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Decking Calculator

Calculate how many deck boards, joists, bearers and fasteners you need for any deck area. Works for hardwood, pine and composite decking. Get quantities and cost estimate instantly.

Quick answer: Deck boards needed = Deck Area ÷ (Board Width − Gap) × Board Width. A 20 m² deck with 90mm boards at 5mm gaps needs 20 ÷ 0.085 × 0.090 ÷ board length boards.

🔨 Decking Calculator

Typical: 90mm, 140mm, 190mm
Deck Boards (lm)
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Number of Boards
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Joists Required
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Timber Cost
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📐 How It Works

Deck board quantity is calculated from the number of rows needed to cover the deck width (at board width plus gap spacing), multiplied by the deck length. Joists are spaced perpendicular to the boards. Add 10% waste for end cuts and defects.

Cover Width = Board Width + Gap Rows = CEILING(Deck Width ÷ Cover Width) Lineal Metres = Rows × Deck Length × 1.10 Joists = CEILING(Deck Length ÷ Joist Spacing) + 1

📋 Worked Example

5m × 4m deck, 90mm boards, 5mm gaps, 4.8m board lengths: Cover width = 95mm. Rows = 4000÷95 = 42.1 → 43 rows. LM = 43 × 5 × 1.10 = 236.5 lm. At $9/lm = $2,129.

❓ Frequently Asked Questions

Hardwood (merbau, spotted gum, blackbutt): durable, attractive, premium price, H3 treatment not required for many species. Treated pine (MGP10, H3 treated): lower cost, widely available, needs maintenance. Composite decking: low maintenance, consistent appearance, higher upfront cost. Choose based on budget, aesthetics and maintenance preference.

Standard gap: 5–6mm for most timber decking. Composite decking: 3–8mm as per manufacturer. Gaps allow water drainage and air circulation which prevents rot. Gaps also allow for timber expansion in wet weather — do not fix boards too tight.

90mm boards: 450mm joist spacing maximum. 140mm boards: 600mm maximum. Composite decking: check manufacturer's specification — typically 400–500mm. In high-traffic or wet conditions reduce spacing for additional rigidity.

Any timber in contact with or close to the ground requires H3 or H4 treatment. Joists, bearers and posts: H3 minimum, H4 for in-ground posts. Deck boards above ground: many hardwood species don't require treatment. Treated pine deck boards: H3 suitable. Always check the treatment level for your exposure conditions.