Concrete Calculator
Work out how much concrete a slab, footing or post hole needs, in cubic yards for ready-mix or in bags of premix.
How the concrete calculator works
Volume (ft³) = length (ft) × width (ft) × thickness (in) ÷ 12. Cubic yards = ft³ ÷ 27.
Round columns: π × (diameter ÷ 2)² × depth, with the diameter converted to feet.
Bag yields used: 80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³ of mixed concrete. Check the yield printed on your brand's bag.
Add 5–10% for spillage, uneven ground and form flex.
Common slab sizes (bags include 10% waste)
| Slab | Cubic yards | 80 lb bags |
|---|---|---|
| 10 × 10 ft, 4 in | 1.23 | 62 |
| 12 × 12 ft, 4 in | 1.78 | 89 |
| 10 × 20 ft, 4 in | 2.47 | 123 |
| 20 × 20 ft, 4 in | 4.94 | 245 |
| 24 × 24 ft, 4 in | 7.11 | 353 |
| 10 × 10 ft, 6 in | 1.85 | 92 |
| 20 × 20 ft, 6 in | 7.41 | 367 |
Step-by-step guide
Related tools
Concrete Calculator FAQ
How much concrete do I need for a 10×10 slab?
A 10 × 10 ft slab 4 inches thick needs 33.3 ft³, or 1.23 cubic yards. With 10% extra, order about 1.36 yd³, or 62 bags of 80 lb premix.
How many 80 lb bags make a yard of concrete?
About 45 bags (27 ft³ ÷ 0.60 ft³ per bag).
How thick should a concrete slab be?
4 inches is standard for patios and walkways; driveways and garage floors are usually 5–6 inches. Follow local code and your engineer for structural work.