Board foot calculator
Board feet are counted on the rough thickness, before surfacing. Measure your board after it has been planed and you will undercount by about a fifth.
The rule that costs people money
A board foot is a piece 12 inches square and one inch thick, or its equivalent in volume. What catches people is which dimensions go into it. The trade rules are unambiguous: board footage is measured before surfacing, on the rough thickness. Buy 4/4 stock and you pay for a full inch, then take delivery of something 13/16 of an inch thick, because the mill sawed an inch and absorbed the planing.
That is a 19% gap between the invoice and the tape measure, and it is not a swindle — it is the convention, published in the grading rules and consistent across the industry. But a calculator that asks for "thickness" without saying which one will quietly cost you the difference on every order.
Two systems that get confused with each other
Hardwood is sold by the board foot under the hardwood grading rules, in the quarter system — 4/4, 5/4, 6/4, 8/4 and up, where the numerator is quarter-inches of rough thickness. Softwood dimensional lumber is a different standard entirely, sold by the piece or the linear foot, where a dry surfaced 2×4 measures 1½ by 3½ inches.
Both systems compute board measure on nominal dimensions. The softwood standard says so directly and carries an explicit warning about the trap. So the widely repeated advice to "use actual dimensions, not nominal" is backwards in both systems — and on a 2×4 it produces an answer 34% too low.
Rounding runs in one direction
Lengths are dropped to the next lower standard length. An 8 foot 4 inch board tallies as 8 feet, never 9. Widths round to the nearest inch, with exact half-inches counted alternately up and down so the error averages out across a pack. Anything under an inch thick is counted as an inch.
Two prominent guides get the length rule backwards — one rounds to the nearest foot, the other rounds up — and one of them also advises rounding thickness up to the nearest inch, which over-bills 5/4 stock by 60%.
Standard thicknesses and what they surface to
| Quarter | Rough | Surfaced (S2S) | You lose |
|---|---|---|---|
| 4/4 | 1" | 13/16" | 19% |
| 5/4 | 1¼" | 1‑1/16" | 15% |
| 6/4 | 1½" | 1‑5/16" | 13% |
| 8/4 | 2" | 1¾" | 13% |
| 12/4 | 3" | 2¾" | 8% |
| 16/4 | 4" | 3¾" | 6% |
Asked at the bench
So am I being overcharged?
No. You are buying the thickness the mill sawed and dried, and the planing that brings it to a usable surface is part of what you are paying for. The rule is published and applies to everyone. It only becomes a problem when a calculator asks you to measure a board you already own, because then you measure the surfaced figure and get a different answer to the yard.
What if I buy rough sawn?
Then the two figures agree and there is nothing to reconcile — you are measuring what you are billed for. Rough stock is also the honest way to buy if you have a planer, because you keep the thickness that would otherwise go into the chip bin.
Does the price per board foot include waste?
It does not. Board footage measures what is in the pack, not what survives your cut list. Ripping to width, jointing an edge and cutting around defects typically costs anywhere from a fifth to two fifths depending on grade, so order accordingly. Lower grades are cheaper per board foot precisely because more of them ends up on the floor.
How do I tally a stack of random widths?
Width by width, rounding each to the nearest inch, with exact half-inches alternating up and down through the pile. That alternation is deliberate: it makes the rounding error cancel across a pack rather than accumulate in the seller’s favour.