or anyone working with lumber—from a professional woodworker building custom furniture to a DIYer tackling a backyard deck—understanding the board foot is non-negotiable. Unlike simple linear measurements, a board foot calculates the volume of lumber, ensuring you buy the right amount and pay a fair price. This guide will demystify the board foot, providing you with clear formulas, practical examples, and the tools you need to master this essential skill.
What is Board Foot?
A board foot is the standard unit of measurement for lumber in North America. It represents a volume of wood that is 1 inch thick, 12 inches wide, and 12 inches long—equivalent to 144 cubic inches.
- Board Feet vs. Square Feet: This is a critical distinction. Square feet measure area (length x width), while board feet measure volume, accounting for thickness. A 1-inch thick board that covers 1 square foot contains 1 board foot. However, a 2-inch thick board covering that same 1 square foot contains 2 board feet.
- Why Divide by 144? The number 144 represents the number of cubic inches in a single board foot (12 in x 12 in x 1 in). When all your measurements are in inches, you are calculating total cubic inches. Dividing by 144 converts that cubic inch volume into the standard unit of board feet.
Basic Board Foot Formula
The formula for calculating board feet changes slightly depending on whether your length measurement is in feet or inches. Use the one that is most convenient for your task.
Primary Formula (Length in Feet)
This is the most common formula, as lumber length is typically quoted in feet.
Board Feet (BF) = (Thickness (in) x Width (in) x Length (ft)) / 12
Alternative Formula (All Measurements in Inches)
Use this when you have precise measurements in inches.
Board Feet (BF) = (Thickness (in) x Width (in) x Length (in)) / 144
Both formulas yield the same result. The first formula simply incorporates the conversion from inches to feet within the calculation.
How to Calculate Board Feet for Common Lumber Sizes
Lumber is often referred to by its “nominal” size (like 2×4), which is its rough-cut dimension before being planed and dried to its smaller “actual” size. For accurate board foot calculations, you should use the actual dimensions. A nominal 2×4, for instance, actually measures approximately 1.5 inches by 3.5 inches.
Let’s calculate with actual dimensions:
Example 1: How many board feet in a 2x4x8?
- Thickness: 1.5 in
- Width: 3.5 in
- Length: 8 ft
- Calculation: (1.5 in x 3.5 in x 8 ft) / 12 = (42) / 12 = 3.5 Board Feet
Example 2: How many board feet in a 2x4x10?
- Thickness: 1.5 in
- Width: 3.5 in
- Length: 10 ft
- Calculation: (1.5 in x 3.5 in x 10 ft) / 12 = (52.5) / 12 = 4.375 Board Feet
Example 3: How many board feet in a 2x2x12?
- Thickness: 1.5 in
- Width: 1.5 in
- Length: 12 ft
- Calculation: (1.5 in x 1.5 in x 12 ft) / 12 = (27) / 12 = 2.25 Board Feet
Using a Board Foot Calculator or App
For projects involving multiple boards or complex calculations, a board foot calculator app or online tool is a huge time-saver. These tools are also essential for loggers and sawyers using a log board foot calculator (like the Doyle or Scribner Log Rules) to estimate the yield from a log.
How to use them:
- Enter the thickness, width, and length of your lumber.
- Specify the units (inches or feet).
- The calculator instantly returns the total board footage.
Many advanced calculators also allow you to input a price per board foot to get an immediate cost estimate.
Board Foot Chart
A board foot chart provides a quick reference for determining the board footage in a single linear foot of a given board size. To use it, find the intersection of the thickness and width, then multiply that value by the total length of your board in feet.
| Thickness (in) | Width (in) | Board Feet per Linear Foot |
|---|---|---|
| 1″ (4/4) | 4″ | 0.33 |
| 1″ (4/4) | 6″ | 0.5 |
| 1″ (4/4) | 8″ | 0.67 |
| 1.5″ (Nominal 2x) | 3.5″ | 0.44 |
| 1.5″ (Nominal 2x) | 5.5″ | 0.69 |
| 1.25″ (5/4) | 6″ | 0.625 |
Example: A 5/4×6 board that is 8 feet long: 0.625 BF/LF x 8 ft = 5 Board Feet.
How to Calculate Board Foot Price
Lumber, especially hardwoods and specialty woods, is often sold by the board foot. Calculating the cost is straightforward once you know the total volume.
Formula:
Total Cost = Total Board Feet x Price Per Board Foot
Example: If you need 25 board feet of cherry wood priced at $6.50 per board foot, your total cost would be 25 BF x $6.50/BF = $162.50.
Always confirm the price unit before purchasing to avoid surprises, and factor in an extra 10-15% for waste and cutting errors.
Converting Between Board Feet and Square Feet
You cannot directly convert board feet to square feet without knowing the thickness. Once the thickness is known, the conversion is simple.
- To convert Board Feet to Square Feet:
Square Feet = (Board Feet x 12) / Thickness (in) - To convert Square Feet to Board Feet:
Board Feet = (Square Feet x Thickness (in)) / 12
Example: Converting 650 board feet to square feet for 1-inch thick lumber:
Square Feet = (650 BF x 12) / 1 in = 7,800 square feet.
The same 650 board feet of 2-inch thick lumber would only cover (650 BF x 12) / 2 in = 3,900 square feet.
Common Questions About Board Feet
Why are board feet divided by 144?
As explained, a single board foot is defined as 144 cubic inches. When you calculate Thickness x Width x Length in inches, your result is in cubic inches. Dividing by 144 is the conversion factor to translate cubic inches into the standard unit of board feet.
What is the difference between nominal and actual sizes?
Nominal size (e.g., “2×4”) is the name of the board when it is first cut from the log. The actual size (e.g., “1.5” x 3.5″”) is its true dimension after drying and planing. Always use actual dimensions for precise board foot calculations.
How do I calculate for 4/4, 5/4, or 6/4 lumber?
The “quarter” system is a common way to describe rough-sawn lumber thickness. Each “quarter” represents 1/4 of an inch.
- 4/4 = 1 inch thick
- 5/4 = 1.25 inches thick
- 6/4 = 1.5 inches thick
Simply plug these decimal thicknesses directly into the board foot formula.
Practical Tips and Common Mistakes When Calculating Board Feet
- Measure Actual Dimensions: Never assume a 2×4 is actually 2 inches by 4 inches. Use calipers or a tape measure for accuracy.
- Convert Units Consistently: The most common error is mixing units without conversion. Ensure all measurements are either in feet or inches before plugging them into the correct formula.
- Account for Waste: Always add 10-20% to your total board foot calculation to cover cutting mistakes, planer snipe, and natural defects in the wood.
- Double-Check Your Math: A simple arithmetic error can lead to a significant over- or under-purchase, costing time and money.
Conclusion
Mastering the board foot calculation is a fundamental skill that brings confidence and efficiency to any woodworking or construction project. By understanding the formulas, using calculators and charts when helpful, and remembering to use actual dimensions, you can accurately estimate your lumber needs and costs. Bookmark this guide for your next project, and never second-guess a lumber order again.