Match the calculator to the geometry
Use a slab, footing, or aggregate page when the project is rectangular. The concrete slab calculator converts length × width × thickness into cubic feet, then adds the editable waste allowance before showing yards and bag counts.
Use a round-form or post-hole page when diameter matters. The post-hole calculator subtracts actual post displacement; a nominal 6 × 6 post is 5.5 in wide in the cited lumber dimensions table.
Match the output to the purchase
Coverage pages round sheets, cartons, packages, or tiles up to whole units after waste. Replace the planning coverage with the selected product label because a 20 ft² carton and a 25 ft² carton produce different orders for the same room.
Span pages are lookups, not universal maximums. The deck joist page requires 11 fields, including species, grade, spacing, loads, treatment, use case, and code edition, before it returns a sourced row.