How to Calculate Board Feet: Formula, Examples, Cost
The unit hardwood lumber is sold in, and how to work it out for any board
If you have bought hardwood from a lumberyard you have seen prices quoted "per board foot" rather than per piece or per linear foot. It is a unit of volume, and it lets a yard price a 4-inch-wide offcut and a 12-inch-wide plank on the same basis. Once you know the formula, estimating a project's lumber bill takes a minute. The Board Feet tab does the arithmetic for you, but understanding it helps you spot a quote that looks wrong.
What a board foot is
One board foot is the volume of a board 1 inch thick, 12 inches wide and 12 inches long: 144 cubic inches. A piece of that size is a square foot of board one inch thick. Everything else is a multiple of it.
The formula
With thickness and width in inches and length in feet:
Board feet = (thickness in × width in × length ft) ÷ 12
This is the formula the calculator displays. If you have all three dimensions in inches instead, divide by 144. To use it, multiply the three numbers and divide by 12, then multiply by the number of identical pieces.
Worked examples
| Board | Working | Board feet |
|---|---|---|
| 1 in × 6 in × 8 ft | (1 × 6 × 8) ÷ 12 | 4.0 |
| 2 in × 8 in × 10 ft | (2 × 8 × 10) ÷ 12 | 13.33 |
| 0.75 in × 5.5 in × 6 ft | (0.75 × 5.5 × 6) ÷ 12 | 2.06 |
| Four pieces of 1 in × 4 in × 3 ft | (1 × 4 × 3) ÷ 12 × 4 | 4.0 |
The last row shows why quantity matters: four small pieces still add up to four board feet, and the yard will charge for the total even if you only need part of each board.
Nominal versus actual thickness
Lumber is sold by its rough, nominal size, not by what you hold in your hand after it has been planed. Hardwood is sold in quarters of an inch: 4/4 (pronounced "four-quarter") is 1 inch rough, 5/4 is 1.25 inches, 6/4 is 1.5 inches and 8/4 is 2 inches. After surfacing, a 4/4 board usually ends up about 13/16 to 7/8 of an inch thick, yet you are still charged as for 1 inch. The calculator takes whatever thickness you type, so for buying use the nominal figure, and for design use the finished one.
Softwood framing lumber is a different story. A "2×4" actually measures about 1.5 × 3.5 inches, and softwood from home centers is typically priced per piece or per linear foot, not per board foot. If you are comparing prices across the two, convert to a board-foot basis using the nominal size.
Turning board feet into a budget
Price per board foot varies widely with species, grade and region, so use a quote from your own supplier. The Lumber & Cuts tab combines three inputs: board feet, price per board foot and a waste factor. The calculator multiplies the board feet by one plus the waste percentage and then by the price.
For the 13.33 board-foot 8/4 piece above at a hypothetical price of $8.00 per board foot and 15% waste:
- Base cost: 13.33 × $8.00 = $106.67
- With waste: 13.33 × 1.15 = 15.33 board feet, and 15.33 × $8.00 = $122.67
The price in this example is made up to show the arithmetic; plug in your own.
How much waste should you add?
Waste covers defects you cut around, saw kerf, end checks, mistakes and boards bought wider or longer than you need. Many woodworkers add 10 to 15 percent for straightforward projects, and 20 percent or more for rough lumber, figured or defect-prone species, or work where grain matching matters. These are rules of thumb, not standards. A project cut from long clear boards wastes less than one made from short, knotty stock.
Common mistakes
- Mixing units. Entering length in inches into a formula that expects feet gives a result 12 times too large. In the calculator, the length field changes to match the unit setting.
- Forgetting quantity. Calculate one piece, then multiply by the count, or enter the quantity.
- Using finished thickness for buying. You pay for rough thickness, so a 3/4-inch finished part usually comes from 4/4 stock.
- Not rounding up. Yards sell by the piece and often in 1-foot or 2-foot increments of length. Round your total up.
Metric
If you work in millimeters, the calculator converts everything to inches and feet before applying the formula. Board feet remain the unit the lumber trade uses in North America, but the metric equivalent in many countries is the cubic meter. One cubic meter is about 424 board feet.
Next, see how to turn a parts list into a shopping list in cut lists, kerf and waste.