Deck Board Calculator — Running Metres, Boards & Waste
Calculate the exact number of decking boards and running metres — or linear feet — needed for any deck size. Enter deck area, board width and gap size to get board count, total length and recommended pack quantities — for timber or composite decking.
How many decking boards do I need?
Divide the deck width by the board width plus the gap — the gap is part of the module, and leaving it out is the usual reason an order falls short. A 120 mm board at 5 mm covers 125 mm per row; a 5½ in board at ¼ in covers 5¾ in. Rows × deck length gives running metres or feet, then add 7–10% for cuts.
How much gap should I leave between boards?
4–6 mm, or about ¼ in, for timber laid dry — it will shrink as it seasons, so a tight joint opens up rather than the other way round. Composite is the opposite and moves with heat: follow the manufacturer's figure, usually 5–8 mm, and leave more on a long run in full sun.
Deck type
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Shopping list — Raised deck
Decking boards and cladding
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How the deck board calculator works
- 1Effective board coverage = board width (mm) + gap width (mm)
- 2Boards per row = deck width ÷ effective coverage (round up)
- 3Total running metres, or linear feet, = boards per row × deck length × waste factor
- 4Waste factor: 10% for straight lay, 15% for diagonal lay
- 5Board count depends on the board length entered in the shared deck calculator; otherwise divide the total running length by your stock board length — 4.8 m, or 16 ft — and round up
- 6Add fascia, starter boards or trim separately where your deck detail requires them; the shared deck calculator estimates the main decking material take-off
Worked example
Deck: 4 m wide × 6 m long (13 × 20 ft). Boards: 28×120 mm with 5 mm gap, 4.8 m (16 ft) board length.
Effective coverage per board: 125 mm, a 5 in cover. Rows: 4000 ÷ 125 = 32. Running metres: 32 × 6 = 192 m + 10% = 211 m — 692 linear feet. Boards: 211 ÷ 4.8 = 44 boards.
Frequently asked questions
What gap should I leave between deck boards?
4–6 mm (⅛–¼ in) is standard for timber decking boards — this allows drainage and seasonal movement. Composite decking manufacturers typically specify 5–8 mm (¼–⅜ in). Use spacers for consistent gaps during installation.
How many deck boards do I need for a 20 m² deck?
For 120 mm wide boards with a 5 mm gap (125 mm effective width, a 5 in cover) and straight lay: approximately 8.8 running metres per m² (8.0 lm/m² × 1.10 waste factor) — about 2.7 linear feet per square foot — so roughly 176 running metres, or 577 linear feet, for 20 m². Divide by your board length to get board count.
Should I use 28 mm or 38 mm decking boards?
28 mm boards are standard for joists at 400 mm centres — 16 in OC. 38 mm boards allow joist spacing up to 600 mm (24 in OC), reducing the number of joists but costing more per board. For decks with diagonal boards, use joists at 300 mm (12 in) centres regardless of thickness.
Do I need to leave a gap at the end of deck boards?
Yes — leave 5–10 mm (¼–⅜ in) between board ends and any abutting structure (house wall, steps). This prevents moisture trapping and allows the boards to expand without buckling.
How much timber do I need for deck joists?
Use the deck calculator above to get joist quantities. As a rule: joists at 400 mm centres — 16 in OC — on a 6 m (20 ft) wide deck need 16 joists (including end joists). Use the framing lumber calculator to convert joist count to total running length.
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