Concrete Footing Calculator
Enter the length, width and depth of your footing and this concrete footing calculator returns the volume in cubic yards and cubic meters, bag counts and a cost estimate. It covers continuous strip footings, isolated pier footings, deck footings and foundation footings.
Concrete Footing Calculator
A footing sits deep in the ground and anchors everything above it. Unlike a slab poured on grade, a footing has to reach below the frost line and land on firm, undisturbed soil. Getting the volume right matters because a footing should be one continuous pour; a cold joint in a footing is a weak point in the structure.
How to calculate concrete for a footing
A footing is a rectangular prism, so the formula is length times width times depth, all in the same unit. For a continuous footing, length is the total run. For an isolated pier footing, length and width are the pad dimensions.
The common form is: volume in cubic feet equals length in feet times width in feet times depth in inches divided by 12. Divide the cubic feet by 27 for cubic yards.
Worked example, a garage perimeter footing
Take a 24 by 24 foot garage with a 20 inch wide footing poured 12 inches deep. First find the perimeter: 24 plus 24 plus 24 plus 24 is 96 linear feet. Convert the width: 20 divided by 12 is 1.67 feet. Multiply: 96 times 1.67 times 1 is 160 cubic feet. Divide by 27 to get 5.93 cubic yards. Add 10 percent for waste to reach 6.52, then order 7 yards. At this size, ready mix is the only practical option, since 6.5 yards is nearly 300 bags of 80 pound mix.
Footing width and depth
Two rules decide the size. Width is usually about twice the width of the wall above it, so an 8 inch wall takes a 16 inch footing and a 10 inch wall takes a 20 inch footing. Retaining walls need wider footings to resist side loads. Depth has two parts: the footing thickness itself, typically 8 to 12 inches, and the trench depth, which must carry the bottom of the footing below the frost line.
| Project | Width | Depth | Concrete needed |
|---|---|---|---|
| Deck pier (single) | 2 × 2 ft | 12 in | 4 cu ft |
| Residential wall (20 ft run) | 16 in | 8 in | 17.8 cu ft |
| Garage perimeter (24 × 24) | 20 in | 12 in | 160 cu ft |
| Retaining wall (20 ft run) | 24 in | 12 in | 40 cu ft |
Frost depth by region
The bottom of a footing must sit below the frost line. If it does not, water in the soil freezes, expands and pushes the structure upward, which cracks and tilts it over time. The International Residential Code sets footing and frost protection rules in section R403. Frost depth rises as you move north, so always confirm the exact figure with your local building department.
| Region | Example states | Typical frost depth |
|---|---|---|
| Deep South | FL, LA, south TX, HI | 0 to 6 in |
| Southern | GA, AL, MS, AZ | 6 to 12 in |
| Mid Atlantic | VA, NC, TN, KY, MO | 12 to 24 in |
| Northern Midwest | NY, PA, OH, IL, KS | 30 to 42 in |
| Northeast | CT, MA, NH, VT, ME | 42 to 54 in |
| Far North | MN, WI, ND, MT, AK | 48 to 72 in |
Footing construction tips
- Dig to undisturbed soil. The trench bottom must sit on original, firm ground, never on fill.
- Keep the bottom level. Uneven bottoms create thin spots that crack under load.
- Set rebar before you pour. Two continuous horizontal bars on chairs about 3 inches from the bottom is a common residential minimum.
- Pour continuously. A footing should be a single pour to avoid a cold joint.
- Measure the trench you actually dug. Sloughed sidewalls and over cut hold more concrete than the drawing shows, so carry a 10 percent allowance.
Related calculators
- Column calculator for piers and posts that sit on footings
- Slab calculator for the floor slab above footings
- Bags calculator to turn footing volume into a bag count
- How to calculate concrete guide for the full method
Frequently asked questions
How deep should a concrete footing be?
The bottom must sit below the frost line for your region, which ranges from a few inches in the Deep South to 6 feet in the far north. The footing thickness itself is usually 8 to 12 inches. Confirm the required depth with your local building department.
How wide should a footing be?
A common rule is twice the width of the wall above. An 8 inch wall takes a 16 inch footing. Retaining walls need wider footings to resist side pressure.
Do footings need rebar?
Most residential footings use at least two continuous horizontal bars set on chairs near the bottom. Retaining wall footings add vertical bars that tie into the wall. Follow your local code and structural drawings.
Can I pour a footing in sections?
It is not recommended. A footing should be one continuous pour, since cold joints create weak points. If you must stop, leave a keyed, roughened surface and add dowel bars across the joint.
How much concrete for a deck pier footing?
A single 2 by 2 foot pad at 12 inches deep holds about 4 cubic feet, roughly 7 bags of 80 pound mix. Multiply by the number of piers.
How long before I can build on a footing?
Wait 2 to 3 days before stripping forms and about 7 days before placing heavy load such as a wall. Keep it moist during the first week.
References
- International Residential Code R403 footings, explained by InterNACHI: https://www.nachi.org/foundation-footings.htm
- American Concrete Institute, ACI 332 Residential Code Requirements for Structural Concrete: https://www.concrete.org
- Quikrete, Concrete Mix 1101 data sheet (bag yields): https://www.quikrete.com/PDFs/DATA_SHEET-Concrete Mix 1101.pdf
- Portland Cement Association, concrete technology library: https://cement.org
Methodology: volume uses length times width times depth in a common unit, divided by 27 for cubic yards; bag counts use published manufacturer yields. Frost depths are typical ranges, not code values for a specific site. Confirm footing size, depth and reinforcement with your local building department and a licensed engineer. Recommended: attribute this page to a named author and add a review date.