loading, because board feet are identical but a warped board is useless.
Save this estimate: use the Copy link button in the results box above to share these exact inputs, or Print results for a paper copy to take to the store.