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🧱Concrete Calculator
Formulas
- Slab volume (ft³) = length (ft) × width (ft) × thickness (in) ÷ 12
- Round column (ft³) = π × (diameter in ÷ 24)² × height (ft)
- Cubic yards = cubic feet ÷ 27
- Bags = cubic feet ÷ bag yield, rounded up
Bag yields used
| Bag size (standard concrete mix) | Yield | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 ft³ | 45 |
| 60 lb | 0.45 ft³ | 60 |
| 40 lb | 0.30 ft³ | 90 |
Yields are the figures printed on common U.S. concrete mix bags (e.g. Quikrete 1101 Concrete Mix, 80 lb = 0.6 ft³). Fast-setting and high-strength mixes can differ slightly; check your bag.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
About 45. One 80 lb bag of standard concrete mix yields about 0.6 cubic feet, and a cubic yard is 27 cubic feet, so 27 ÷ 0.6 = 45 bags.
How thick should a concrete slab be?
4 inches is typical for patios, sidewalks and shed floors. Driveways that carry cars are usually 4 to 6 inches. Follow local code or an engineer's plan for structural work.
Should I order extra concrete?
Yes. Add 5–10% for uneven ground, spills and form bulging. Running short in the middle of a pour is far more costly than a few extra bags.
When should I order ready-mix instead of bags?
As a rough guide, jobs over about 1 cubic yard (45+ bags of 80 lb) are usually faster and often cheaper with a ready-mix truck. Many suppliers have a minimum order or a short-load fee.
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Last reviewed: 2026-10-17