Deck Board Calculator
Area (sq ft)
120.0
Boards needed
22
How it works
Calculating the number of deck boards for a timber deck requires accounting for board width, gap spacing between boards, joist direction, and waste from cuts at deck edges. Over-buying is costly; under-buying causes mid-build delays. The Deck Board Calculator handles rectangular and L-shaped decks, outputs board count and linear metres, and estimates waste.
**Board layout** Deck boards run perpendicular to joists. Standard board widths: 90mm, 140mm, 145mm (after planing), or imperial 2×4 (89mm actual), 2×6 (140mm). Gap between boards: 5–8mm (allows drainage, accounts for wood expansion). Effective spacing = board width + gap.
**Calculation** Number of boards = ceil(deck_width / (board_width + gap)). For a 4m wide deck with 140mm boards and 6mm gaps: 4000 / (140 + 6) = 4000 / 146 = 27.4 → 28 boards. Linear metres = 28 × deck_length. Add 10–15% waste for end cuts, defect removal, and board selection.
**Fascia boards** Fascia boards (face-mounted on the perimeter) require separate calculation: 2 × length + 2 × width (for a rectangular deck perimeter), adjusted for corner overlaps. Many builders use the same deck board species for fascia.
**Board orientation angle** Diagonal decking (45° boards) is more visually appealing but requires ~15–20% more material (longer diagonal span, more end cuts). The calculator supports 90°, 45°, and custom angle orientations.
**Material types** Softwood (treated pine): cheapest, requires regular staining. Hardwood (ipe, cumaru): expensive, dense, very durable (25+ year lifespan). Composite: no maintenance required, higher initial cost, available in hollow or solid profiles with different spanning capabilities.
Privacy: all calculations run in the browser. No data is transmitted.
Frequently Asked Questions
- Gap recommendations: wet or green timber (will shrink as it dries): 3–5mm or even butted tight (it will open as it dries). Dry kiln-dried timber: 5–8mm (allows water drainage, prevents debris accumulation, accounts for minimal expansion). Composite decking: follow manufacturer specifications — typically 5–6mm for most products (composite expands less than wood but still expands, especially in sun-exposed conditions). The gap serves two functions: water drainage (pooling water rots timber faster) and ventilation (air flow beneath the deck accelerates drying after rain, reducing rot risk).
- Deck boards typically run perpendicular to the house and parallel to the main viewing direction — this visually makes the deck appear wider. Running boards toward the house (perpendicular to the facade) makes the space appear longer and deeper. For safety: boards should always run perpendicular to the joists (so every board spans across multiple joists). Never run boards parallel to joists — this leaves spans unsupported. For diagonal decking: joists should be placed 25–30% closer together (to compensate for the longer board span across the diagonal) than the standard 400mm or 600mm spacing.
- Treated softwood (pressure-impregnated pine/spruce): cheapest option, requires staining every 2–3 years, 15–25 year lifespan if maintained. Cedar/redwood: naturally rot-resistant, attractive grain, more expensive than treated pine, needs oiling every 2–3 years. Hardwood (ipe, cumaru, garapa): expensive, very dense (may need pre-drilling), extremely durable (25–50+ year lifespan), natural oils provide inherent rot resistance. Composite (Trex, Millboard, Cedral): plastic/wood fibre composite, no maintenance needed, 25–30 year warranties, higher initial cost, can be slippery when wet unless texture is adequate, limited colour options vs. natural wood.
- Joist spacing depends on deck board thickness and span capability: 19mm (¾ inch) composite: max 400mm joist spacing. 25mm (1 inch) softwood: max 400mm spacing. 38mm (1½ inch) hardwood: max 500–600mm spacing. 47mm (2×4 softwood): can span 600mm (24 inch). For diagonal decking: reduce joist spacing by approximately 25% — boards span a longer distance at 45° than at 90°. Always check manufacturer specifications for composite products — they may have specific joist spacing requirements that void the warranty if not followed.