Board feet in a 4×4×8
There are two honest answers. The wood you actually carry out of the store is 8.17 board feet — a 4×4 really measures 3.5″ × 3.5″. But if you're pricing or trading lumber, the industry counts this board as 10.67 BF, because board feet are by convention figured on the nominal 4″ × 4″ name.
The math, both ways
trade (nominal): 4″ × 4″ × 96″ ÷ 144 = 10.67 BF
Which one do you want? If you're figuring weight, volume, or "how much wood is that really," use 8.17. If you're comparing against hardwood priced per board foot, or reading a mill tally, use 10.67 — 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 4×4 is for
The 4×4 is the fence and deck post — nearly always pressure treated, because posts meet dirt. Treated ratings matter here: "ground contact" stock carries roughly twice the preservative of "above ground" boards, and a post set in concrete needs it.
Its 3-1/2″ actual dimension is why post-base hardware, anchors, and caps all say "for 4×4 (3-1/2″)". Building codes have been steadily pushing decks toward 6×6 posts, so check span tables before framing anything tall on 4×4s. Unlike smaller sizes, a 4×4 usually contains the tree's center — expect checking cracks as it dries, which look alarming and are mostly cosmetic.
Eight feet is the default length — it matches the ceiling, fits inside a pickup bed with the tailgate down, and wastes the least when cut for typical walls. It's usually the cheapest per foot, too, because it's the volume seller.
Heft check: at its true 3.5″ × 3.5″ section, a 8′ board holds 0.68 ft³ of wood — about 24 lb kiln-dried, and closer to 37 lb dripping fresh off the pressure-treated rack.
What that volume costs at hardwood prices
| Price per BF | Actual wood (8.17 BF) | Trade tally (10.67 BF) |
|---|---|---|
| $3.00 | $24.50 | $32.00 |
| $5.00 | $40.83 | $53.33 |
| $7.00 | $57.17 | $74.67 |
| $10.00 | $81.67 | $106.67 |