Footing calculator
Concrete for strip footings and pad footings, in cubic yards and 80 lb bags — plus the typical sizes and depths that pass inspection.
Footing concrete calculator
cubic yards, including waste
Typical footing sizes
| Application | Width | Depth | Notes |
|---|---|---|---|
| Deck post (6 × 6) | 12 in | 36–48 in | Below frost line, rebar cage |
| Fence post | 10–12 in | 30–36 in | Drain gravel at base |
| Shed slab edge | 12 in | 12 in | Monolithic with slab |
| House strip footing | 16–20 in | 8–12 in | Two #4 bars continuous |
| Garage stem wall | 18–24 in | 12 in | Engineer per soil bearing |
How deep? It depends on frost
Exterior footings on a heated building must bear below the frost line so frozen ground cannot lift them. Approximate maximum frost depths in the continental US:
| Region | Typical frost depth | Examples |
|---|---|---|
| Deep South | 0–6 in | FL, LA, TX Gulf Coast |
| Mid-South | 12–24 in | TN, AR, OK, VA |
| Mid-Atlantic / Ohio Valley | 24–36 in | PA, OH, MO, KS |
| Upper Midwest / Northeast | 36–60 in | MN, WI, MI, NY, ME |
| Mountain West | 36–60 in | CO, UT, MT, WY (varies by elevation) |
These are planning ranges. Your local building department publishes the adopted frost depth — use that number on a permitted job.
Pouring a footing
- Dig to undisturbed soil. Loose fill compresses and cracks the footing above it.
- Compact and level. A 4-inch gravel base helps drainage and keeps the pour level.
- Set forms and rebar. Two continuous #4 bars with 3 inches of cover, chairs or dobies to hold elevation.
- Pour in one lift. Cold joints in a footing weaken the load path.
What a footing is actually doing
A footing has two jobs, and neither of them is holding a volume of concrete. It spreads the load of the structure over enough soil that the pressure stays under what the soil can carry, and it sits deep enough that weather cannot move it. Get either wrong and the cubic yards are irrelevant.
Sizing from soil bearing
Undisturbed soil carries a limited load per square foot. The IRC presumptive values run from 1,500 psf for clay and silt up to 3,000 psf for gravel, with sand around 2,000 psf. When nobody has tested the soil, residential design usually assumes 1,500 psf, which is the conservative end of that table.
Footing area follows directly: area = load ÷ allowable bearing. A deck post carrying 2,500 lb on soil rated at 1,500 psf needs 1.67 square feet, which is a 16-inch square pad or an 18-inch round one — not the 12-inch tube that gets installed by default. That 12-inch pad has 0.785 square feet and carries 1,178 lb. It is undersized by half on a loaded corner, and the failure shows up as a post that sinks a little more each spring.
This is the calculation worth doing before digging. Volume is a consequence of the size you choose, not an input to it.
Frost depth is a legal number, not an engineering one
Frost heave happens when a frost-susceptible soil — silt and fine sand, mostly — holds water, freezes, and grows ice lenses that lift whatever sits on top. Footings avoid it in one of two ways: bear below the depth where frost reaches, or replace the susceptible soil under the footing with free-draining gravel that will not hold water.
The depth to use is not the one on a regional map. It is the one your building department has adopted, because that is the number an inspector will check and the number a lender will assume. The maps on this page are for planning a project before you have pulled a permit.
Stepping a footing down a slope
On sloping ground, footings step rather than follow the grade. Each step has to keep the bars continuous through it and keep the required bearing depth on the downhill side; the step height itself should not exceed the footing depth, so the load path never narrows. Stepped footings are the most common place to see reinforcement that was cut and lapped instead of bent and continued, which quietly turns a continuous footing into a series of blocks.
What this calculator cannot see
It computes volume. It does not know your soil, your frost depth, your water table, or whether the clay under the site is expansive. Expansive clay is the one to watch: it swells when wet and shrinks when dry, and it moves footings that were sized correctly for bearing. Sites with expansive soils get deeper footings, bell-shaped piers, or a structural slab designed for the movement — none of which a volume calculator can tell you to do.
Frequently asked questions
How much concrete does a 20 ft footing need?
At 16 × 8 inches: 20 × 1.33 × 0.67 = 17.8 cubic feet, or 0.66 cubic yards — about 30 bags of 80 lb mix.
Do I need rebar in a footing?
Yes, for anything structural. Codes typically require two continuous #4 bars in strip footings; pad footings use a small grid or cage.
Can I pour a footing in one day with bags?
A 20 ft footing is about 30 bags — manageable for two people. Longer runs (over 40 ft) are better with ready-mix.
How big a footing does a deck post need?
Load divided by soil bearing. A post carrying 2,500 lb on soil rated at 1,500 psf needs about 1.7 sq ft, so a 16-inch square or 18-inch round pad. A 12-inch pad carries only about 1,180 lb.
How do I find the frost depth for my address?
Ask your local building department for the adopted frost depth. That is the number an inspector checks; regional maps are for planning before you pull a permit.
Why does a footing need to be below frost?
Frost heave lifts anything sitting in soil that freezes and grows ice lenses. Bearing below the frost line, or replacing the susceptible soil with free-draining gravel, removes the movement.
Does a footing need gravel underneath?
A 4-inch gravel base helps drainage and gives a level surface to pour against. It does not replace compacting the soil below it, which is what actually prevents settlement.
Note: Footing size and depth are structural decisions. This calculator estimates volume only; have a licensed professional or your building department confirm dimensions for a permitted project.
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