Decking Calculator

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

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

Gap Width Guide

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:

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.

How Much Does Decking Cost?

Decking costs depend on the material you choose, the deck size, foundation requirements, and whether you build it yourself or hire a professional. This section breaks down every cost so you can budget your project confidently.

Cost by Material Type

Total material cost includes the subframe (joists, bearers), foundation blocks, fixings, weed membrane, and fascia. Board cost alone is misleading because the subframe and fixings add 40-60% on top.

Full Project Cost by Deck Size

DIY vs Professional Cost Breakdown

DIY decking saves EUR 50-100 per m2 in labour. A 15 m2 deck saves EUR 750-1500 by doing it yourself. The trade-off is time — expect a DIY build to take a full weekend for a medium deck, versus one day for a professional team.

Softwood vs Composite: 15-Year Cost of Ownership

The upfront saving of softwood disappears within 5-7 years of annual maintenance. If you factor in the time spent sanding, oiling, and replacing boards, composite becomes even more attractive. The break-even point is typically around year 6-8. See our composite vs wood decking comparison for a full side-by-side.

Cost per m2 Quick Reference

For fast budgeting, use these all-in rates:

Multiply your deck area in m2 by these rates for a quick estimate, then use the calculator above for a precise materials list.

Extra Costs to Plan For

1. Steps — Each step costs EUR 50-120 in materials (or EUR 120-250 installed) depending on width and material
2. Railing or balustrade — EUR 40-100 per linear metre for timber, EUR 80-200 for glass or metal
3. Built-in seating — EUR 100-300 per bench, depending on length and design
4. Lighting — Recessed deck lights cost EUR 5-15 each, plus EUR 50-100 for a transformer and cabling
5. Building regulations — Decks over 300mm above ground level may require building control notification in some jurisdictions, with fees of EUR 200-400
6. Old deck removal — EUR 10-20 per m2 DIY, EUR 25-45 per m2 professional

Ways to Save on Your Decking Project

Choose treated softwood for the lowest upfront cost, build it yourself to cut EUR 50-100 per m2 in labour, and run boards parallel to the house to minimise offcut waste. Buying boards and subframe timber as a single bulk order often unlocks a trade discount, and reusing sound foundation blocks from an old deck avoids the EUR 50-150 foundation spend.