How lumber is measured
A board foot is a piece of wood 12 inches long, 12 inches wide and 1 inch
thick: thickness × width × length in inches, divided by 144. The pages ask for
the three the way a yard says them — thickness and width in inches, length in
feet — then the number of boards. A bare 8 in the length box is 8
feet, 8' 6" reads as well, and the thickness box takes hardwood
quarters as written: 4/4, 5/4, 8/4. Five
boards 1¼ by 10 inches and 8 feet long are 41.67 board feet.
A log is measured across the small end inside the bark, and along its length, and scaled at the whole inch and whole foot below what you measure. Then a log rule turns it into lumber, and the rules disagree: a log 16 inches by 16 feet scales 144 board feet by Doyle, 160 by Scribner Decimal C and 180 by International ¼ inch. The page prints all three, the rule you pick largest.
Which size you pay for
Softwood is billed by the nominal size on the tag, and its board feet are counted on that name. A 2×4 dried and planed is 1½ by 3½ inches, so a 2×4 8 feet long is 5.33 board feet on the invoice and 3.5 board feet of wood, and the pages print both, each saying which size it is. The five 5/4×10 boards above are dressed to 1 by 9¼ inches: 30.83 board feet of wood in the 41.67 you pay for.
Hardwood is tallied instead. The length drops to the whole foot below, width times those feet over 12 is the surface measure, rounded to the nearest whole square foot, and the board feet are that times the thickness. A 4/4 board 7½ inches by 9 feet 6 inches is 5.94 board feet exactly and 6 on the ticket. A board on exactly half a square foot is counted up and the next one down, in turn; wood thinner than an inch is measured as an inch; and a board surfaced two sides is billed at its rough quarter — the 4/4 above planes to 13/16 inch, 4.82 board feet of wood billed as 6.
Rounding once, and a waste figure
Every decimal is kept until the money: 41.67 board feet at $4.15 is $172.92, where a calculator that rounds the board feet first gets $172.93. One price is said four ways: for a 5/4×10 board 8 feet long, $4.15 a board foot is $4.32 a linear foot, $34.58 a board and $4,150 a thousand board feet.
No calculator we checked adds any waste. These pages start at 10%, the low end of what suppliers add for knots, checks and saw cuts, and the five boards become six on the list. Softwood is listed by the board at the shortest length a yard sells that holds yours, and a longer run in the longest it sells: 100 feet of 2×6 is five 20 foot boards, six with the waste. Hardwood is listed by the whole board foot: 5.94 with 10% is 6.53, so 7.
Where the numbers come from
The softwood rules are NIST's PS 20-20, read in the published text: §2.2 counts board feet on the nominal size, and Table 3 gives the dressed sizes. One simplification: PS 20 dresses the ⅜, ½ and ⅝ inch boards 1/16 inch under, and these pages use ⅛ for every board under an inch. The hardwood rules are the National Hardwood Lumber Association's 2019 rule book, read in a copy held by a third party because the association's own PDF is gone, and checked against the American Hardwood Export Council's grading guide. Lengths sold are the Southern Forest Products Association's 8 to 20 feet for dimension lumber, and the even feet from 6 to 16 for 1× boards, within NeLMA's grading rules.
The log rules are Purdue's FNR-191. Doyle and International are worked from its formulas, because its printed tables disagree with them in a few cells. Scribner Decimal C was drawn rather than computed, so it is read from FNR-191's Table 2, and a log the table has no cell for is left out and named, never given the nearest cell. The one convention is the 10% waste. There are no reference prices, since species and grade move a board foot too far: the price box is empty until you type your yard's, except on the lumber price page, which opens on its worked example.
What these pages do not do
They do not plan the cuts. The list counts boards; where the joints fall in a 100 foot run is yours to lay out.
They do not have a measure for every way wood is sold. Hardwood surfaced four sides and molding have no mode of their own, and a live-edge slab has no box for its wavy edge: type its average width, at actual size.
They do not grade lumber or say what a board can span. No code check runs here.
And the log page does not price a log or say what it will saw out. It has no buying list and no price box, and the mill's overrun or underrun is its own. International ¼ inch scales a log longer than 20 feet as two, so type it as two.