Decking Calculator
Enter the deck size, board width and joist spacing — we work out the board count, joist count, linear metres of timber and the approximate screws you need.
Deck boards needed
62
- Deck area (m²)
- 20
- Board rows (across width)
- 28
- Boards per row (along length)
- 2
- Boards before wastage
- 56
- Linear metres of board
- 223.2
- Joists needed
- 14
- Linear metres of joist
- 56
- Approx. deck screws
- 784
Board rows = ceil(deck width ÷ (board width + 5 mm gap)). Boards per row = ceil(deck length ÷ board length). Total boards = ceil(rows × per-row × (1 + wastage%)). Joists = ceil(deck length ÷ joist spacing) + 1, each spanning the deck width. Fasteners assume 2 screws per board–joist intersection (Trex/TimberTech installer guidance).
How to use this calculator
Measure the deck length (along the boards) and width (across them). The default board is 140 mm — the universal "nominal 6-inch" size for both pressure-treated softwood and composite. Pick a board length you can actually buy: 3.6 m (≈ 12 ft) is the most common in DIY sheds. Joist spacing defaults to 400 mm, which sits on the same construction grid as the IRC residential standard of 16" on centre; tighten to 300 mm for diagonal or picture-frame layouts. Wastage of 10% is the rectangular-deck default — push it to 15% if you are cutting a non-rectangular shape or laying boards on the diagonal.
How the calculation works
Board rows = ceil(deck width ÷ (board width + 5 mm gap)). The 5 mm gap is the published Trex / TimberTech default for composite decking, leaving room for thermal movement; pressure-treated softwood is fitted slightly tighter (≈ 3 mm) because it shrinks as it dries. Boards per row = ceil(deck length ÷ board length). Total boards = ceil(rows × per-row × (1 + wastage%)). Joists run perpendicular to the boards at the chosen on-centre spacing, with one extra at the open end: joist count = ceil(deck length ÷ joist spacing) + 1. Linear metres of joist = joists × deck width. Approximate deck screws = rows × joists × 2 (one screw at each board–joist intersection, on each face of the board, per the Trex installation guide).
Worked example
A 5 m × 4 m deck with 140 mm boards × 3.6 m long, joists at 400 mm centres, 10% wastage. Board pitch = 140 + 5 = 145 mm, so rows = ceil(4.0 ÷ 0.145) = 28 rows. Boards per row = ceil(5.0 ÷ 3.6) = 2. Before wastage: 28 × 2 = 56 boards. With 10%: ceil(56 × 1.10) = 62 boards. Linear metres = 62 × 3.6 = 223.2 m. Joists: ceil(5.0 ÷ 0.4) + 1 = 14 joists × 4 m = 56 m of joist timber. Screws: 28 × 14 × 2 = 784. Sanity check against the square-metre rule of thumb (≈ 3.5 boards per m² for 140 mm boards at 3.6 m lengths): 20 m² × 3.5 ≈ 70 boards including wastage — within rounding of our 62-board figure once you take wastage out.
Frequently asked questions
What joist spacing should I use for my deck?
The default residential spacing under the International Residential Code (IRC R507) and the Trex Composite Decking Installation Guide is 16 inches on centre — about 406 mm, which the calculator rounds to a 400 mm metric grid. Step down to 300 mm (12") if you are laying boards diagonally, using a thinner board (under 25 mm finished), or working with a less rigid composite. Step up only if you are using thicker hardwood boards and have a structural engineer signing off — wider spacing is the most common cause of bouncy or sagging decks.
Why does the calculator add a 5 mm gap between boards?
Composite manufacturers (Trex, TimberTech, Millboard) all spec a 4–6 mm gap to leave room for thermal expansion in summer and to let rainwater drain through. The calculator uses 5 mm as the universal default. Pressure-treated softwood is laid tighter — around 3 mm — because the timber shrinks as it dries and the gap opens up on its own within the first season. If you fit treated softwood with no gap at all, the board count drops by roughly 3–4% on a typical deck; everything else in the math stays the same.
Does the board count include wastage?
Yes. The "Deck boards needed" headline figure is the count after applying the wastage percentage you entered, then rounded up to a whole board — this is what you order from the supplier. The "Boards before wastage" line in the breakdown is the bare-minimum count if every cut went perfectly. Use 10% for a clean rectangle, 15% for diagonals, picture-frame edging, or any deck with curved or irregular cuts. Running short mid-job usually means a second delivery charge and a colour or batch mismatch on composite, which is the most common DIY mistake on a deck build.
How does the calculator estimate the number of screws?
It uses 2 deck screws per board per joist intersection, the standard pattern in the Trex face-fix and most hidden-clip installation guides (one clip serves as a fixing on each side of the board joint, equivalent to two screws). So a deck with 28 board rows and 14 joists needs roughly 28 × 14 × 2 = 784 screws. That is the structural fixing only — it does not include screws for the joist hangers, the rim joist, or trim and fascia, which typically add another 5–10% to the total.
What board length should I pick?
Pick the shortest stock length that fully spans your deck without a join in any row. For a 5 m deck, a 3.6 m board needs a join — every row is two boards — which is fine but doubles the number of cuts. A 4.8 m board would mean one board per row with a 0.2 m offcut, which is cleaner but more expensive per metre. Composite boards do not splice end-to-end without a joist underneath, so always plan joins to land on a joist centre.
Does this work for octagonal, curved, or multi-level decks?
Not directly — the math assumes a single rectangular plan. For an L-shape, calculate each rectangle separately and add the board counts. For octagonal or curved decks, calculate the bounding rectangle and bump wastage to 20–25% to allow for the diagonal cuts. Multi-level decks are simply two or more rectangles plus the steps between them; price the step boards separately at about 1 board per linear metre of step nosing.