Decking Calculator
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How to Calculate Decking Boards, Joists and Screws
An accurate materials list saves you money and avoids mid-project supply runs. This guide walks you through the board count formula, joist spacing requirements, screw quantities, and the full checklist of everything you need to build a deck. If you are also putting up a boundary around the garden, our fence calculator helps you work out panels, posts, and fixings.
The Boards-Needed Formula
Boards across the width = Deck width (mm) / (Board width + Gap width), rounded up
For a 3m (3000mm) wide deck using 145mm boards with 5mm gaps:
3000 / (145 + 5) = 3000 / 150 = 20 boards
If each board is 3.6m long and the deck is 3m deep, each board spans the full depth with some trimming. If the deck is longer than the board length, you will need multiple boards per row, joined over a joist.
Always add 10% to the board count for waste from cuts, defective boards, and future repairs.
Joist Spacing Table
| Decking Material | Recommended Spacing | Maximum Spacing | Board Thickness |
|---|---|---|---|
| Treated softwood | 400mm | 450mm | 28-32mm |
| Hardwood | 450mm | 500mm | 19-25mm |
| Composite (solid core) | 400mm | 450mm | 22-25mm |
| Composite (hollow core) | 300mm | 350mm | 22-25mm |
| Diagonal softwood layout | 300mm | 350mm | 28-32mm |
Closer joist spacing makes the deck feel more solid underfoot. If in doubt, go with 400mm — it works for all material types and handles foot traffic, furniture, and the occasional hot tub without issues.
Board Size Options
| Board Type | Finished Size | Common Lengths | Coverage per Board (with 5mm gap) |
|---|---|---|---|
| Softwood (standard) | 28mm x 145mm | 2.4m, 3.0m, 3.6m, 4.8m | 0.54 m2 (at 3.6m length) |
| Softwood (wide) | 28mm x 195mm | 3.0m, 3.6m, 4.8m | 0.72 m2 (at 3.6m length) |
| Hardwood (standard) | 19mm x 140mm | 2.1m, 2.7m, 3.3m | 0.48 m2 (at 3.3m length) |
| Composite (standard) | 25mm x 140mm | 3.0m, 3.6m, 4.0m | 0.52 m2 (at 3.6m length) |
| Composite (wide) | 25mm x 190mm | 3.0m, 3.6m, 4.0m | 0.70 m2 (at 3.6m length) |
Gap Width Guide
| Material | Minimum Gap | Recommended Gap | Why |
|---|---|---|---|
| Treated softwood | 4mm | 5-6mm | Moderate expansion, drainage |
| Hardwood | 5mm | 6-7mm | Dense wood expands less but drainage is important |
| Composite | 5mm | 6-8mm | Higher thermal expansion than timber |
| Near a wall | 10mm | 10-15mm | Allows for board expansion toward the building |
Gaps serve two purposes: drainage and expansion. Water must drain between boards quickly to prevent pooling and algae growth. Timber and composite materials expand when warm and damp, so gaps prevent boards from buckling against each other.
Complete Materials Checklist
For a 3m x 3m deck (9 m2) using treated softwood boards:
| Item | Quantity | Purpose |
|---|---|---|
| Decking boards (28mm x 145mm x 3.6m) | 22 (20 + 10% waste) | Deck surface |
| Joists (47mm x 150mm x 3.0m) | 8 (at 400mm spacing + outer pair) | Subframe support |
| Bearer beams (47mm x 150mm x 3.6m) | 3 | Span under joists, on blocks |
| Concrete deck blocks | 12 (4 per bearer) | Foundation pads (use our concrete calculator for poured pads) |
| Decking screws (65mm, coated) | 400 (2 per board per joist) | Fixing boards to joists |
| Coach bolts (M10 x 100mm) | 24 (for joist-to-bearer connections) | Structural fixings |
| Weed membrane | 10 m2 | Weed suppression under deck |
| Gravel (10mm pea shingle) | 0.5 tonne | Drainage base |
| Joist hangers (if wall-mounted) | 8 | Connecting joists to ledger board |
| Fascia board | 6m linear | Trim around deck edges |
Planning Your Deck Layout
Before ordering materials, decide on the board direction. Boards running parallel to the house wall are the simplest and most economical layout — fewer cuts and no angled waste. A diagonal layout creates a more dynamic look but increases waste by 15-20% and requires closer joist spacing (300mm).
For best drainage, orient boards so they slope very slightly (1-2%) away from the house. This is usually achieved by setting the joists at a slight fall rather than shimming individual boards. Once the deck is built, protect the timber with an exterior stain or paint — our exterior paint calculator can estimate the quantity for deck staining and sealing.
These calculations are estimates only. Actual requirements may vary depending on surface conditions, product specifications, and installation methods. Always consult a qualified professional for precise measurements.
Prices updated: 2026-03Frequently Asked Questions
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