Material Summary
- Footing size—
- Dig depth (frost)—
- Soil bearing—
- Concrete—
- Rebar—
- Estimated concrete cost—
Soil Bearing Capacity
| Soil type | Assumed bearing |
|---|---|
| Clay | 1,500 psf |
| Loam | 1,500 psf |
| Sand | 2,000 psf |
| Gravel | 3,000 psf |
| Rock | 6,000+ psf |
Conservative presumptive values; a soils report or local code may differ.
Frost Depth Guide
| Region | Typical frost depth |
|---|---|
| Warm / southern | 0 – 12 in |
| Temperate | 24 – 36 in |
| Cold / northern | 36 – 48 in |
| Far north | 48 in+ |
Footings must bear below the local frost line to avoid frost heave. Check your code.
How Big Does My Footing Need to Be?
A footing is sized to spread the load onto the soil without overloading it. The required bearing area equals the load on that footing divided by the soil bearing capacity. For example, a 5,000 pound load on clay rated at 1,500 pounds per square foot needs about 3.3 square feet of bearing, which rounds to a 24 inch square footing. This calculator does that structural math first, then sizes the footing, checks it against the soil, and converts the concrete into cubic yards and 80 pound bags so you know both how big to dig and how much to buy.
Footing Size, Soil, and Frost Depth
Three things drive a footing: the load it carries, the soil under it, and the frost line. Weak soil like clay needs a wider footing than strong gravel for the same load. The bottom of the footing must sit below the frost line, since water freezing under it will heave and crack whatever sits above. That is why a deck post in a cold climate is poured to 36 or 48 inches deep even though the bearing area only needs the bottom to be a certain width. Pick your soil and frost depth and the calculator handles both the width and the depth.
How Much Concrete for a Footing?
Once the footing dimensions are set, the concrete is just volume: the footprint times the depth. A pier poured to frost depth uses far more concrete than the shallow pad it sounds like, which is the surprise that empties many a wheelbarrow. The calculator reports cubic yards and the equivalent number of 80 pound bags, where each bag yields about 0.6 cubic feet, so you can decide between bagging it or ordering ready-mix.
Frequently Asked Questions
How big should a deck footing be? Size it from the post load and soil; commonly 16 to 24 inches square or a 12 inch pier, poured below frost.
How deep should a footing be? Below the local frost line, often 36 to 48 inches in cold climates, 12 inches or less where it does not freeze.
What soil bearing should I assume? Use 1,500 psf for clay or unknown soil as a safe default, more for sand, gravel, or rock.
Do footings need rebar? Spread footings usually get a light grid; small deck piers often use one or two vertical dowels.