Concrete volume and bag count for a slab or footing, plus board-foot lumber calculation. The formulas trades actually use — verified against real bag-yield figures, not guessed.
How it works
Concrete volume is length × width × thickness (converted from inches to feet), then divided by each bag size's published yield in cubic feet — 0.30 for a 40 lb bag, 0.45 for 60 lb, 0.60 for 80 lb — and rounded up, since you can't buy a fraction of a bag. Lumber uses the standard board-foot formula, thickness (in) × width (in) × length (ft) ÷ 12, multiplied by quantity and summed across every board row you add.
FAQ
Why do the three bag sizes give different total bag counts?
Because each bag size yields a different volume of mixed concrete per bag (0.30, 0.45, and 0.60 cubic feet for 40, 60, and 80 lb bags respectively) — heavier bags cover more volume per bag, so you need fewer of them for the same slab, even though the total concrete volume is identical.
What exactly counts as a "board foot"?
One board foot is the volume of a board 1 inch thick, 12 inches wide, and 1 foot long (144 cubic inches) — it's a volume measurement, not a length, which is why a thicker or wider board racks up more board feet per linear foot than a thin one.
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