Deck Calculator
Estimate decking boards, joists and materials cost for a deck.
About the Deck Calculator
Enter the deck's length (the direction the boards run) and width, the board width, the gap between boards, the joist spacing and a materials cost per square unit. The calculator gives the area, the rows of boards, the total board length, the joists and the cost.
Boards run across the joists. Joists are typically 400 mm (16 in) apart, closer (300 mm / 12 in) for composite or diagonal boards. Leave 3–6 mm gaps for drainage and expansion.
The formula
Rows = ⌈width ÷ (board + gap)⌉; joists = ⌊length ÷ spacing⌋ + 1
Worked example
A 4.8 × 3.6 m deck with 14 cm boards, 5 mm gaps, joists every 40 cm
- Rows: 3.6 ÷ 0.145 = 24.8 → 25 rows, 120 m of board
- Joists: 4.8 ÷ 0.4 + 1 = 13, each 3.6 m long
- Area 17.3 m², about 864 of materials at 50 a m²
Things to keep in mind
- Posts, beams, fixings and footings aren't counted; check span tables for joist sizes.
Questions people ask
How far can joists span?
Depends on size and timber grade; check span tables in your building code.
Do I need permission?
Raised or large decks often need a permit.
How much extra board should I buy?
About 10% for cuts and defects, more for diagonal layouts.