Concrete Footing Calculator
Ensure structural stability and eliminate material shortages. Accurately calculate the exact volume of concrete required for continuous trenches, rectangular spread footings, and circular pier holes.
Project Estimate
A structure is only as strong as the earth and concrete beneath it. Footings are the critical interface between a building’s foundation walls and the ground, designed specifically to spread concentrated loads over a wider area of soil and prevent settling.
Whether you are pouring a continuous trench footer for a retaining wall, an isolated pad footing for a steel column, or a circular pier for a residential deck, guessing your concrete requirements is a high-risk gamble. Running short during a pour compromises the monolithic strength of the footing (creating “cold joints”), while ordering too much wastes hundreds of dollars.
Use our concrete footing calculator above to instantly determine the exact volume you need, and follow the guide below to understand the structural rules that govern foundation depth, load distribution, and material costs.
How to Calculate Concrete for Footings
The foundation of all concrete foundation calculations relies on finding the geometric volume of the trench or hole in cubic feet, and then dividing by 27 to convert that measurement into cubic yards.
Trench & Pad Footing Formula (Rectangular Prism)
Volume (Cubic Feet) = Length (ft) × Width (ft) × Depth (ft)
Example: A 20 ft long trench that is 16 inches wide (1.33 ft) and 8 inches deep (0.66 ft) is calculated as: 20 × 1.33 × 0.66 = 17.55 cubic feet.
Circular Pier Footing Formula (Cylinder)
Volume (Cubic Feet) = π × Radius² (ft) × Depth (ft)
Example: A 12-inch diameter hole (6-inch / 0.5 ft radius) dug 36 inches deep (3 ft) is calculated as: 3.14159 × (0.5)² × 3 = 2.35 cubic feet.
Once you calculate volume concrete in cubic feet, you determine cubic yards by dividing 27 cu into your total (since there are exactly 27 cubic feet in one cubic yard). For example, 54 cubic feet ÷ 27 = 2 cubic yards.
Footing Rules of Thumb: Width and Depth
A footing size calculator is only helpful if you input structurally sound dimensions. Before pouring, you must consult your local building code, but general engineering principles dictate the following:
- Footing Width: A concrete footing should generally be twice as wide as the foundation wall or column it is supporting. If you are building an 8-inch wide masonry block wall, your continuous trench footer should be 16 inches wide. This ensures the weight is adequately distributed.
- Footing Depth (Thickness): The thickness of the footing should be at least equal to the width of the wall it supports, and never less than 6 to 8 inches for structural loads.
- Below the Frost Line: In cold climates, water in the soil freezes and expands, causing “frost heave” which can literally lift a house. Your foundation depth calculator estimates must account for burying the bottom of the footing at least 12 inches below the deepest known frost line in your region.
Will a 4 inch thick concrete footer hold up a house?
Absolutely not. A 4-inch thick concrete pad is suitable for a pedestrian sidewalk or a small tool shed, but it is entirely inadequate for bearing the immense, concentrated structural load of a house. Standard residential building codes generally require footings to be a minimum of 8 to 12 inches thick, depending on soil bearing capacity.
How Many Bags of Concrete Do I Need for a Footing?
For smaller projects, such as a backyard retaining wall or deck piers, buying dry premixed bags is common. You will need to divide your total cubic footage by the yield of the bag size you choose.
| Bag Weight | Volume Yield (Cubic Feet) | Bags to make 1 Cubic Yard |
|---|---|---|
| 40 lb | ~ 0.30 ft³ | 90 bags |
| 50 lb (Fast Set) | ~ 0.375 ft³ | 72 bags |
| 60 lb | ~ 0.45 ft³ | 60 bags |
| 80 lb | ~ 0.60 ft³ | 45 bags |
Example Calculation: How much concrete for a 12×12 footer?
A common scenario is a 12-inch by 12-inch trench. Because 12 inches is exactly 1 foot, the math is incredibly straightforward. The trench requires exactly 1 cubic foot of concrete for every linear foot of length. Therefore, a 20-foot long 12×12 trench needs 20 cubic feet of concrete.
If you are buying 80 lb bags (which yield 0.60 cu ft), you divide 20 by 0.60 to get 33.3 bags. Add a 10% waste factor, and you should buy 37 bags 80lb bags.
Buying Bags vs. Ready-Mix Delivery
A crucial decision in footing construction is whether to use a concrete mixer and bags, or to call a ready-mix delivery truck.
The Break-Even Point: The general rule is that any project requiring more than 1.5 to 2 cubic yards (about 90 bags of 80lb concrete) should be handled by a ready-mix truck. Hand-mixing 90 bags is backbreaking labor, and by the time you mix the final bag, the first bag will have already started curing, compromising the structural monolith of your footing.
Cost Factors: While the raw footing cost calculator math might show bags are slightly cheaper for small volumes, ready-mix companies often charge “short load” delivery fees for anything under 3 or 4 yards. Always quote both options for mid-sized projects.
Structural Considerations: Rebar and Soil
Concrete has immense compressive strength (it can withstand heavy weights pushing down), but poor tensile strength (it cracks easily when pulled or bent). Because soils settle unevenly, footings are subjected to bending forces.
Does a 6 inch concrete slab need rebar? Yes, a 6-inch concrete slab or footing almost always requires rebar reinforcement. Thicknesses of 6 inches are typically used for load-bearing applications, and a grid of #3 or #4 rebar provides the necessary tensile strength to prevent the footing from cracking in half if the dirt beneath it settles unevenly.
What is poor man’s concrete? “Poor man’s concrete” refers to the highly unadvisable DIY practice of filling a footing trench with alternating layers of large field rocks and small amounts of cement mix to save money on material. While it fills the hole, it severely compromises the structural integrity of the pour and should never be used for load-bearing footings.
Frequently Asked Questions
Reference: https://en.wikipedia.org/wiki/Wall_footing