Estimating Deck Boards
Building a new deck is one of the highest ROI (Return on Investment) exterior home improvement projects you can undertake. However, ordering the decking materials is often where DIYers and even seasoned contractors make expensive mistakes.
Deck boards (the actual surface you walk on) are typically sold by the linear foot, but deck sizes are measured in square feet. Converting between the two requires accounting for the exact width of the boards you choose and the necessary gaps between them.
The Math Behind Deck Board Calculation
To figure out how many linear feet of decking you need, you first have to determine how much physical surface area a single linear foot of your chosen board actually covers.
Understanding Nominal vs. Actual Width
Lumber is sold in "nominal" sizes, not actual sizes. For example, a standard 5/4x6 deck board is nominally 6 inches wide, but its actual width is 5.5 inches.
Furthermore, you cannot install deck boards tightly against each other. They must have a gap (usually 1/8 to 1/4 inch) to allow rain to drain and the wood to expand and contract with the weather. Therefore, a 5.5-inch board with a 0.25-inch gap covers 5.75 inches of deck space.
The Formula
- Find the Total Square Footage of your deck by multiplying Length × Width.
- Determine the Effective Board Width by adding your board's actual width to your desired gap size (e.g., 5.5 in + 0.25 in = 5.75 in).
- Convert the Effective Board Width into feet by dividing by 12.
- Divide the Total Square Footage by the Effective Board Width (in feet) to get the total Linear Feet required.
Linear Feet = (Deck Length × Deck Width) ÷ ((Board Width + Gap) ÷ 12)
Example Calculation
Imagine you are building a 16x12 foot deck. You are using standard treated pine boards (5.5 inches actual width) and leaving a 1/4-inch gap.
- Total Square Footage:
16 × 12 = 192 sq ft. - Effective Board Width:
5.5 + 0.25 = 5.75 inches. - Effective Width in feet:
5.75 ÷ 12 ≈ 0.479 ft. - Total Linear Feet:
192 ÷ 0.479 = 400.8 linear feet.
The Rule of Waste
If you perfectly mapped out exactly 400.8 linear feet, you would undoubtedly run short. This is because boards must end in the center of a floor joist (to be screwed down). If a board overhangs a joist by 6 inches, you have to cut it off and throw that piece away.
Additionally, lumber is imperfect. You will encounter warped boards, split ends, or massive knots that must be cut out. The industry standard is to add 10% to 15% waste factor to your final linear foot calculation. If your deck has angled corners or a diagonal decking pattern, you should increase that waste factor to 20%.