Lumber Calculator
Building a deck, wall framing, or woodworking project? Enter your lumber dimensions and piece count to calculate board feet (bf) and total material cost.
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A board foot is a volume, not an area
This is the confusion the unit causes. A board foot is 144 cubic inches of lumber — nominally a piece 1 inch thick, 12 inches wide and 1 foot long. It measures how much wood there is, not how much surface it covers, which is why two boards with the same face area can hold very different board footage.
Formula: board feet = (thickness × width × length) ÷ 12
Thickness and width are in inches, length in feet. The 12 converts the mixed units into a single figure. A 2 × 4 × 8 works out to (2 × 4 × 8) ÷ 12 = 5.33 board feet.
Nominal size is not actual size
Board foot calculations use the nominal dimension — the 2 and the 4 in "2 × 4" — even though a dressed 2 × 4 actually measures 1.5 × 3.5 inches. The difference is planing and drying: the board was cut to full size green and lost about half an inch each way when it was surfaced. The lumber trade prices the rough size, so nominal is the correct input here, and this calculator expects it.
| Nominal | Actual |
|---|---|
| 1 × 4 | 3/4 × 3 1/2 in |
| 2 × 4 | 1 1/2 × 3 1/2 in |
| 2 × 6 | 1 1/2 × 5 1/2 in |
| 2 × 8 | 1 1/2 × 7 1/4 in |
| 2 × 10 | 1 1/2 × 9 1/4 in |
| 4 × 4 | 3 1/2 × 3 1/2 in |
Note that the 1/4 in shrinkage grows on wider boards: a 2 × 8 loses 3/4 in of width, not 1/2. Framing to actual dimensions matters as soon as you are fitting between studs or sizing a header.
When lumber is priced by the board foot
Dimensional softwood at a home center is priced per piece or per linear foot, so board feet are a comparison tool there rather than a purchase unit. Hardwood is different: walnut, oak, maple and cherry are sold by the board foot at a lumberyard, and that is where the calculation earns its keep — it is the only way to compare a wide short board against a narrow long one.
Hardwood thickness is quoted in quarters of an inch, rough: 4/4 is one inch, 8/4 is two. A board billed as 4/4 will surface down to about 13/16 in, and you are charged for the 4/4.
Order more than the cut list says
Add 15 to 20% to any hardwood order and 10% to framing lumber. Boards arrive with knots, checks, wane and bow, and a cut list assumes clear material of full length. On rough-sawn hardwood the allowance goes higher still, because you lose thickness to jointing and planing before the piece is even square.
Sight down every board before it goes in the cart. A bowed or twisted stud is not worth its discount, and crowned joists have to be installed crown-up whether you noticed the crown or not.
Frequently asked questions
How do you calculate board feet of lumber?
Multiply thickness in inches by width in inches by length in feet, then divide by 12. Use the nominal dimensions — the 2 and the 4 in a 2 × 4 — not the actual 1.5 × 3.5 in measurement, because the lumber trade prices the rough size before planing.
How many board feet are in a 2x4x8?
A 2 × 4 × 8 is 5.33 board feet: (2 × 4 × 8) ÷ 12. A 2 × 6 × 8 is 8 board feet and a 2 × 4 × 10 is 6.67, which shows why board feet rather than piece count is the fair way to compare prices.
Why is a 2x4 not actually 2 by 4 inches?
It was cut to a full 2 × 4 inches while green, then lost about half an inch each way to drying and surfacing. A dressed 2 × 4 measures 1.5 × 3.5 in. The shrinkage grows on wider boards — a 2 × 8 is actually 7 1/4 in, not 7 1/2.
How much extra lumber should I order?
Ten percent over the cut list for framing lumber and 15 to 20% for hardwood. Cut lists assume clear boards of full length, and real stock arrives with knots, checks, wane and bow. Rough-sawn hardwood needs more still, since jointing and planing take thickness before the piece is square.