Key Takeaways
- A footing’s volume uses length in feet but width and depth in inches
- Footing depth is set by the local frost line, not a universal number
- Rebar quantity is a linear footage estimate, not a structural design
- Always check local code before digging or pouring a footing
A footing is a different shape question from a slab. It runs the length of a wall or fence line, but its cross-section, the width and depth that actually determine how much concrete it takes, is usually a small number measured in inches rather than feet.
Length in feet, cross-section in inches
An 8 inch wide, 12 inch deep footing running 20 feet needs 20 x (8/12) x (12/12) = 13.3 cubic feet of concrete, about 0.49 cubic yards before waste. Keeping width and depth in inches for the calculation, rather than converting everything to feet by hand, avoids the rounding errors that creep in when a 6 inch dimension becomes 0.5 feet and then gets multiplied through several steps.
Footing depth is a code question, not a formula
Unlike volume, footing depth is not something you calculate from the job’s dimensions. It exists to place the base of the footing below the local frost line, so seasonal freeze-thaw cycles in the soil do not heave the footing and whatever it supports. Frost depth varies enormously by region, from a few inches in mild climates to several feet in cold ones, and is set by local building code. There is no substitute for checking with your local building department before digging.
Rebar is a quantity estimate, not a design
A simple way to estimate rebar for a footing is linear footage: the number of horizontal bar runs you plan to place, multiplied by the footing length. Two runs along a 20 foot footing is 40 feet of bar. That tells you how much to buy, but not how much the footing actually needs structurally. Bar size, spacing, and placement depend on the load the footing carries and on local code, which is a question for your local building department or a structural professional, not a materials calculator.
Use the concrete footing calculator to work out volume, bags and an optional rebar estimate for your footing. If you are specifically footing a deck rather than a wall or fence, the deck footing calculator works out both the footing count from your deck’s beam and post layout and the concrete for each one.