Concrete Calculator
Enter your slab dimensions to get the concrete volume in cubic yards and cubic feet, the number of 60 lb and 80 lb pre-mix bags, and an optional cost estimate. A waste allowance is included because forms over-fill and sub-grades are never perfectly level.
Your measurements
Ready-mix runs roughly $125–$165/yd³. Leave at 0 to skip cost.
Volume
- Cubic feet
- 44 ft³
- Total slab area
- 120 ft²
- Includes waste
- 10%
Materials
- 60 lb bags
- 98 bags
- 80 lb bags
- 74 bags
Concrete needed
1.63 yd³
≈ 74 × 80 lb bags
Estimates only. Order a little extra for waste and verify against your supplier's coverage figures.
The formula
Cubic yards = (Length ft × Width ft × Thickness ft) ÷ 27 × (1 + waste %)
Example: A 10 ft × 12 ft patio at 4 in thick = 10 × 12 × 0.333 = 40 ft³ → 40 ÷ 27 = 1.48 yd³. With 10% waste that's ~1.63 yd³, or about 74 × 80 lb bags.
How it works
- 1Convert thickness from inches to feet (divide by 12).
- 2Multiply length × width × thickness to get cubic feet.
- 3Divide cubic feet by 27 to get cubic yards (a yard is 3×3×3 ft).
- 4Add a waste allowance (10% is typical) for spillage and uneven sub-grade.
- 5Divide the cubic feet by the bag yield (0.45 ft³ for 60 lb, 0.6 ft³ for 80 lb) to get bag counts.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
An 80 lb bag yields about 0.6 ft³ of mixed concrete, and a cubic yard is 27 ft³, so you need 27 ÷ 0.6 = 45 bags per cubic yard.
How thick should a concrete slab be?
4 inches is standard for patios, sidewalks and shed floors. Use 5–6 inches for driveways and anywhere vehicles or heavy loads sit.
Should I order ready-mix or use bags?
Bags are practical up to roughly 1 cubic yard (about 45 × 80 lb bags). Beyond that, ordering ready-mix delivered by truck is usually cheaper and far less labor.