How to Calculate Board Feet (Formula and Examples)

Hardwood lumber is usually sold by the board foot, not by the piece. If you’re buying oak for a tabletop or walnut for shelves, you need to know how to calculate board feet to compare prices and order the right amount.

What is a board foot?

A board foot is a volume of wood equal to 12 in × 12 in × 1 in, or 144 cubic inches.

The formula

Board feet = thickness (in) × width (in) × length (ft) ÷ 12

Or, if all three measurements are in inches: thickness × width × length ÷ 144.

Worked examples

Quarter sizes (4/4, 5/4, 8/4)

Hardwood thickness is often given in quarters of an inch. 4/4 (“four-quarter”) is 1 in rough thickness, 5/4 is 1¼ in and 8/4 is 2 in. Use the rough thickness in the formula, because that’s how the lumber is measured for sale.

Nominal vs actual sizes

Construction lumber such as a “2×4” is sold by nominal size; the actual dressed size is about 1½ × 3½ in. Board-foot pricing generally uses the nominal size. When you’re working out how much finished wood you’ll have, use the actual size.

Working out the cost

Cost = board feet × price per board foot

Example: three boards of 4/4 × 8 in × 8 ft red oak at $6.50 per board foot.

1. Each board: 1 × 8 × 8 ÷ 12 = 5.33 bf.
2. Three boards: 16 bf.
3. Cost: 16 × $6.50 = $104.

How much extra to buy

Allow for defects, knots and offcuts. Many woodworkers add 15–20% for straightforward projects and up to 30% when grain matching matters.

Use the calculator

The Board Feet Calculator handles multiple boards and pricing at once. Building a deck instead? See the Deck Calculator.