Decking Project Estimator
Calculate decking boards, joists, posts, screws and ground preparation materials for a complete deck build.
Decking materials for a 4.2m by 3m deck
For a deck measuring 4.2 metres by 3 metres, this estimate recommends 47 boards (3.6m lengths) plus joists at 400mm centres, 150mm noggins, posts, screws, weed membrane and sub-base using the default 2026 UK assumptions shown below.
- Deck area12.60 m²
- Deck boards47 boards (3.6m lengths)
- Joists9 joists / 37.80 lm
- Noggins16 pieces (150mm timber)
- Posts8 posts
- Structural bolts16 bolts
- Decking screws416 screws
- Weed membrane13.86 m²
- Sub-base1.26 m³ / 2.27 tonnes
Results - Decking Project Estimator
- Deck area12.60 m²
- Deck boards47 boards (3.6m lengths)
- Joists9 joists / 37.80 lm
- Noggins16 pieces (150mm timber)
- Posts8 posts
- Structural bolts16 bolts
- Decking screws416 screws
- Weed membrane13.86 m²
- Sub-base1.26 m³ / 2.27 tonnes
How this decking estimator works
This estimator builds a practical bill of materials for a ground-level deck using common 2026 UK assumptions: decking laid over joists at 400mm centres, 150mm noggins, two structural bolts per post, plus weed membrane and sub-base for ground preparation. Pick a board length of 2.4m, 3.6m or 4.8m and choose a wastage allowance of 5%, 10% or 15% depending on how simple or complex your layout is.
Popular sizes
These decking presets cover the board-and-joist sizes people actually price up for garden platforms, seating areas and raised decks.
Garden decking searches usually revolve around standard entertainment areas, hot tub surrounds and raised seating platforms. Pages such as 2.4m × 1.8m, 3m × 1.8m, 2.4m × 2.4m, 3.6m × 1.8m create direct landing pages for board counts, joists, posts, screws and ground preparation materials.