What is the Deck Board Calculator?
The Deck Board Calculator estimates how many decking boards and screws you need to cover a rectangular deck. By accounting for the gap between boards and a waste allowance for cuts and defects, it gives a realistic shopping list before you head to the lumber yard.
How to use it
Enter your deck's length and width in feet. Then enter the width of a single board in inches (a nominal "2x6" deck board is about 5.5 in actual), the board length in feet, the gap you want between boards (commonly 1/8 in to 1/4 in for drainage), and a waste allowance percentage (10% is typical). The calculator returns the number of boards, your total deck area, and an approximate screw count.
The formula explained
First the deck area is found: \(\text{area} = \text{length} \times \text{width}\). Each board covers a strip whose effective width is the board width plus the gap, converted from inches to feet: \(\frac{w + g}{12}\). Multiplying by the board length gives the coverage area of one board. Dividing the deck area by this coverage and rounding up gives the base board count. Finally we add the waste allowance and round up again.
$$\text{Boards} = \left\lceil \left\lceil \frac{A}{C} \right\rceil \cdot \left(1 + \frac{\text{Waste \%}}{100}\right) \right\rceil$$ $$\text{where}\quad \left\{ \begin{aligned} A &= \text{Deck Length (ft)} \times \text{Deck Width (ft)} \\ C &= \frac{\text{Board Width (in)} + \text{Gap (in)}}{12} \times \text{Board Length (ft)} \end{aligned} \right.$$ $$\text{Screws} = \left\lceil 3.5 \times \text{Deck Length (ft)} \times \text{Deck Width (ft)} \right\rceil$$
Worked example
A 16 ft × 12 ft deck = 192 sq ft. Using 5.5 in boards with a 0.25 in gap, the effective width is \((5.5 + 0.25) / 12 = 0.4792\) ft. With 16 ft boards, one board covers \(0.4792 \times 16 = 7.667\) sq ft. So \(192 / 7.667 = 25.04\), rounded up to 26 boards. Adding 10% waste gives \(26 \times 1.10 = 28.6 \to\) 29 boards. Screws \(\approx 192 \times 3.5 =\) 672.
FAQ
What gap should I use? Most pressure-treated and composite decks use a 1/8 in to 1/4 in gap so water drains and boards can expand.
How many screws per board? The estimate uses about 350 screws per 100 sq ft (two screws per joist intersection at 16 in centers). Buy a little extra.
Does this handle angled or odd-shaped decks? It assumes a rectangle. For complex shapes, split the deck into rectangles and add the results.
