Concrete Footing Calculator
Calculate concrete for footings, post holes, piers and sonotubes. Rectangular or round shapes. Get cubic yards and bags.
Footing Dimensions
Concrete Required
Bags Needed
How to Calculate Concrete for a Footing
Calculating the amount of concrete needed for footings is essential to prevent structural settling and to ensure a strong foundation. The calculation method depends entirely on the shape of the footing you are pouring: continuous rectangular footings (trenches) or cylindrical footings (post holes, piers, and sonotubes).
Because trenches are rarely perfectly straight and post holes often flare out at the bottom, calculating the exact volume mathematically will usually leave you short on concrete. It is highly recommended to include a 10% waste factor to account for uneven digging, spillage, and variations in the soil.
Concrete Footing Formulas
Rectangular Footing Formula
Volume = Length × Width × Depth
Convert all measurements to feet before multiplying. To get cubic yards, divide the total cubic feet by 27.
Round Footing (Cylinder) Formula
Volume = π × r² × Depth
Pi (3.14) times the radius squared times the depth. Convert the diameter and depth to feet before calculating.
Sonotube & Post Hole Reference Table
| Diameter | Depth | Cubic Feet | 80lb Bags Needed |
|---|---|---|---|
| 8 inches | 3 feet (36") | 1.05 cu ft | 2 bags |
| 10 inches | 3 feet (36") | 1.64 cu ft | 3 bags |
| 12 inches | 4 feet (48") | 3.14 cu ft | 6 bags |
| 16 inches | 4 feet (48") | 5.58 cu ft | 10 bags |
Frequently Asked Questions
How do I calculate concrete for a footing?
To calculate concrete for a footing, determine the cubic footage (Length × Width × Depth in feet) and divide by 27 to get cubic yards. Always add a 10% waste factor to ensure you have enough material.
How much concrete do I need for a sonotube?
To find concrete for a sonotube, calculate the volume of a cylinder. Multiply Pi (3.14) by the radius squared (in feet) by the depth (in feet). Divide by 27 for cubic yards. A standard 12-inch sonotube 4 feet deep takes about 3.14 cubic feet, or just over 5 80lb bags.
What is the standard footing depth?
Standard footing depth depends on local building codes and the frost line. Footings must typically be poured at least 12 inches below the frost line to prevent frost heave. This can mean 12 inches deep in warm climates, or 48+ inches deep in northern climates.
How many bags of concrete per post hole?
A typical 10-inch diameter post hole dug 3 feet deep will require approximately three to four 80lb bags of concrete. The exact amount depends on the hole diameter, depth, and the size of the post displacing the concrete.