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 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)