Board feet, explained
A board foot is the unit lumber is quoted in when the cross-section matters as much as the length — hardwood, custom milling, and any bill of framing stock you want priced by volume rather than by the piece. This guide explains what the unit measures, walks the formula the board foot calculator runs, shows why a nominal "2×4" and a dressed 2×4 give different totals, and follows a cut list through waste and rounding to a priced order — two steps the tool itself does not take, applied here by hand with the site's shared helpers. Every figure below is computed at build time by the same engine that powers the tool, so the copy cannot disagree with the calculator.
What a board foot measures
One board foot is the volume of a piece of wood 1 ft long, 1 ft wide, and 1 in thick: 12 × 12 × 1 = 144 cubic inches. A cubic foot is 12 × 12 × 12 = 1,728 cubic inches, so a cubic foot holds exactly 12 board feet and a board foot is one twelfth of a cubic foot. That twelfth is where the ÷ 12 in the working formula comes from.
The unit is a volume, not a length. Two pieces of the same length are the same number of linear feet however their sections differ; in board feet they differ by the ratio of those sections. An 8 ft piece of dressed 2×6 (5.5 × 1.5 in) is 5.5 board feet, the 8 ft anchor board at 6 × 1 in is 4, and both are 8 linear feet. That is what the unit is for: a bill of mixed widths and thicknesses collapses to one volume figure that a single per-board-foot price can be applied to.
The formula, step by step
The engine computes length (ft) × width (in) × thickness (in) ÷ 12. The mixed
units are deliberate: lengths are measured in feet on site and cross-sections in inches, and one
division by 12 reconciles them.
The definition anchor: an 8 ft board, 6 in wide and 1 in thick, is 8 × 6 × 1 ÷ 12 = 4 board feet. Check it against the definition — 8 ft of a 6 in face has the same area as 4 ft of a 12 in face, and 4 ft of a 12 in wide, 1 in thick board is 4 board feet by definition, one per foot of length. The build asserts that the two routes agree.
Three things happen before that multiplication in the calculator. A length entered as feet plus inches is converted to decimal feet first: 10 ft 6 in becomes 10.5 ft, so 10 ft 6 in of dressed 2×6 (5.5 × 1.5 in) is 10.5 × 5.5 × 1.5 ÷ 12 = 7.219 board feet. Widths and thicknesses stay in inches as decimals — 1½ in is entered as 1.5. And nothing is rounded until display: the per-piece figure is carried at full precision into the piece count, so an order of forty pieces does not accumulate forty rounding errors.
Nominal versus dressed sizes
Softwood lumber is named by its nominal size and sold at a smaller dressed size. Under PS 20-20, the American Softwood Lumber Standard published by NIST (Table 3), the minimum dry dressed size of a nominal 2×4 is 1.5 × 3.5 in, a 2×6 is 1.5 × 5.5 in, a 4×4 post is 3.5 × 3.5 in, a 6×6 is 5.5 × 5.5 in, and a 5/4×6 deck board — five quarters of an inch, 1.25 in nominal — dresses to 1 × 5.5 in. These are the sourced sizes the site's deck, fence, and post-hole tools default to.
The formula does not care which set you use, but the answer does. Hardwood dealers figure rough stock from nominal (quarters) sizing, so a dealer's board-foot total is built on the nominal cross-section, while the wood in a dressed softwood piece is the smaller dressed section. The engine takes no side: it computes the dimensions you give it, and the choice is yours. Reproduce a quote with the convention it was written in; measure the wood itself with the dressed size.
| Nominal size | Dry dressed (PS 20-20) | Length | Nominal board feet | Dressed board feet | Dressed ÷ nominal |
|---|---|---|---|---|---|
| 2×4 | 1.5 × 3.5 in | 8 ft | 5.333 | 3.5 | 65.6% |
| 2×6 | 1.5 × 5.5 in | 12 ft | 12 | 8.25 | 68.8% |
| 4×4 | 3.5 × 3.5 in | 10 ft | 13.333 | 10.208 | 76.6% |
| 6×6 | 5.5 × 5.5 in | 8 ft | 24 | 20.167 | 84% |
| 5/4×6 | 1 × 5.5 in | 12 ft | 7.5 | 5.5 | 73.3% |
For an 8 ft 2×4 the nominal figure is 5.333 and the dressed figure 3.5 — the dressed piece holds about 65.6% of the volume its name implies. The gap narrows on larger sections because the 0.5 in taken off each nominal dimension of 2, 4, and 6 in stock — the same offset on every dimension of the 2×4, 2×6, 4×4, and 6×6 in the data, checked at build time — is a smaller share of a wide face than of a narrow one: a 6×6 keeps 84% of its nominal volume, a 2×4 only 65.6%. The 5/4 board loses 0.25 in of thickness and keeps 73.3%.
