Understanding the Board Foot
If you walk into a big-box hardware store, you buy framing lumber (like a 2x4) by the linear foot. However, if you visit a commercial lumberyard, a sawmill, or buy expensive rough-sawn hardwoods (like oak, walnut, or cherry), the wood is sold by a completely different unit of measurement: the Board Foot (abbreviated as BF or bd ft).
Unlike a linear foot (which only measures length) or a square foot (which measures surface area), a board foot is a measurement of volume.
Because raw hardwood logs are milled into highly irregular thicknesses and widths to maximize the yield of the tree, lumberyards use the board foot to charge you for the exact cubic volume of solid wood you are purchasing.
The Definition of a Board Foot
One board foot is defined as a piece of rough wood that is exactly:
- 1 inch thick
- 12 inches wide
- 1 foot long
This equals exactly 144 cubic inches of wood. Any piece of wood can be converted into board feet based on this volume metric. For example, a piece of wood that is 2 inches thick, 6 inches wide, and 1 foot long is ALSO exactly 1 board foot, because it contains the exact same volume of wood material.
How to Calculate Board Feet
Calculating board feet requires mixing measurements. Thickness and width are measured in inches, while length is typically measured in feet.
The Formula
To find the board footage of a piece of lumber:
- Multiply the Thickness (in inches) × Width (in inches) × Length (in feet).
- Divide that result by 12.
- Multiply by the Quantity (number of pieces) if you are buying a bundle of identically sized boards.
Board Feet = (Thickness × Width × Length) ÷ 12
Example Calculation
You are at a sawmill buying rough-sawn maple for a dining room table. You select 4 beautiful planks. Each plank is 2 inches thick (often called "8/4" lumber), 8 inches wide, and 10 feet long.
- Multiply dimensions:
2 in × 8 in × 10 ft = 160 - Divide by 12:
160 ÷ 12 = 13.33 board feet per plank - Multiply by 4 planks:
13.33 × 4 = 53.33 total board feet
If the mill charges $1.00 per board foot for maple, your total cost will be roughly $1.
The "Quarter" System (4/4, 8/4, 12/4)
When buying hardwoods, you will rarely see thickness listed in inches. Hardwood thickness is expressed in quarters of an inch.
- 4/4 ("Four-Quarter"): Wood that is roughly 1 inch thick.
- 5/4 ("Five-Quarter"): Wood that is roughly 1.25 inches thick.
- 8/4 ("Eight-Quarter"): Wood that is roughly 2 inches thick.
- 12/4 ("Twelve-Quarter"): Massive slabs that are roughly 3 inches thick.
When calculating board feet, simply convert the quarter system back to decimal inches (e.g., use 1.25 inches for 5/4 wood) before running the formula.