Where to start
Formulae, assumptions, limits and treatment of L-shapes, diagonal layouts, spare material, supports and costs. First record actual dimensions, the house wall, thresholds, drains and every area that must remain accessible. Nominal product width is not always coverage width, so keep the element width separate from the working gap.
Link the calculations
One calculator result should feed the next. Board row count helps estimate fasteners, board direction sets joist direction, and exposed edge length affects fascia. Save at least two options and compare material, waste and cost.
What must be confirmed
Support spacing, expansion gaps, fastening method, permitted overhangs and base requirements depend on the exact product and local conditions. The manufacturer manual and structural design take precedence over defaults in the service.
Practical example
For a 5 × 4 m deck, changing board direction can alter the number of joints and the use of stock lengths. The option with fewer boards is not always cheaper if it needs more joists, doubled support at butt joints or expensive fascia.
Next step
After calculating, print the report, attach the drawing and mark assumptions that need confirmation. Purchasing should use a verified list rather than deck area alone.