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 many decking boards do I need?
Divide the deck width by the board width plus the gap between boards — the gap is part of the module, and leaving it out is the usual reason an order comes up short. A 145 mm board at a 5 mm gap covers 150 mm per row; a 5½ in board at ¼ in covers 5¾ in. Multiply rows by deck length for running metres or feet, then add 10% for cuts.
How far apart should deck joists be?
As a common estimating profile, 600 mm (24 in) centres suit 28 mm (5/4 in) boards and 400 mm (16 in) centres suit thinner 21 mm stock. Go closer — 300–400 mm, or 12–16 in — for a diagonal or herringbone layout, where the board spans further between bearings. Product data and local span tables override any rule of thumb.
Deck type
Plan view (from above)
Shopping list — Raised deck
Decking boards and cladding
Structural timber
Fasteners and brackets
Foundations
Railing
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How the deck calculator works
- 1Deck area = length × width (m²)
- 2Board running metres — or linear feet — = deck area ÷ (board width + gap); board count = running length ÷ board length, rounded up with waste
- 3Joist count: deck dimension perpendicular to joists ÷ 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 (13 × 20 ft, 258 sq ft). Decking boards 28×120 mm with 5 mm gap, joists at 400 mm centres — 16 in OC.
You need 206 running metres of decking boards — 676 linear feet — plus 16 joists at 4 m (13 ft) length, 6 posts and approximately 0.35 m³ (12.4 cu ft) of concrete for footings.
Frequently asked questions
What joist spacing should I use for a deck?
Joist spacing depends on board thickness, species, product data and local span tables. As a common estimating profile, the calculator uses 600 mm centres (24 in OC) for 28 mm decking and 400 mm centres (16 in OC) for thinner 21 mm boards; use closer spacing such as 300–400 mm, or 12–16 in, for diagonal layouts or where the product/code requires it.
How deep should deck footings be?
Below the frost line — typically 600 mm (24 in) in southern regions to 1200 mm (48 in) 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, a 5 in cover. You need 160 running metres of board per 20 m² at standard 1 m board length, about 2.4 linear feet per square foot — adjust for your board length.
What is the best gap between deck boards?
4–6 mm (⅛–¼ in) is standard for timber decking. This allows drainage and air movement while preventing trip hazards. Composite decking manufacturers typically specify 5–8 mm (¼–⅜ in).
Do I need a permit to build a deck?
In most jurisdictions, decks over 30 cm (12 in) from the ground or attached to the house require a permit. Check your local planning authority before starting.
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