How do you calculate concrete for a slab?
For a rectangular slab, concrete volume = length × width × thickness with every dimension in feet, then divide by 27 to convert cubic feet to cubic yards. A 10 × 10 ft slab at 4 inches (0.333 ft) thick is 33.3 ft³ ≈ 1.23 yd³. Add about 10% for spillage, sub-grade irregularity, and over-excavation.
Source: Volume geometry (V = L × W × T) · Verify against your AHJ and the adopted code edition.
A slab is just a flat box, so the volume is length × width × thickness — the only trick is units.
- Put every dimension in feet. Thickness is usually given in inches, so divide by 12 first: a 4-inch slab is 4 ÷ 12 = 0.333 ft.
- Multiply to get cubic feet: 10 ft × 10 ft × 0.333 ft = 33.3 ft³.
- Divide by 27 to get cubic yards (concrete is ordered by the yard): 33.3 ÷ 27 ≈ 1.23 yd³.
Then add a waste allowance — about 10% is typical for spillage, an uneven sub-grade, and over-excavation, so you’re not caught a wheelbarrow short.
Ready-mix is ordered by the cubic yard; most suppliers have a minimum load (often around a yard) and a short-load fee below it, so round up sensibly.
Work it out: the concrete takeoff handles slabs, footings, walls, columns, and round piers — with the unit conversions, a waste allowance, and a bag count — so you don’t have to juggle inches and feet.