The concrete volume formula
Concrete is sold by the cubic yard. The volume of a slab is length × width × thickness, with the thickness converted from inches to feet and the result divided by 27 (cubic feet per cubic yard):
cubic yards = ( length(ft) × width(ft) × thickness(ft) ) ÷ 27
thickness(ft) = thickness(in) ÷ 12
order qty = cubic yards × (1 + waste%)
Worked example
A 10 ft × 10 ft slab, 4 inches thick:
Thickness in feet: 4 ÷ 12 = 0.333 ft.
Volume: 10 × 10 × 0.333 = 33.3 cubic feet.
Cubic yards: 33.3 ÷ 27 ≈ 1.23 yd³ (1.36 yd³ with 10% waste).
Bags vs ready-mix
For small jobs, bagged concrete is convenient: an 80 lb bag yields about 0.6 cubic feet and a 60 lb bag about 0.45 cubic feet. For anything past roughly 1 cubic yard (around 45 80-lb bags) a ready-mix delivery is usually cheaper and far less work.
Tip: always add a waste margin (5–10%) for spillage, uneven subgrade and over-excavation. Running short mid-pour means a cold joint, so it's better to order slightly over.