# Decking Calculator

> Calculate the exact number of decking boards, joists, and screws needed for your project. Covers joist spacing, board gaps, and a full materials checklist.

Canonical: https://renoquant.com/decking-calculator
Prices updated: 2026-03

## At a glance

A 3×3 m deck with 145 mm boards and 5 mm gaps takes 20 boards across 8 joists — about 320 outdoor-rated screws (2 per board per joist) plus 10% waste. Joist spacing is 400 mm centres for softwood and 450 mm for composite (300 mm under diagonal layouts or heavy traffic). Composite boards expand more in heat, so use 6–8 mm gaps; softwood needs 5 mm. European material cost runs EUR 30–60 per m² for treated softwood and EUR 80–180 per m² for composite.

## 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](/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](/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](/exterior-paint-calculator) can estimate the quantity for deck staining and sealing.

## Frequently Asked Questions

### How many decking boards do I need?

Divide the deck width by the board width plus the gap width. For 145mm boards with a 5mm gap: a 3m-wide deck needs 3000 / 150 = 20 boards. Then multiply by the deck length divided by board length if boards do not span the full length. Always add 10% for waste.

### What is the correct joist spacing for decking?

Standard joist spacing is 400mm centre-to-centre for softwood decking and 450mm for composite. For diagonal decking or heavy-traffic areas, reduce spacing to 300mm. Closer spacing reduces board flex and bounce underfoot.

### How big should the gap between decking boards be?

Leave a 5-8mm gap between boards for drainage and expansion. Use a 5mm spacer for treated softwood and 6-8mm for composite decking, which expands more with temperature changes. Without proper gaps, water pools on the surface and boards can buckle.

### What are the standard decking board sizes?

The most common softwood decking board is 32mm x 150mm (finished size around 28mm x 145mm) in lengths of 2.4m, 3.0m, 3.6m, and 4.8m. Composite boards are typically 25mm x 140mm or 25mm x 150mm in lengths up to 4m.

### How many screws do I need for decking?

Use 2 screws per board per joist. A 3m x 3m deck with 20 boards and 8 joists needs 320 screws (20 x 8 x 2). Buy a box of 200 stainless-steel or coated decking screws per 5 m2 of deck area. Always use screws rated for outdoor use — standard interior screws corrode within months.

### Do I need a weed membrane under decking?

Yes. A weed membrane under the subframe prevents weeds from growing through the deck and makes maintenance easier. Lay it over compacted ground or gravel before building the frame. It costs EUR 1-2 per m2 and saves significant effort in the long run.

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Interactive version with calculator: https://renoquant.com/decking-calculator. Figures are estimates; final quotes vary by site conditions, materials, and region.