Lumber Calculator

How the lumber calculator works

Enter the length of one piece, its thickness and width, the number of pieces and the price per foot or metre. On the US units side the sizes are nominal, the way lumber is named and sold (a 2×4 is entered as 2 and 4), and the result is board feet. On the metric side the sizes are the actual finished section in millimetres, the way timber is sold in the UK, Europe and Australia, and the result is cubic metres. Both sides give the total length and the cost.

The US formula is board feet = thickness (in) × width (in) × length (ft) ÷ 12 × pieces. The metric formula is volume = thickness (m) × width (m) × length (m) × pieces. Cost is the total length times the price per foot or metre.

What a board foot is

The American Softwood Lumber Standard, PS 20-20, published by NIST, defines board measure as nominal thickness in inches times nominal width in feet times length in feet. One board foot is therefore a piece 1 in thick, 12 in wide and 1 ft long, in nominal terms. A 10 ft 2×4 is 2 × 4 × 10 ÷ 12 = 6.67 board feet, a 16 ft 2×10 is 2 × 10 × 16 ÷ 12 = 26.67, and an 8 ft 1×6 is exactly 4.

Board feet are counted on the nominal size, not the actual size. The same standard warns that converting board feet to cubic metres needs great care for that reason, because a board foot is not a well-defined volume. So the calculator does not turn board feet into cubic metres; the metric side works from actual sizes instead.

Nominal and actual sizes

Dimension lumber is dressed smaller than its name. PS 20 Table 3 gives the minimum dressed dry sizes: a nominal 2 in thickness is 1½ in (38 mm), 4 is 3½ in (89 mm), 6 is 5½ in (140 mm), 8 is 7¼ in (184 mm), 10 is 9¼ in (235 mm) and 12 is 11¼ in (286 mm). The US result shows the dressed size for any standard dimension size you enter. A 2×4 is actually 1½ × 3½ in, a cross-section of 5.25 in² against the 8 in² its name suggests, about two thirds.

Metric sizes work differently. PS 20 notes that its own metric sizes are based on the dressed size, so a metric section is its real size: 38 × 89 mm is the metric size of a dressed 2×4. Timber sold in millimetres is quoted this way too, so a 47 × 100 mm stud is close to a US 2×4 but not the same. Enter the finished size from the supplier on the metric side.

Green and dry sizes

The dressed sizes above are for dry lumber, which PS 20 defines, for lumber under 5 in nominal thickness, as seasoned or dried to a moisture content of no more than 19%. Green lumber has slightly larger minimum sizes: the standard gives 1 9/16 in for a nominal 2 and 3 9/16 in for a nominal 4, against 1½ and 3½ in dry. Board feet are the same either way, since they use the nominal size.

Lengths and pricing

The USDA Forest Products Laboratory Wood Handbook notes that softwood lumber is made in multiples of 1 ft, that 2 ft multiples are common for construction lumber, and that dimension lumber is often found in 8 to 18 ft lengths (2.4 to 5.5 m) in 2 ft (0.6 m) steps. Choosing lengths that divide well into the cuts you need leaves smaller offcuts.

Lumber is priced per piece, per linear foot or metre, or per thousand board feet. To compare them, a 2×4 has 2 × 4 ÷ 12 = 0.667 board feet per linear foot, so $1.10 per foot is $1.65 per board foot, or $1,650 per thousand. For a price per piece, divide by the piece length to get a price per foot.

A worked example

Ten 10 ft 2×4s come to 10 × 6.67 = 66.67 board feet and 100 linear feet, which at $1.10 per foot is $110. In metric, ten 3 m lengths of 47 × 100 mm timber have a volume of 0.047 × 0.1 × 3 × 10 = 0.141 m³ and a total length of 30 m, which at £3.20 per metre is £96.

What this tool does not do

It does not work out how many pieces a job needs, add wastage, check grade or strength, or size members for a span. Use a framing or joist calculator for the piece count, add extra pieces for cutting losses, and check spans against your local code or a structural engineer.

Using this calculator with other BuildMetricLab tools

The stud wall calculator counts studs and plates for a wall, the floor joist and I-joist calculators help with framing members, and the deck framing calculator covers decks. All BuildMetricLab tools run in your browser with no sign-up, and every formula is shown on the page so the maths can be checked.