How Much Lumber Do I Need? (Board Feet)

By RugCalc Editorial Team · Last updated: 2026-10-13 · 8 min read

Quick answer: board feet = (thickness in × width in × length ft) ÷ 12 per board, summed and rounded up. Here's how to count framing, add waste, and order for any project.

Board feet formula

Lumber is priced in board feet: (Thickness in inches × Width in inches × Length in feet) ÷ 12. A 2×4×8 is 2×4×8 ÷ 12 = 5.33 board feet. A 2×6×10 is 2×6×10 ÷ 12 = 10 board feet.

Board feet measures volume, not length — that's why a long, skinny 1×6 can be cheaper than a short, thick 4×4 even though the 4×4 is shorter. For a rough estimate, a 2×4×8 is about 5.33 bf and a 2×4×10 is about 6.67 bf; scale from there.

Pieces vs board feet

You'll buy lumber two ways: by the piece for dimensional stock (2×4s, 2×6s) and by board feet for larger timber. For most home projects, counting pieces is simpler — board feet matters for pricing and for larger beams.

Lumber yards quote price per board foot on the sign and per piece on the shelf. When you get a quote, confirm which unit it's in — a "cheap" price per board foot can still add up fast on a big beam order.

Worked example

For a small deck frame needing 20 pieces of 2×6×10:

• Board feet per piece = 2×6×10 ÷ 12 = 10 bf
• Total = 20 × 10 = 200 board feet

That's the bare framing. Add waste and you're closer to 220 board feet for the order — the difference between a single trip and a frustrating second run.

Framing counts

For walls, studs typically go 16 in on center, so a wall roughly needs 1 stud per foot of length plus corners and openings. For decks, joists at 16 in spacing need about 1 joist per 1.5 ft of span width. Lay out the exact members before ordering — a materials list beats a guess.

Doors and windows add header material above the opening (typically doubled studs and a header), so a wall with several openings needs noticeably more lumber than its length alone suggests. Count each opening and add its framing, don't just multiply wall length.

Adding waste

Add 5–10% waste for cuts, knots, and bad boards. When in doubt, buy one extra of each size — a single trip to the lumber yard costs more than a spare 2×4. Wood moves, too: boards can be bowed or split at the yard, so plan for a couple of rejects in every order.

Common mistakes

• Forgetting the ÷12. Board feet is per 12-in "foot" — get the formula right.
• Counting nominal vs actual size. A 2×4 measures 1.5×3.5 in; board-foot math uses nominal.
• No waste. Knots and cuts always eat into the count.
• Ordering per-piece without a layout. Map the framing, then count.
• Not confirming price unit. Board foot vs per-piece pricing can differ by a lot.

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