Worked examples
Each row is one run of the calculator: a length, a cross-section, and a piece count. The order total is the per-piece figure times the count, rounded only for display.
| Piece | Length × width × thickness | Pieces | Board feet per piece | Order total |
|---|---|---|---|---|
| Definition anchor | 8 ft × 6 × 1 in | 1 | 4 | 4 |
| Dressed 2×4 framing | 8 ft × 3.5 × 1.5 in | 10 | 3.5 | 35 |
| The same ten pieces at nominal size | 8 ft × 4 × 2 in | 10 | 5.333 | 53.33 |
| Dressed 4×4 posts | 10 ft × 3.5 × 3.5 in | 6 | 10.208 | 61.25 |
| Dressed 6×6 posts | 8 ft × 5.5 × 5.5 in | 4 | 20.167 | 80.67 |
| Dressed 5/4×6 deck boards | 12 ft × 5.5 × 1 in | 30 | 5.5 | 165 |
| Rough 5/4 stock at nominal size | 6 ft × 8 × 1.25 in | 12 | 5 | 60 |
| 10 ft 6 in of dressed 2×6 | 10.5 ft × 5.5 × 1.5 in | 1 | 7.219 | 7.22 |
| Metric: 2.4 m × 89 mm × 38 mm | 7.874 ft × 3.504 × 1.496 in | 1 | 3.44 | 3.44 |
Read the second and third rows together: the same ten 8 ft studs come to 35 board feet at their dressed size and 53.33 at their nominal size. The rough 5/4 row shows the hardwood convention at work — 12 pieces at 6 ft × 8 in × 1.25 in nominal are 60 board feet — and the post rows show why square stock adds up quickly: a single 8 ft 6×6 is 20.167 board feet, the volume of 5.8 dressed 2×4 studs of the same length.
From a cut list to an order: waste, rounding, and your price
The board foot calculator has no waste field and no rounding step. It reports pieces × board
feet per piece, and a cost only if you type a price — of the five-step pipeline on the
methodology page it runs the geometry step and the user-priced cost step, nothing in between.
The waste and whole-package helpers that the site's other estimators run (the
deck calculator applies the decking convention to its boards for you)
are applied here by hand, so you can see what they add before you put a rounded count
back through the tool. Take 24 dressed 2×6 boards at 12 ft. Geometry, from the tool: 8.25 board feet each, 198 for the bill — that
is the calculator's answer, and it stops there. Waste, by hand: the decking convention
the deck calculator starts from is 10% — a trade convention,
adjustable — and quantity × (1 + waste% ÷ 100) makes it 217.8 board feet.
But lumber is bought as whole boards, so the margin belongs on the piece count instead: 24 × 1.1 = 26.4. Rounding, by hand: whole
boards round up, to 27 boards. Back through the tool: 27 pieces are 222.75 board feet, and if the price
you type is $4.50 per board foot the cost line reads $1,002.38; type nothing
and no cost is shown, because the tool never supplies a price of its own.
Two things to notice when you carry the margin yourself. Round once, on the whole-board count, after the waste is applied — padding the board-foot total and then padding the count again would compound the margin. And the waste is a single flat percentage: it stands for end cuts, culls, and mistakes in aggregate, not for the specific offcuts of your cut list.
Metric entry
This tool's metric mode takes the length in metres and the width and thickness in millimetres — the form labels say which — converts them to the engine's canonical feet and inches with exact factors — 1 ft = 0.3048 m and 1 in = 25.4 mm — and then runs the identical formula. A 2.4 m length at 89 × 38 mm is 7.874 ft × 3.504 in × 1.496 in ÷ 12 = 3.44 board feet. As a check, the same piece measured directly — the millimetre sides converted to metres by the site's unit converter — is 2.4 × 0.089 × 0.038 = 0.008117 m³, and the board-foot total, carried unrounded as 3.439705 board feet, divided by 12 gives 0.286642 cu ft, which times 0.028316846592 is 0.008117 m³. The two routes agree because every factor is exact; the displayed 3.44 is a rounding of the figure the conversion used, not the figure itself.
