How to Calculate Board Feet

What a board foot is

A board foot is a volume: a piece of wood 1 foot long, 1 foot wide and 1 inch thick, which is 144 cubic inches (NIST Handbook 130, §2.12.1.1). Hardwood lumber is bought and sold by the board foot under the NHLA rules, so a lumberyard price like “$9.25/BF” means $9.25 for each 144 in³.

The formula

With every size in inches:

Board feet = T × W × L ÷ 144

With the length in feet (how lumber lengths are usually given), one foot is 12 inches, so 12 of the 144 cancel:

Board feet = T (in.) × W (in.) × L (ft) ÷ 12

Both give the same answer. Multiply by the number of pieces for a stack of identical boards.

Worked examples

These are the published sources' own examples, and the calculator's tests check against each one.

The third example is a “2×6” measured at its actual dressed size, 1-1/2″ × 5-1/2″: 6.875 BF rather than 10. Which size to use depends on what's being sold, which is what the next sections are about.

Try one board

For a whole list of boards, with species, prices and a printable tally sheet, use the board foot calculator.

Hardwood thickness in quarters

Hardwood is sold by its rough thickness in quarters of an inch: the number before the slash is how many quarters. 4/4 (“four quarter”) is 1″, 8/4 is 2″. Board feet are figured on this nominal rough thickness, even if you buy the board already surfaced (NHLA ¶16).

S2S sizes: NHLA Rules ¶14 and USDA Wood Handbook, Table 6-4. Surfacing takes 3/16″ off rough stock up to 6/4 and 1/4″ off 7/4 to 16/4. Above 16/4 the surfaced size isn't standard and is set by special contract.

Thin stock counts as 1″

In hardwood board measure, standard thicknesses under 1″ count as 1″ (NHLA ¶16). A 1/2″ × 12″ × 12″ board is tallied as 1 board foot, not 1/2.

Rough vs surfaced

Rough lumber is as it left the sawmill. Surfaced lumber has been planed: S2S on both faces, S4S on faces and edges. Because hardwood is measured before surfacing, a 4/4 board planed to 13/16″ is still sold as 1″ thick. When you plan a project, work out which rough quarter will plane down to your finished thickness: 3/4″ parts come from 4/4, 1-3/4″ parts from 8/4.

Softwood: nominal sizes

Construction softwood is sold by nominal size: a 2×4 is actually 1-1/2″ × 3-1/2″ when dry. Under the American Softwood Lumber Standard, board measure is computed from the nominal size (NIST PS 20-25, §2.2), so a 2×4 that's 8 feet long is 2 × 4 × 8 ÷ 12 = 5.33 BF. That figure is computed from §2.2; the standard itself has no worked example.

Metric

An inch is exactly 25.4 mm (NIST Handbook 44, Appendix C), so a board foot is exactly 144 × 0.0254³ = 0.002359737216 m³, and one cubic metre is about 423.776 board feet. To get board feet from metric sizes, convert to inches first (mm ÷ 25.4; metres × 1000 ÷ 25.4), or work out cubic metres (m × m × m) and divide by 0.002359737216.

Price per board foot and per linear foot

Cost is board feet × price per board foot. Some yards price by the linear foot instead; one linear foot of a board is T × W ÷ 12 board feet, so:

$/linear ft = $/BF × T × W ÷ 12

For example, a 1×6 board at $9.25/BF is $9.25 × 1 × 6 ÷ 12 = $4.63 per linear foot. This is derived from the board-foot formula, not quoted from a source.

From a finished cut list to rough lumber

Plans give finished sizes. To know what to buy, work backwards:

1. Thickness: pick the smallest quarter whose surfaced (S2S) thickness covers the finished thickness (the table above).
2. Width and length: add the rough-mill allowances, 1/4″ of width and 1″ of length per part (USDA Forest Service GTR-NE-329).
3. Board feet on the rough quarter thickness, rough width and rough length.
4. Waste: add about 20% for defects, grain and offcuts. That's trade guidance from Popular Woodworking, which suggests about 50% when the mill picks the boards (random widths and lengths), not a rule.

Here's a Shaker end table in cherry, the calculator's finished → rough example:

Grouped by species and quarter, with 20% waste: Cherry 8/4 needs 3.03 BF → 3.63 BF, so buy about 4 BF; Cherry 4/4 needs 5.15 BF → 6.18 BF, so buy about 7 BF. In all: 9.81 BF, about $95.32 at the example prices (8/4 at $10.50/BF, 4/4 at $9.25/BF). Rounding each group up to whole board feet (11 BF) would cost $106.75.

Open the calculator's Finished → rough mode to do this for your own cut list and print it as a shopping list.