Yard&Board

Board Feet Explained (and the Nominal Lumber Trap)

A board foot is a volume of wood, not a length. One board foot equals a piece 1 inch thick, 12 inches wide, and 12 inches long. Once you know the formula, you can price and order lumber the way hardwood dealers do, and you can catch the mistakes that make people over-order or under-order without realizing it.

The board-foot formula

The standard formula, confirmed by Woodworkers Source, a lumber-industry retailer:

Board feet = thickness (in) x width (in) x length (ft) / 12

Woodworkers Source publishes two other versions of the same math: thickness x width x length, all three in inches, divided by 144, or thickness in inches times the piece's square footage. All three give the same answer. Yard & Board's board foot calculator uses the inches-and-feet version, since that is how most lumber is labeled at the yard: thickness and width in inches, length in feet.

The key rule buried in that formula: use the nominal (rough-sawn) thickness, not the actual thickness after milling. A hardwood board sold as "4/4" (four-quarter, meaning 1 inch nominal) is billed as 1 inch thick for board-foot purposes, even though the actual planed thickness is closer to 13/16 inch. If you calculate using the planed thickness instead of the nominal thickness, your number will not match what the dealer charges.

Worked example

Say you are buying six pieces of nominal 2x8 lumber, each 10 feet long, for a framing job.

One board: 2 in x 8 in x 10 ft / 12 = 160 / 12 = 13.33 board feet.

Six boards: 13.33 x 6 = 80 board feet.

Now compare that to a hardwood order. A 4/4 walnut board, nominal 6 inches wide, 8 feet long:

1 in x 6 in x 8 ft / 12 = 48 / 12 = 4 board feet.

Notice the thickness in that second example is 1, the nominal figure, not 0.8125 (13/16 inch), the actual planed figure. Plug in the actual thickness by mistake and you get 3.25 board feet instead of 4, a 19% undercount on that single board.

Why a 2x4 isn't 1.5 inches by 3.5 inches at the store, but is in the formula

This is the part that trips up most first-time buyers. A "2x4" is a nominal size: the name the mill gives the board when it is rough-cut, before drying and planing shrink it down. The size on the tag and the size on the tape measure are two different numbers.

Cross-verified against two independent lumber-industry sources, ProWood (a manufacturer) and Archtoolbox (an architecture reference), which agree exactly on the figures below:

ProWood's table also lists a 6x6 at actual 5-1/2 in x 5-1/2 in, though Archtoolbox's table did not include that size, so treat it as single-source. The same goes for two decking-specific sizes ProWood publishes and Archtoolbox does not cover: nominal 5/4x6 decking measures 1 in x 5-1/2 in actual, and nominal 6/4x6 measures 1-5/16 in x 5-1/2 in actual.

These figures trace back to the American Softwood Lumber Standard, PS 20, the governing federal standard for dressed softwood sizing. The PDF of that standard would not render as text for this research pack, so it is cited here for provenance only. The ProWood and Archtoolbox tables are the sourced numbers.

For board-foot math on dimensional softwood like a 2x4, use the nominal number (2 and 4), not the actual milled number (1.5 and 3.5). The nominal size is what the formula and the lumberyard both use for volume and pricing, even though the board in your hands measures smaller.

Where board-foot pricing shows up

Board-foot pricing is standard practice at hardwood and specialty lumber dealers. Woodworkers Source, the lumber-industry retailer whose formula this guide uses, is one example: walk into a hardwood dealer for walnut, cherry, or oak, and the price tag or invoice will be dollars per board foot, calculated with nominal thickness the way this guide describes.

Home centers selling framing lumber and decking, by contrast, typically price by the piece or by the linear foot. A stud or a deck board gets one price for the whole length, no board-foot math involved. That does not mean board feet are useless for that lumber. Converting a shopping list of dimensional lumber into total board feet is still the fastest way to compare the true wood volume, and rough cost, of two different framing plans, or to sanity-check a quote that mixes board sizes.

Confirm how your specific yard bills before placing a large order. This guide covers the standard nominal-thickness formula, but individual dealers can apply their own conventions.

Mistakes that throw off your order

Two mistakes account for most board-foot errors:

Mixing up the inches formula and the feet formula. The three versions of the formula Woodworkers Source publishes are mathematically identical, but only if you keep the units straight. If you plug a length in inches (say, 96) into the version of the formula built for length in feet (which divides by 12, not 144), you inflate your board-foot count by 12 times. That is a large enough error to double- or triple-order lumber without noticing, since the number still looks plausible on paper.

Using actual thickness instead of nominal thickness for hardwoods. As shown in the worked example above, plugging in the planed thickness instead of the nominal thickness undercounts board feet. That means your self-calculated total comes in lower than what the dealer will actually invoice, which either leaves you short on wood mid-project or blows the budget you planned around your own math.

Run your numbers through the board foot calculator before you order, and pick a common nominal size from the dropdown when one applies, so you are not hand-typing actual dimensions into a formula built for nominal ones.

FAQ

Do I use nominal or actual dimensions to calculate board feet?

Nominal. The board-foot formula and lumberyard pricing both use the rough-sawn nominal size, such as 1 inch for a 4/4 hardwood board or 2 inches for a 2x4, even though the actual planed dimensions are smaller.

Is a board foot the same as a linear foot?

No. A linear foot measures length only, regardless of the board's thickness or width. A board foot measures volume: thickness times width times length, divided by 12 when length is in feet. A wide, thick board contains more board feet per linear foot than a narrow, thin one.

Why is a 2x4 actually 1.5 inches by 3.5 inches?

The 2x4 name refers to its nominal, rough-sawn size before the mill dries and planes it smooth. Drying shrinkage and planing remove roughly a half inch from each dimension, leaving an actual size of 1.5 in x 3.5 in. This applies across the nominal-to-actual table sourced from ProWood and Archtoolbox above.

Does every lumber seller price by the board foot?

No. Hardwood and specialty dealers commonly price by the board foot. Home centers selling dimensional softwood framing lumber and decking typically price by the piece or the linear foot instead. Board-foot math still works as a way to total up wood volume across a shopping list, even when the receipt itself is priced per piece.

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