Most decking calculators, including the ones timber retailers run themselves, just give you a board count from a flat area. That’s the easy part. The joists, bearers, posts and footings underneath, the actual substructure that holds a deck up, are what people forget, and they’re usually the bigger part of the cost anyway.
This one asks for your deck’s length, width and which way the boards run, since joists always cross the boards at right angles and bearers run under the joists parallel to the boards, resting on the posts. From that it works out a proper directional grid rather than an area sum, joist spacing set automatically from your board thickness and material (SANS 1008 guidance is roughly 20 to 22 times the board’s thickness, tighter again for composite), bearer and post layout, and screws or hidden clips counted at every real board-joist crossing.
It handles timber (treated pine or balau hardwood) and composite, an attached deck with a ledger board correctly drops one whole bearer line and its posts since the house structure carries that edge instead, and every post’s footing gets counted and handed straight across to our Concrete & Mortar Calculator, pre-filled with the exact post count and a sensible footing size for your post, so you’re not doing that part by hand either.
Board sizes, prices, joist/bearer/post spacing and stock lengths are all editable to match your own supplier’s quote. Building an L-shaped deck? Split it into two rectangles and run each through separately, the substructure at the shared edge comes out a bit conservative rather than short.
Let me know if the spacing defaults or post pricing look off for a job you’ve actually quoted.