How to Calculate Board Feet: The Complete Lumber Math Guide
Master lumber estimation with our guide on calculating board feet, linear feet, and square feet for woodworking and construction.
Whether you are building a custom dining table or buying hardwood lumber for a carpentry project, lumber yards and sawmills quote prices by the board foot. Understanding how to calculate board feet ensures you buy the correct volume of wood and pay the correct price.
This guide covers the formulas, board foot calculation steps, and how to convert other lumber units.
What is a Board Foot?
A board foot (BF) is a unit of volume measurement for lumber. One board foot represents a volume equivalent to a board that is 12 inches wide, 12 inches long, and 1 inch thick:
$$\text{1 Board Foot} = 12“\text{ wide} \times 12“\text{ long} \times 1“\text{ thick} = 144\text{ cubic inches}$$
Lumber sizes are classified by nominal dimensions (e.g., a “2x4” is actually 1.5 inches by 3.5 inches), but board foot calculations for rough-sawn hardwoods always use the nominal size before planed finishing.
The Board Foot Formula
Depending on how your measurements are taken, use one of the two standard formulas:
Formula 1: Using Length in Feet
If the length of your board is in feet, the thickness in inches, and the width in inches: $$\text{Board Feet} = \frac{\text{Thickness (in)} \times \text{Width (in)} \times \text{Length (ft)}}{12}$$
Formula 2: Using Length in Inches
If all three dimensions (thickness, width, and length) are measured in inches: $$\text{Board Feet} = \frac{\text{Thickness (in)} \times \text{Width (in)} \times \text{Length (in)}}{144}$$
Worked Examples
Example 1: Standard Construction Board
Suppose you have a rough-sawn hardwood board that is 2 inches thick, 8 inches wide, and 10 feet long:
- Using Formula 1: $$\text{Board Feet} = \frac{2 \times 8 \times 10}{12} = \frac{160}{12} = \mathbf{13.33\text{ BF}}$$
Example 2: Woodworking Plank (Inches)
Suppose you have a walnut board that is 1.5 inches thick, 6 inches wide, and 96 inches long:
- Using Formula 2: $$\text{Board Feet} = \frac{1.5 \times 6 \times 96}{144} = \frac{864}{144} = \mathbf{6.00\text{ BF}}$$
Hardwood Thickness Notation (Quarters)
When buying rough-sawn hardwood lumber, sawmills use a “quarters” notation to express thickness:
- 4/4 (four-quarter): 1 inch rough thickness
- 5/4 (five-quarter): 1-1/4 inches rough thickness
- 6/4 (six-quarter): 1-1/2 inches rough thickness
- 8/4 (eight-quarter): 2 inches rough thickness
- 12/4 (twelve-quarter): 3 inches rough thickness
Always substitute the decimal equivalent (e.g., 1.25 for 5/4, 1.5 for 6/4) as the thickness value in your formulas.
Estimating and Buying Tips
- Deduct for Planing and Jointing: Planing rough wood to get smooth faces removes material. A 4/4 board (1“ thick rough) typically finishes out at 3/4“ thick. A 8/4 board finishes at 1-3/4“ thick. Always buy lumber matching the rough thickness you need.
- Add a 15-20% Waste Factor: Hardwood boards are rarely straight and clear of defects. Plan for knots, checks, splits, and grain matching by ordering extra board footage.
- Linear Feet vs. Board Feet: Linear footage only measures the length of the board, ignoring width and thickness. To convert linear feet to board feet: $$\text{Board Feet} = \text{Linear Feet} \times \frac{\text{Thickness (in)} \times \text{Width (in)}}{12}$$