Board Foot Calculator
Calculate Board Foot
What is a Board Foot?
A board foot (BF or FBM for foot, board measure) is a unit of volume for lumber in the United States and Canada. It represents the volume of a piece of wood that is one foot long, one foot wide, and one inch thick before surfacing. More precisely, one board foot is equivalent to 144 cubic inches (1 ft × 1 ft × 1 in = 12 in × 12 in × 1 in = 144 cu in).
Woodworkers, lumber yards, and anyone dealing with rough-sawn or hardwood lumber use the board foot to measure and price wood. It’s especially common for hardwoods, while softwoods are often sold by linear foot or by the piece after being milled to standard dimensions. Understanding how to calculate board foot is crucial for estimating project costs and material needs.
Common misconceptions include thinking a board foot is a linear measure or that it always refers to the actual dimensions after planing and jointing. The board foot calculation is typically based on the nominal (rough-sawn) dimensions, although actual dimensions are used when they are known for surfaced lumber.
Board Foot Formula and Mathematical Explanation
The standard formula to calculate board foot when the length is given in feet is:
Board Foot (BF) = (Thickness (inches) × Width (inches) × Length (feet)) / 12
If the length is given in inches, the formula is:
Board Foot (BF) = (Thickness (inches) × Width (inches) × Length (inches)) / 144
The division by 12 (or 144) is necessary to convert the volume from (inches × inches × feet) or (inches × inches × inches) into the standard board foot unit, which is 144 cubic inches.
Variables Table:
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| T | Thickness | Inches | 0.25 – 8+ (often 1-4 for common lumber) |
| W | Width | Inches | 1 – 12+ (often 2-12 for common lumber) |
| L (feet) | Length | Feet | 1 – 24+ (often 6-16) |
| L (inches) | Length | Inches | 12 – 288+ |
Variables used in the board foot calculation.
Practical Examples (Real-World Use Cases)
Example 1: Calculating Board Feet for a Hardwood Plank
Suppose you are buying a piece of rough-sawn cherry that is 2 inches thick, 8 inches wide, and 10 feet long.
- Thickness (T) = 2 inches
- Width (W) = 8 inches
- Length (L) = 10 feet
Using the formula: BF = (2″ × 8″ × 10′) / 12 = 160 / 12 = 13.33 Board Feet.
If the cherry costs $8 per board foot, this single plank would cost 13.33 * $8 = $106.64.
Example 2: Board Feet for Multiple Boards
You need five pieces of oak, each 1.5 inches thick, 6 inches wide, and 8 feet long.
- Thickness (T) = 1.5 inches
- Width (W) = 6 inches
- Length (L) = 8 feet
- Number of Pieces = 5
Board Feet per piece = (1.5″ × 6″ × 8′) / 12 = 72 / 12 = 6 Board Feet per piece.
Total Board Feet = 6 BF/piece * 5 pieces = 30 Board Feet.
Knowing how to calculate board foot helps you compare prices and ensure you’re getting the volume of wood you expect.
How to Use This Board Foot Calculator
Our board foot calculator is simple to use:
- Enter Thickness: Input the actual thickness of your lumber in inches. For nominal lumber (like a “2×4”), use the actual thickness (e.g., 1.5 inches).
- Enter Width: Input the actual width of your lumber in inches (e.g., 3.5 inches for a “2×4”).
- Enter Length: Input the length of the lumber in feet.
- Enter Number of Pieces: Specify how many identical pieces of lumber you have.
- View Results: The calculator instantly shows the board foot per piece and the total board feet for all pieces, along with cubic inch and cubic foot volumes. The table and chart also update to give you more context.
The results allow you to quickly calculate board foot volume, essential for purchasing and project planning. You can also compare different board sizes to see how their board foot content varies.
Key Factors That Affect Board Foot Results
- Actual vs. Nominal Dimensions: The most significant factor is whether you use actual (surfaced) or nominal (rough) dimensions. Board foot calculations traditionally use nominal dimensions for rough lumber, but for surfaced lumber (S4S), using actual dimensions is more accurate for the wood you receive.
- Thickness: Directly proportional to the board foot volume. Doubling thickness doubles the board feet.
- Width: Also directly proportional. Doubling width doubles the board feet.
- Length: Directly proportional when measured consistently (either all feet or all inches in the respective formulas).
- Waste Factor: When buying lumber for a project, you often need more board feet than the final project volume due to cuts, knots, and defects. Factor in a waste percentage (e.g., 10-20%) on top of the calculated board foot requirement. You might find our project calculator helpful for this.
- Lumber Grade and Species: While not part of the volume calculation, grade (e.g., FAS, Select, No. 1 Common) and species (e.g., Oak, Walnut, Pine) drastically affect the price per board foot.
- Kerf Width: The width of the saw blade (kerf) removes material with each cut, which can add up over many cuts, effectively reducing the usable board foot volume from your initial purchase.
Frequently Asked Questions (FAQ)
- Q: What is the difference between a board foot and a linear foot?
- A: A board foot is a measure of volume (144 cubic inches), while a linear foot is a measure of length (12 inches). Softwood dimensional lumber (like 2x4s) is often sold by the linear foot, while hardwoods are more commonly sold by the board foot.
- Q: How do I calculate board foot for lumber thinner than 1 inch?
- A: For hardwoods thinner than 1 inch (e.g., 3/4″ or 1/2″), the thickness is often rounded UP to 1 inch for the board foot calculation when purchasing. However, our calculator uses the actual thickness you enter for a precise volume. Always clarify with your supplier how they calculate board foot for thin stock.
- Q: What about lumber wider than 12 inches?
- A: The formula remains the same. A board that is 1″ thick, 14″ wide, and 12′ long would be (1 * 14 * 12) / 12 = 14 board feet.
- Q: Is board foot used for plywood or sheet goods?
- A: No, plywood and other sheet goods are typically sold by the square foot of surface area, with the thickness specified.
- Q: Why is board foot based on 144 cubic inches?
- A: Because it represents a board 1 foot (12 inches) wide, 1 foot (12 inches) long, and 1 inch thick: 12″ × 12″ × 1″ = 144 cubic inches.
- Q: How do I measure the thickness and width of rough-sawn lumber accurately?
- A: Rough-sawn lumber can be uneven. Measure at several points and use an average, or be conservative and use the thickest/widest points if you are buying, and the thinnest/narrowest if you are selling or planning for minimal waste after surfacing.
- Q: Does the way I calculate board foot change for different wood species?
- A: No, the volume calculation to calculate board foot is the same regardless of species. However, the price per board foot varies significantly between species like pine, oak, walnut, or exotic woods.
- Q: What does “4/4” or “8/4” lumber mean in relation to board foot?
- A: This notation refers to the nominal thickness in quarters of an inch. 4/4 (four-quarter) is nominally 1″ thick, 5/4 is 1.25″, 6/4 is 1.5″, 8/4 is 2″, etc. You’d use these nominal thicknesses (or the actual measured thickness after drying/surfacing) to calculate board foot.
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