Formulas
Slab, wall or footing: ft³ = length × width × (thickness in ÷ 12)
Round column: ft³ = π × (diameter in ÷ 12)² ÷ 4 × height
Cubic yards = ft³ ÷ 27
Worked example
A 20 × 12 ft slab, 4 in thick: 20 × 12 × 0.333 = 80 ft³ = 2.96 yd³. Adding 10% for waste and subgrade variation gives 3.26 yd³, so order 3.5 yards.
Bag yields
| Bag | Yield | Bags per yd³ |
|---|---|---|
| 80 lb | 0.60 ft³ | 45 |
| 60 lb | 0.45 ft³ | 60 |
| 40 lb | 0.30 ft³ | 90 |
Past about 1 yd³, ready-mix is usually cheaper and gives a more consistent pour than mixing bags.
Ordering tips
- Round up to the nearest half yard. Running short mid-pour leaves a cold joint.
- Check the actual subgrade depth in several places. An extra half inch over a large slab adds up fast.
- Footings and piers poured against earth often need 10–15% extra.
Frequently asked questions
- How many bags of concrete for a 10x10 slab?
- At 4 in thick, 33.3 ft³ (1.23 yd³). That's 56 bags of 80 lb, or about 62 with 10% waste.
- How much does a yard of concrete weigh?
- About 4,000 lb (2 tons) for normal-weight concrete at 150 lb/ft³.
Results are estimates for planning and education. Verify against the current code edition your jurisdiction has adopted, manufacturer data and a licensed professional before you build or install.