Concrete Calculator
Enter your dimensions and see how many cubic yards of concrete and how many bags of pre-mix you need, with an allowance for waste.
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Choose the shape, enter the dimensions and the calculator gives the cubic yards of concrete and the number of 80, 60 and 40 lb bags you need.
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How to calculate how much concrete you need
Concrete is sold by volume, in cubic yards in the United States and cubic meters elsewhere. For a slab or footing the volume is length × width × thickness. Convert everything to feet, multiply, and divide by 27 (there are 27 cubic feet in a cubic yard). For a round column or post hole, volume is π × (diameter ÷ 2)² × height. Then add 5-10% extra for uneven ground, spillage and over-excavation.
Worked example: a 10 × 10 ft patio, 4 inches thick
Thickness 4 in = 0.333 ft. Volume = 10 × 10 × 0.333 = 33.3 cubic feet = 1.23 cubic yards. With 10% extra you need about 1.36 cubic yards, or roughly 62 bags of 80-lb mix.
How many bags of concrete per cubic yard?
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | about 0.60 cu ft | 45 |
| 60 lb | about 0.45 cu ft | 60 |
| 40 lb | about 0.30 cu ft | 90 |
Yields vary slightly by brand, so check the label of the product you buy and round up. Bags make sense for small jobs such as post holes, steps and small pads; for more than about 1 cubic yard, ready-mix delivered by truck saves a lot of mixing time and gives a more consistent pour.
Typical thicknesses
| Project | Common thickness |
|---|---|
| Sidewalk or patio | 4 inches |
| Garage floor or shed slab | 4-6 inches |
| Driveway (cars) | 5-6 inches |
| Heavy-duty driveway | 6 inches or more, often reinforced |
| Footing for a small structure | 8-12 inches thick and wider than the wall |
These are general guides, not engineering advice. Local building codes, soil conditions and reinforcement requirements decide the right size for structural work, so check with your local building department or a qualified contractor.
How to use the concrete calculator
- Pick the shape: slab, footing or round column.
- Enter the dimensions and their units, the quantity and a waste allowance.
- Read the cubic yards and the number of 80, 60 and 40 lb bags to buy.
Frequently asked questions
How do I calculate cubic yards of concrete?
Multiply length × width × thickness in feet to get cubic feet, then divide by 27. A 10 × 10 ft slab at 4 inches thick is 33.3 cu ft, or 1.23 cubic yards.
How many 80 lb bags of concrete make a cubic yard?
About 45 bags of 80 lb (0.60 cu ft each). With 60-lb bags you need about 60, and with 40-lb bags about 90.
How much extra concrete should I order?
Add 5-10% for waste and uneven subgrade; some contractors add more for rough excavations. Running short in the middle of a pour is far worse than having a little left.
How thick should a concrete slab be?
4 inches is standard for patios and sidewalks, and 5-6 inches for driveways and garage floors. Heavier loads need thicker, reinforced slabs.
How do I calculate concrete for a round hole or column?
Use π × radius² × depth. A 12-inch diameter, 3-foot deep hole needs 0.785 × 1 × 3 = 2.36 cubic feet, or about 4 bags of 80 lb.
Is it cheaper to mix bags or order ready-mix?
Bags cost more per cubic yard but have no delivery minimum. For about 1 cubic yard or more, ready-mix is usually cheaper and much less work.
Related tools: Square footage and area calculator · Volume calculator · Volume converter