Decking Calculator — Boards, Joists & Fixings

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.

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.

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.

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 for quantities. Always lay weed membrane under the decking frame. If you prefer a paved patio instead, see our Paving Slab Calculator.

The TDCA (Timber Decking & Cladding Association) specification requires two fixings at every joist crossing point, positioned at the outer quarter points (25% in from each edge). The total is therefore board rows × number of joists × 2 — this calculator works it out from your dimensions. For the common 140mm board at 400mm joist centres that works out at roughly 35 screws per m², not the 20/m² rule of thumb you sometimes see. Use stainless steel or coated decking screws at least 2.5× the board thickness (typically 4mm × 50–65mm), as stainless prevents rust staining on the timber. For fencing around your deck, see our Fence Post Spacing Calculator.

Choose L-shaped as the deck shape, then split the L into two rectangles: Leg A is the main deck and Leg B carries on past one end of Leg A, covering part of the width, with the boards running the same way across both legs. Enter each leg's length (along the boards) and width (across the boards), measured up to the junction without overlapping. The calculator works each leg out as its own rectangular deck using the same board, joist and fixing rules, then merges the order: the joist line at the junction is shared, because Leg B's subframe butts into Leg A's end joist, so one joist is deducted from the combined total. Boards are counted per leg with wastage rounded up per leg, which matches board joints breaking on the junction joist. Screws are not reduced at the junction: board ends on each side of the joint still need their own two fixings into the shared joist, exactly as the TDCA rule requires at any butt joint. If you build the two legs as completely separate frames, add one joist back.

This calculator is built around timber decking: face-fixed screws at 400mm joist centres by default. The board-row and joist-count maths also works if you enter a composite board's width and length, and you can adjust the joist spacing in Advanced Options: the site's Composite Decking Calculator uses 350mm as its standard composite spacing, with 300mm reserved for heavy-traffic areas, hot tubs or diagonal board layouts. What it will not get right for composite is the fixings: composite boards are normally secured with hidden clips, not face screws, so the fixings total shown here is a timber screw count, not a clip count. For clip numbers, starter and end clips and fascia sized to composite decking, use our Composite Decking Calculator. For UK cost comparisons between composite and timber, see Composite vs Timber Decking.

Composite decking typically costs £40-80 per m² supply-only (roughly £60-100 per m² installed), against about £15-30 per m² supply-only for softwood (£30-60 per m² installed). This calculator does not price composite boards directly: once you know your board count from the Composite Decking Calculator, enter your supplier's price into the Price Per Board field here (or on that calculator) to get a materials cost. For the full cost, maintenance and lifespan breakdown, see our Composite vs Timber Decking comparison.

This calculator works out board, joist and fixing quantities for a standard garden-level timber deck. If any part of a balcony or raised deck platform will sit higher than 600mm above the ground, the TDCA (Timber Decking and Cladding Association) treats it as a raised deck, a structural design problem needing heavier engineered joists rather than just a different joist-spacing choice, and Building Regulations require a safety barrier at least 1100mm high (Approved Document K, cross-checked against BS 6180) on any platform above that height. Use the board and joist counts from this calculator for materials ordering, but get the frame for a balcony or raised deck designed and signed off by a structural engineer or your local building control office, not from this calculator alone. See our guide on decking joist spacing for the full raised-deck breakdown.

That depends on your deck's area, board size and layout: enter your deck length and width above (or Leg A/B for an L-shape) and this calculator works out the exact boards, joists, fixings and weed membrane for your project. For a worked example on a typical 4m x 3m deck, see our answer: How Much Decking Do I Need?