How many decking boards do I need per m²?

For standard 140mm wide timber decking boards with a 5mm gap, you need approximately 7 boards per metre of width. A 3.6m long board covers 0.504 m² (3.6m x 0.14m). So roughly 2 boards per m² for 3.6m boards at 140mm width. This decking board calculator works this out precisely for your chosen board size and includes wastage.

What spacing should I use for decking joists?

Decking joists should be spaced at 400mm centres (centre to centre) for standard softwood decking boards. For thinner boards or composite decking, check the manufacturer's specifications — some require 300mm centres. Joists should be 47x100mm minimum, or 47x150mm for spans over 1.5m between supports. For interior floor joists, see our Floor Joist Calculator (/flooring/floor-joist-calculator).

What gap should I leave between decking boards?

Leave a 5-8mm gap between decking boards to allow for water drainage and timber expansion. Use a 5mm spacer (a pencil works well) for consistent gaps. Boards should also be spaced 10mm from walls or other fixed structures to allow for expansion.

Do I need a sub-base under decking?

Decking frames can sit on concrete pads, deck blocks or a compacted sub-base. For a level base, consider a Type 1 sub-base — use our Type 1 Sub-Base Calculator (/landscaping/type-1-sub-base-calculator) for quantities. Always lay weed membrane under the decking frame. If you prefer a paved patio instead, see our Paving Slab Calculator (/landscaping/paving-slab-calculator).

How many screws do I need for decking?

Allow approximately 20 stainless steel or coated decking screws per m² of decking. Each board needs 2 screws at every joist crossing point. Use 4mm x 50mm (or 60mm for thicker boards) decking screws. Stainless steel screws are recommended to prevent rust staining on the timber. For fencing around your deck, see our Fence Post Spacing Calculator (/landscaping/fence-post-spacing-calculator).

TradeCalcs

Decking Calculator — Boards, Joists & Fixings

Calculate decking boards, joists, fixings and all materials needed for your timber deck. Uses standard UK decking sizes.

Deck Length

Length of the deck area

Deck Width

Width of the deck area

Board Width

140mm is the most popular UK size

Board Length

Choose to minimise waste and cuts

Include wastage allowance

Adds 10% extra for cuts and offcuts

Wastage Percentage

10% recommended for rectangular decks

Price Per Board (optional)

Price per deck board for cost estimate

How We Calculate This

Our decking calculator works out all the materials you need by calculating the number of deck boards to cover your area, joists at 400mm centres, and the appropriate fixings.

How we calculate deck boards

Board rows = Deck width ÷ (Board width + 5mm gap)

Boards per row = Deck length ÷ Board length (rounded up)

Total boards = Board rows × Boards per row

Joist calculation

Joists are spaced at 400mm centres running perpendicular to the deck boards. The number of joists = (Deck length ÷ 0.4) + 1, rounded up.

Standard UK sizes

Board widths: 120mm, 140mm (most popular), 150mm
Board lengths: 2.4m, 3.0m, 3.6m, 4.8m
Board thickness: 28-32mm (standard softwood)
Joist size: 47×100mm or 47×150mm treated softwood
Joist spacing: 400mm centres
Fixings: ~20 screws per m² (4mm × 50mm stainless steel)

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.