How it's calculated
volume = L × W × H × (1 + waste%)
L, W, H = slab length, width and thickness (converted to metres), waste% = extra allowance. Cubic metres ÷ 0.764555 = cubic yards. 80 lb bag ≈ 0.60 ft³, 60 lb bag ≈ 0.45 ft³.
Common questions
How many 80-lb bags make a cubic yard?
An 80-lb bag yields about 0.6 cubic feet, so a cubic yard (27 cubic feet) needs roughly 45 bags.
When should I order ready-mix instead of bags?
Bags work for small slabs, but past about one cubic yard, ready-mix delivery is usually cheaper and far less labor.