The slips that swing a total twelvefold
The formula mixes feet and inches, so a unit slip changes the answer by a factor of 12, because there are 12 inches to the foot — the same error the slab tools see when a 4 in thickness is typed as 4 ft. Enter the anchor board's 8 ft length as 96 (its inches) and it becomes 48 board feet instead of 4, 12 times too many. Enter its 6 in width as 0.5 (its feet) and it drops to 0.333, 12 times too few. Neither result looks absurd on its own — which is why the sanity check is the definition itself: a 1 in board covering one square foot is one board foot.
The second slip is mixing conventions — pricing a quote figured on nominal size with dressed dimensions, or the reverse — which moves the total by the shares in the table above. The third is reading a board-foot total as a length: it is a volume, and how many pieces of stock it takes depends on the stock length you cut from.
Where the model stops
The board-foot engine is pure arithmetic on three dimensions and a count. It does not know the species, grade, or moisture state of the wood, it does not deduct saw kerf or defects, and it does not choose stock lengths for a cut list. It has no waste field and rounds nothing up — the boards to buy come from your count plus the waste you choose to add, rounded to whole boards by you. It carries no price: cost appears only when you enter a price per board foot, and the site never scrapes, averages, or invents one. And a board-foot total is a quantity, nothing more — it says nothing about structural adequacy, span capacity, load, or code compliance. The site estimates material quantities only; whether a joist, post, or stringer is adequate for its job is a decision for a licensed engineer, a code official, or a qualified contractor.
Frequently asked questions
Is a board foot the same as a linear foot?
No. A linear foot is a length, and every 8 ft piece is 8 linear feet whatever its section. A board foot is a volume — 144 cubic inches, one twelfth of a cubic foot — so the same 8 ft length is a different number of board feet for each section it comes in: 4 at 6 × 1 in, 5.5 at the dressed 2×6 section of 5.5 × 1.5 in.
Should I enter nominal or dressed dimensions?
Whichever the figure you are checking was built from — the engine applies no convention of its own. A hardwood quote figured from rough (quarters) sizing is reproduced by entering the nominal section; the wood actually present in a dressed softwood piece is found by entering its dressed size, which for a 2×4 is 1.5 × 3.5 in under PS 20-20. The table on this page gives the gap between the two for each size the site carries, so a total that is off by about that share usually means the conventions were mixed.
How many board feet are in a cubic foot?
Exactly 12. A cubic foot is 1,728 cubic inches and a board foot is 144, so dividing a board-foot total by 12 gives cubic feet — the volume-equivalent line the calculator shows under every result.
How do I total a cut list with several sizes?
One run of the tool handles one cross-section, so a list with several sizes goes through once per size and the board-foot totals are added by hand. Waste and rounding are also yours to add: put the margin on each size's piece count, round that count up to whole boards, and run the rounded count — the 24-board bill on this page shows the sequence — rather than padding the grand total, because whole boards of each size are what you buy.
Can I enter metric sizes?
Yes. This tool's metric mode takes the length in metres and the width and thickness in millimetres, as its form labels them, converts them with exact factors (1 ft = 0.3048 m, 1 in = 25.4 mm), and runs the identical formula: a 2.4 m piece at 89 × 38 mm is 3.44 board feet, 0.008117 m³.
Where does the cost figure come from?
Only from the price per board foot you type: the engine multiplies it by the unrounded board-foot total and shows the product. There is no price table, no default, and no regional figure behind the field, so leaving it empty removes the cost line entirely. In the bill on this page, 27 boards came to $1,002.38 because $4.50 was typed in, not because the site knows a price.
The board foot calculator runs every board-foot figure on this page interactively, the deck calculator counts the joists and 5/4 boards whose volume you would price here, and how material estimates are calculated walks the five-step pipeline every estimator on the site shares. All guides are listed on the guides index.
Board dimensions never leave your browser — the arithmetic runs locally. The full estimation pipeline is documented on the methodology page.