How to Calculate Concrete for Footings
Calculating the concrete volume for footings follows the same basic formula used for any rectangular solid: multiply the length by the width by the depth, all in feet, to get the volume in cubic feet. Then divide by 27 to convert to cubic yards, the unit used when ordering ready-mix concrete. For multiple footings of the same size, multiply the single-footing volume by the quantity before dividing by 27.
For example, if you need 8 footings that are each 2 feet long, 1.5 feet wide, and 1 foot deep, the calculation is: 2 × 1.5 × 1 = 3 cubic feet per footing, times 8 footings = 24 cubic feet total, divided by 27 = 0.89 cubic yards. The calculator above performs all of these conversions automatically, so you can enter each dimension in your preferred unit without manual math.
Footing Types
Different construction scenarios call for different footing configurations. The four most common types found in residential and light commercial construction are:
- Spread footings (isolated pad footings): Square or rectangular pads that support individual columns or posts. Common for deck footings, fence posts, and freestanding structures. The calculator above is ideal for estimating spread footings.
- Continuous strip footings: Long, continuous concrete strips that run beneath load-bearing walls. House perimeter foundations almost always use continuous strip footings. Calculate volume using the total wall length as the footing length.
- Grade beams: Similar to strip footings but typically deeper and reinforced with substantial rebar. Grade beams transfer loads horizontally and are often used with helical piers or drilled piers in areas with poor soil bearing capacity.
- Pier footings:Cylindrical or belled-bottom footings drilled into the ground to reach stable soil or bedrock. Common in areas with expansive clay soils or high frost depth requirements. Concrete volume for round pier footings uses the formula: radius squared × pi × depth.
Frost Line and Minimum Depth Requirements
The frost line is the most critical factor determining how deep your footings must be. Soil that contains moisture expands when it freezes, a process called frost heave. Footings installed above the frost line will be lifted, shifted, or cracked each winter as the soil freezes and thaws. Below the frost line, the soil remains at a stable temperature year-round and footings can be sized solely for bearing capacity.
Frost depth varies widely across the United States. Here are typical minimum footing depths by region:
- Deep South (Florida, coastal Gulf states): 12 inches minimum, no meaningful frost risk
- Mid-South and Southwest (Tennessee, Arkansas, New Mexico): 12 to 18 inches
- Mid-Atlantic and Southeast (Virginia, the Carolinas): 18 to 24 inches
- Midwest and Great Plains (Ohio, Illinois, Kansas): 30 to 42 inches
- Northeast (New York, Pennsylvania, New England): 42 to 48 inches
- Northern states (Minnesota, Wisconsin, Michigan, Montana): 48 to 60 inches or more
Always verify the required frost depth with your local building department before excavating. Codes are based on historical frost depth data and carry the force of law in permitted construction.
Standard Footing Sizes
A common rule of thumb for sizing continuous footings is that the footing width should be twice the supported wall thickness, and the footing depth should equal the wall thickness. Here are standard footing dimensions for common wall sizes:
- 6-inch wall:12 inches wide × 6 inches deep footing
- 8-inch wall:16 inches wide × 8 inches deep footing
- 10-inch wall:20 inches wide × 10 inches deep footing
- 12-inch wall:24 inches wide × 12 inches deep footing
- Deck posts (4x4 or 6x6): 10 to 12 inch diameter tube footing, minimum 12 inches deep below frost line
- Deck posts on pad footings:18 to 24 inches square × 12 inches deep, depending on load
These dimensions represent minimums for light residential construction on soil with adequate bearing capacity. Engineered drawings may specify larger footings for heavier loads, expansive soils, or areas with poor bearing capacity. When in doubt, always follow the plans prepared by a licensed engineer or your local prescriptive code tables.
Adding a Waste Factor
Even with precise measurements, real-world footing pours almost always use more concrete than the calculated volume. Soil excavations are rarely cut to perfect dimensions. Forms can shift slightly. Rebar chairs and tie wire displace small amounts. Slightly overfilling forms before leveling is standard practice. For these reasons, always add a minimum of 10 percent to your calculated volume before ordering concrete.
For example, if your calculation yields 0.89 cubic yards, order at least 1 cubic yard to account for waste and overage. Running short mid-pour creates a cold joint, a weak plane in the footing where fresh concrete bonds poorly to concrete that has already begun to set. Cold joints are a structural defect that can allow water infiltration and reduce the load-carrying capacity of the footing. Ordering slightly more than you need is always the safer choice.