Concrete slab formula
A rectangular slab is a volume problem. Convert thickness to feet, multiply length × width × thickness, then divide cubic feet by 27 to get cubic yards. The allowance is applied after the net geometry so you can see how much is pure slab volume and how much is extra.
ft³ = length(ft) × width(ft) × thickness(in ÷ 12)
yd³ = ft³ ÷ 27
order volume = net volume × (1 + allowance)
Bag counts
Bag yield is a product specification, so the dropdown is editable by selection rather than treated as a universal constant. Check the yield printed by the manufacturer for the exact mix and bag size you are buying.
Reference note: The example yields—0.30 ft³ for 40 lb, 0.45 ft³ for 60 lb, and 0.60 ft³ for 80 lb—match QUIKRETE Concrete Mix No. 1101 technical data. Other mixes can differ. QUIKRETE technical data ↗