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Lumber Calculator

Use this free lumber calculator to work out how many board feet your project needs and what it will cost. Add as many pieces as you like — studs, joists, posts, decking, or plywood sheets — pick a size preset or enter custom dimensions, and it totals the board feet for every piece, adds your waste allowance, and prices the whole list at your price per board foot.

How it works

A board foot is the standard unit lumber is measured and sold by. One board foot is a piece of wood 1 inch thick, 12 inches wide, and 12 inches long — 144 cubic inches of wood. Any board, whatever its shape, can be converted into board feet, which is why yards price hardwood this way and why it is the fairest way to compare the cost of different sizes. The formula is short: multiply thickness in inches by width in inches by length in feet, then divide by 12.

Take a standard 2 by 4 that is 8 feet long. Two times four times eight is 64, divided by 12 is 5.33 board feet per board. Ten of them come to 53.33 board feet. The same formula works for plywood: a 4 by 8 foot sheet of 3/4 inch plywood is 0.75 times 48 times 8 divided by 12, which is 24 board feet per sheet. In practice plywood is usually priced by the sheet rather than by the board foot, but the volume still converts the same way, so the calculator handles sheets with the same math.

One thing that confuses almost everyone is nominal versus actual size. Lumber is sold by its nominal size — the size of the rough-sawn board before it is planed smooth and dried. A 2 by 4 is actually 1.5 by 3.5 inches, a 2 by 6 is actually 1.5 by 5.5 inches, and a 1 by 6 is actually 0.75 by 5.5 inches. Board foot calculations always use the nominal size, so you enter 2 and 4 for a 2 by 4, not 1.5 and 3.5. The presets in the calculator already use nominal sizes, and the diagram notes the difference so your framing layout still uses the real dimensions.

The calculator totals every piece in your list, then adds your waste allowance. Ten percent is a good default for most projects: boards have knots, splits, and warps, and some pieces will be too crooked to use. If you are cutting many short pieces out of long boards, or working with an expensive hardwood where every offcut matters, raise it to 15 percent. Finally it multiplies the board feet, including waste, by your price per board foot. That price varies a lot — common softwoods such as pine and spruce often run about $1 to $2 per board foot, while hardwoods such as oak, maple, and walnut can run $5 to $15 or more, depending on species, grade, and region. The price field is editable, so enter your yard's actual price for a realistic total.

Before you order, walk the list once more against your cut list. A common mistake is ordering the right total of board feet in the wrong lengths — twenty 8-foot boards do not help if your project needs ten 16-foot joists. The calculator keeps each piece separate, so check the length column as carefully as the total, and remember that many yards charge extra for lengths over 12 or 16 feet.

Worked example

A builder needs ten 2 by 4 boards, each 8 feet long, for a wall frame. How many board feet is that, and what does it cost at $1.20 per board foot with a 10 percent waste allowance?

Step 1: Board feet per piece — 2 × 4 × 8 ÷ 12 = 64 ÷ 12 = 5.33 board feet.
Step 2: Total for ten boards — 5.33 × 10 = 53.33 board feet.
Step 3: Add 10 percent waste — 53.33 × 1.10 = 58.67 board feet.
Step 4: Cost — 58.67 × $1.20 = $70.40.

Enter one piece of 2 by 4 by 8 feet with a quantity of 10 in the calculator, set the price to $1.20 and the waste to 10 percent, and it gives exactly the same result: 53.33 board feet before waste, 58.67 board feet with waste, and an estimated cost of $70.40. Add a second line — say four 2 by 6 boards at 10 feet, which is 40 board feet — and the total updates to 93.33 board feet before waste, showing how the list builds up piece by piece.

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