Board feet in a 2×12×20
There are two honest answers. The wood you actually carry out of the store is 28.13 board feet — a 2×12 really measures 1.5″ × 11.25″. But if you're pricing or trading lumber, the industry counts this board as 40.00 BF, because board feet are by convention figured on the nominal 2″ × 12″ name.
The math, both ways
trade (nominal): 2″ × 12″ × 240″ ÷ 144 = 40.00 BF
Which one do you want? If you're figuring weight, volume, or "how much wood is that really," use 28.13. If you're comparing against hardwood priced per board foot, or reading a mill tally, use 40.00 — the whole trade runs on nominal, for the historical reasons on our nominal vs actual page. (Construction lumber itself is usually priced per piece, so at the store neither number appears on the shelf tag.)
What a 2×12 is for
The 2×12 is the biggest common dimensional board, and it has one famous job: stair stringers. Cutting the sawtooth for treads and risers eats so much width that only a 2×12 leaves enough uncut wood (the code minimum "throat") to carry the load. It's also the long-span joist, the ridge board, and the site-built beam.
Big boards concentrate the drying stresses of the tree, so a stack of 2×12s hides more crook and wane than a stack of 2×4s. Sight down every one, and plan cuts around the heart center if a board contains it.
Twenty-footers are the frontier of stock lumber — many yards keep a few, others make them a special order. They exist for the spans where a splice would be a sin: long floor joists, ridge boards, and built-up beams. At this length delivery stops being a luxury; so does a helper. The consolation: one uncut 20′ board is often cheaper, straighter, and stronger than the two shorter boards plus splice hardware it replaces.
Heft check: at its true 1.5″ × 11.25″ section, a 20′ board holds 2.34 ft³ of wood — about 82 lb kiln-dried, and closer to 129 lb dripping fresh off the pressure-treated rack — a genuine two-person lift.
What that volume costs at hardwood prices
| Price per BF | Actual wood (28.13 BF) | Trade tally (40.00 BF) |
|---|---|---|
| $3.00 | $84.38 | $120.00 |
| $5.00 | $140.63 | $200.00 |
| $7.00 | $196.88 | $280.00 |
| $10.00 | $281.25 | $400.00 |