A board foot is a volume
One board foot is a piece 1 ft long, 1 ft wide and 1 in thick — 144 cubic inches. That single fact explains most of the confusion, because lumber is priced per board foot but stocked in linear feet and cut to a list in inches. The working formula is thickness in inches × width in inches × length in feet ÷ 12, where the 12 is just inches-per-foot carried through from the 144. Put every dimension in inches instead and you divide by 144. The two forms are the same calculation and must agree; this calculator asserts that in its test rather than trusting it.
The mistake that shows up on the invoice
A 2×4 measures 1½ by 3½. Almost everyone knows that. The trap is assuming board measure should therefore use 1½ × 3½ — it does not. Board measure for dimensional softwood is figured on the nominal call-out, so a 2×4×8 tallies 5.33 BF. Run it on the dressed section and you get 3.5 BF, a 34% shortfall on every piece. Order a truckload that way and the delivery ticket will not match the takeoff.
The dressed sizes themselves are standardized in NIST PS 20, the American Softwood Lumber Standard, and the allowance is not uniform: nominal 1 in loses ¼ in, 2 in through 6 in lose ½ in, and 8 in and wider lose ¾ in. That is why a 2×8 dresses to 7¼ rather than 7½. The calculator shows the dressed pair next to each nominal size, but deliberately keeps it out of the math — it is there to tell you what will arrive, not what you will be charged for.
Hardwood is a different trade
Rough hardwood is not sold in fixed sizes at all. It arrives in random widths and lengths, thickness is called out in quarters of an inch, and the tally is taken on the rough board before it is surfaced. So 4/4 means nominally 1 in, 8/4 means 2 in, and the width is whatever that particular board happens to be — which is why the hardwood mode here asks you to measure the width instead of picking it from a list.
Surfacing then takes material off. AHEC puts the allowance at 3/16 in for nominal thicknesses of 1½ in and under and ¼ in above that, so 4/4 comes out around 13/16 in. You still pay for the full 4/4. Anyone budgeting a project in finished thickness rather than rough thickness will come up short, and the gap widens on thicker stock where the allowance steps up.
Surface measure is the other number on a hardwood tally sheet: width in inches × length in feet ÷ 12, thickness ignored. It exists for grading — the NHLA rules judge how many clear cuttings a board yields against its surface measure, not its board footage. At 4/4 the two figures happen to be equal, which is exactly why they get confused. At 8/4 the board footage is double the surface measure.
What this tally leaves out
This is a board-measure calculator for one size at a time, not a cut-list optimizer. A real yield calculation for rough stock depends on the grade you buy, the length and width of the parts you need, whether grain has to match across a panel, and how much of each board is clear. Those variables move usable yield far more than any percentage field can capture, so the overage here is an ordering allowance rather than a yield prediction.
It also does not price shorts, premiums for specific widths or lengths, kiln-drying charges, or the difference between rough and S2S pricing — all of which appear on real quotes. Enter your own price per board foot to cost the order, and treat the result as the volume to ask a yard to quote against, not as the quote itself. For structural framing takeoffs the relevant questions are span and spacing rather than volume: deck joist span and deck board counts handle those separately.