Concrete Calculator
Work out how much concrete you need for a slab, footing, wall or round column. Enter your dimensions and get cubic yards, cubic feet and the number of premix bags, with waste included.
Results include the waste allowance you enter. Ready mix is usually ordered in quarter or half yard steps, so round up.
How to Calculate Concrete
Concrete is ordered by volume. Multiply the length, width and thickness of the pour in feet to get cubic feet, then divide by 27 to get cubic yards. Thickness is usually measured in inches, so divide it by 12 first.
For example, a 20 ft by 20 ft slab that is 4 inches thick needs 20 x 20 x 4 / 12 = 133.3 cubic feet. Divide by 27 and you get 4.94 cubic yards. Add 10 percent for waste and order about 5.5 cubic yards.
Round columns and sonotubes
For a round column, find the area of the circle and multiply by the height. Convert the diameter to feet, divide it by 2 to get the radius, then use area = 3.1416 x radius x radius.
How many bags of concrete do I need?
Premix bags are a good choice for small pours under about one cubic yard. Each bag size yields a set volume once mixed.
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cubic feet | 45 |
| 60 lb | 0.45 cubic feet | 60 |
| 40 lb | 0.30 cubic feet | 90 |
Tips for an accurate order
- Add 5 to 10 percent for waste, uneven subgrade and spillage.
- Measure the thickness at several points. A slab that is half an inch thicker than planned needs about 12 percent more concrete.
- Include thickened slab edges, grade beams and footings in your total.
- Ready mix trucks usually carry up to about 10 cubic yards. Plan your pour around full loads when you can.
Concrete Calculator Questions
How many cubic yards are in a 10 x 10 slab?
A 10 ft by 10 ft slab that is 4 inches thick needs about 1.23 cubic yards, or about 1.36 cubic yards with 10 percent waste.
How thick should a concrete slab be?
Most patios, walkways and garage floors are 4 inches thick. Driveways for heavier vehicles are often 5 to 6 inches. Always follow the thickness shown on your structural drawings.
Should I use bags or ready mix?
Bags work well for small jobs like fence posts and pads under about one cubic yard. For larger pours, ready mix is faster and gives a more consistent result.
Does this calculator include rebar?
No. It gives concrete volume only. Our concrete estimating service adds rebar, mesh, formwork, finishing and labor pricing from your plans.
Can you estimate the concrete for my whole project?
Yes. Send us your structural plans and we will prepare a full concrete takeoff and estimate in an editable Excel sheet, usually within 24 to 48 hours.
Need a Full Concrete Estimate?
Footings, slabs, walls, rebar and formwork priced from your plans in 24 to 48 hours.
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