Deck Calculator — Boards, Joists, Posts & Footings
Calculate all the materials for a timber or composite deck: decking boards, joists, posts, beam sizes and concrete footings. Works for ground-level, raised and sloped decks — supports metric and imperial dimensions.
How the deck calculator works
- 1Deck area = length × width (m²)
- 2Board running metres = deck area ÷ (board width + gap); board count = running metres ÷ board length, rounded up with waste
- 3Joist count: deck length ÷ joist spacing, plus one (round up)
- 4Post count: determined by span and post spacing grid
- 5Footing volume per post: π × (diameter/2)² × depth below frost line
- 6Beam sizing based on span and tributary load (follows standard span tables)
Worked example
Deck: 4 m × 6 m = 24 m². Decking boards 28×120 mm with 5 mm gap, joists at 400 mm centres.
You need 206 running metres of decking boards, 16 joists at 4 m length, 6 posts and approximately 0.35 m³ of concrete for footings.
Frequently asked questions
What joist spacing should I use for a deck?
400 mm centres is standard for 28 mm decking. Use 600 mm only with 38 mm or thicker boards. Closer spacing (300 mm) is recommended for diagonal board layouts.
How deep should deck footings be?
Below the frost line — typically 600 mm in southern regions to 1200 mm in colder climates. Always check local building codes. Use the calculator to estimate concrete volume per footing.
How many decking boards do I need for 20 m²?
For 120 mm wide boards with a 5 mm gap, the effective coverage per board is 125 mm. You need 160 running metres of board per 20 m² at standard 1 m board length — adjust for your board length.
What is the best gap between deck boards?
4–6 mm is standard for timber decking. This allows drainage and air movement while preventing trip hazards. Composite decking manufacturers typically specify 5–8 mm.
Do I need a permit to build a deck?
In most jurisdictions, decks over 30 cm from the ground or attached to the house require a permit. Check your local planning authority before starting.