Concrete Footing Guide: How to Calculate Volume, Depth, and Width
Footings are the buried concrete strips that spread a wall or foundation’s weight over enough soil to prevent settling. Because they’re poured below grade, running short mid-pour is one of the worst places on a job site to discover a shortage. This guide covers how footing volume is calculated and how local code determines the depth and width you should be building to in the first place.
Calculating Footing Volume
Footing width and depth are usually specified in inches, while length is measured in feet — the calculation converts everything to a single unit before multiplying, then reports the result in cubic yards, the way ready-mix suppliers price it. The formula is simply length × width × depth, with all dimensions in the same unit, then divided by 27 to convert cubic feet to cubic yards.
How Deep Should a Footing Be?
Footing depth is usually set by your local frost line to prevent frost heave — the freeze-thaw cycle that can crack or shift a footing that’s too shallow. Commonly this ranges from 12–42 inches depending on climate, with colder regions requiring the deeper end of that range. Always check your local building code rather than assuming a standard depth applies everywhere.
How Wide Should a Footing Be?
A common rule of thumb is twice the wall width sitting on top of it, often working out to 12–24 inches for residential walls. That said, engineered footings can vary significantly based on soil bearing capacity and the load the footing needs to support, so this rule of thumb is a starting point for estimating material — not a substitute for an engineer’s spec on load-bearing structures.
Confirm Dimensions Before Ordering
Footing dimensions aren’t arbitrary — they’re typically set by your local building code based on the load above and your region’s frost line. Always confirm width and depth against your permit drawings before ordering concrete, since a mismatch between your estimate and the approved plans can mean a failed inspection.
Worked Example
A 20 ft footing, 12 in wide, 12 in deep.
Convert to feet: width = 1 ft, depth = 1 ft.
Volume: 20 × 1 × 1 = 20 cu ft.
Cubic yards: 20 ÷ 27 = 0.74 cu yd — round up to 1 cu yd minimum order, or add a waste allowance if ordering ready-mix for a larger footing run.
Common Mistakes to Avoid
- Mixing units (feet and inches) without converting before multiplying, which throws off the whole volume.
- Using a generic footing depth instead of checking the local frost line requirement.
- Assuming the twice-the-wall-width rule applies to engineered, load-bearing footings without an engineer’s confirmation.
- Ordering concrete based on an estimate that doesn’t match the dimensions on the approved permit drawings.
Related Guidence
Frequently Asked Questions
How deep should a footing be?
Footing depth is usually set by your local frost line to prevent frost heave, commonly 12–42 inches depending on climate. Always check your local building code.
What is the standard width for a footing?
A common rule of thumb is twice the wall width, often 12–24 inches for residential walls, but engineered footings can vary based on soil bearing capacity and load.
Why are footings poured below the frost line?
Soil below the frost line doesn’t go through the freeze-thaw cycle, so a footing set below it won’t be pushed or cracked by frost heave over time.
Try the Concrete Footing Calculator
Ready to run your own numbers? Use the free Concrete Footing Calculator on our site — just enter your measurements and get an instant, accurate estimate.
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