Deck Calculator
Building a deck? Enter the size in feet or metres, your board width and gap, and the cost per square foot to find out how many decking boards you need — and what it will cost.
Estimated Deck Cost
How It Works
A deck is a rectangle of boards laid side by side, so the maths starts with the area: length × width. Then the calculator works out how many boards fit across the width — the width divided by one board plus its gap — and rounds up, because a part-board still needs a full board. Each board runs the full deck length, so total board length is the board count times the length, and estimated cost is the area times your cost per square foot.
The gap between boards matters more than it looks. Boards swell when wet and shrink when dry, so a small gap — a quarter of an inch is typical — lets water drain and stops boards buckling. The gap also changes the board count: wider gaps mean fewer boards across the same width.
Worked Example
A 16 ft × 12 ft deck is 16 × 12 = 192 sq ft. Boards 5.5 inches wide with a 0.25-inch gap each take up 5.75 inches across the width. The 12-foot width is 144 inches, so 144 ÷ 5.75 = 25.04, meaning 26 boards. Each board is 16 ft long, so the total decking is 26 × 16 = 416 linear feet, and at $15 per sq ft the estimated cost is 192 × $15 = $2,880.
Assumptions
- Boards run the full deck length in single pieces — no joins.
- The deck is a single rectangle; L-shaped decks need measuring in two parts.
- The gap is the same between every pair of boards.
- The cost covers decking only as you define the rate — framing, footings, railing and stain are extra.
How to Use This Calculator
Choose imperial (feet and inches) or metric (metres and centimetres) first — switching converts your values. Enter the deck length and width, the board width and gap, and your cost per square foot. Results update live, showing the area, board count, total board length and estimated cost.
Tips and Practical Advice
Order a few spare boards — a warped board in the pile is common. Let timber boards acclimatise on site for a few days before fixing. And check the joist spacing under the deck — most decking needs joists no more than 16 inches apart. A deck that feels bouncy is usually a framing problem, not a board problem.
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