Decking Calculator — Boards, Joists & Fixings

Our decking calculator works out all the materials you need by calculating the number of deck boards to cover your area, joists at your chosen spacing (400mm centres by default), and the fixings to the TDCA two-screws-per-crossing rule.

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 run perpendicular to the deck boards, at your chosen joist spacing (default 400mm centres). The number of joists = (Deck length ÷ joist spacing) + 1, rounded up.

Fixings calculation

Following the TDCA specification, every board is secured with two screws at each joist crossing point. So total fixings = board rows × number of joists × 2.

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 (28mm softwood boards)
• Fixings: 2 screws per joist crossing (4mm × 50mm stainless steel)

L-shaped decks

Choose L-shaped to plan a deck made of two rectangles: Leg A is the main deck and Leg B carries on past one end of Leg A, covering part of the width and sitting flush with one edge, with the boards running the same way across the whole deck. Each leg is worked out with exactly the rules above (board rows across the leg's width, boards per row along its length, joists at your chosen spacing along its length). The orders are then merged: the junction line runs across the boards, so it is a joist line, and Leg B's subframe butts into Leg A's end joist there. That joist is shared, so total joists = Leg A joists + Leg B joists minus 1. Boards are counted per leg and wastage is rounded up per leg, which matches how the deck is laid, with board joints breaking on the junction joist. Fixings are not reduced at the junction: the TDCA two-screws rule applies at every crossing, and board ends from both legs each need their own pair of screws into the shared joist. Weed membrane covers the combined area of the two legs plus the 10% overlap allowance. If you build the two legs as fully separate frames instead, add one joist back.