Concrete Footing Calculator

Footings are the buried concrete that carries the load of a deck, fence, porch or wall down to stable ground, and they are usually poured as round piers or square pads. Estimating them means finding the volume of one footing, multiplying by the count, and adding a margin. This calculator handles both round and square footings and converts the result to bags or cubic yards.

How footing volume is calculated

The shape changes the formula: round footings use the area of a circle, square ones a simple length-by-width. All dimensions go into the same units (the calculator works in inches internally, then converts to cubic feet and yards).

Sizing and placing footings

• Diameter or size: wide enough to spread the load on the soil — deck piers are often 10–16 inches across, larger for heavier loads.
• Depth: below the local frost line, so seasonal freeze-thaw cannot lift the footing and the structure on it.
• Count: one per post or pier, plus any continuous footing under a wall.

Bags of concrete per footing

A worked example

Six round footings, 12 in diameter, 36 in deep:

• Per footing = π × 6² × 36 = 4,072 cu in = 2.36 cu ft
• Total with 5% = 6 × 2.36 × 1.05 = 14.9 cu ft
• In 80 lb bags = 14.9 ÷ 0.6 = 25 bags (about 0.55 cu yd)

Forming, rebar and the pour

Round footings are commonly formed with cardboard tube forms set in the hole and braced plumb; square footings use timber forms. Most footings carry rebar for crack resistance — vertical bars in piers, a grid in larger pads — sized with the related rebar calculator. Set anchor bolts or post bases in the wet concrete so the structure ties to the footing. For many footings or a continuous run, ready-mix beats mixing bags by hand.

Footings as the base of the build

Footings are the first concrete of a project and everything above depends on them being the right size, depth and position. They commonly support the posts of a deck or fence — sized by the related deck and fence calculators — and the related concrete calculator handles slabs and the larger pours that may sit alongside. Get the footings right and level, and the structure rises true from a solid base.

Sizing footings to the load and soil

A footing's job is to spread a concentrated load — a post, a column, a wall — over enough soil that the ground can carry it without settling. Heavier loads and softer soils need wider footings. A deck post carrying a corner of a roof needs a bigger footing than one carrying open deck alone; a wall footing is sized to the wall's load per foot. The diameter or width in the calculator should reflect the load and the soil's bearing capacity; when in doubt, or for anything structural, follow the local code's prescriptive sizes or an engineer's design rather than guessing, since an undersized footing settles and cracks the structure above.

Depth and the frost line

In any climate that freezes, footings must extend below the frost line — the depth to which the ground freezes in winter — so that the freeze-thaw cycle cannot heave them. Frost-heaved footings lift and rack decks, porches and walls, often irreparably. The required depth varies hugely by region, from a foot in mild areas to four feet or more in cold ones, and the local building department sets it. Even in non-freezing climates, footings should reach firm, undisturbed soil below the loose topsoil. Never bottom a footing on fill or soft ground; dig to bearing soil.

Forming and pouring

Round pier footings are commonly formed with cardboard tube forms set in the augered hole, braced plumb and cut to height; square footings use timber forms. Set the form so its top is at the planned elevation, since that controls the height of the post or column above. Most footings carry rebar — vertical bars in piers, a small grid in larger pads — positioned with cover all around, sized by the related rebar calculator. Set anchor bolts or post bases into the wet concrete so the structure ties mechanically to the footing rather than just resting on it.

Bags versus ready-mix, and curing

For a handful of post footings, bagged concrete mixed on site is practical — the calculator gives the bag count. Once the total passes roughly a cubic yard, a continuous footing or many piers, ready-mix delivered by the truck is cheaper, more consistent and far less labour than mixing dozens of bags by hand. Whichever you use, consolidate the concrete to remove air pockets, and let it cure — keep it damp for the first days — before loading it, since concrete gains strength over weeks. Footings are the first concrete of a project and everything above depends on them being the right size, depth, position and strength, so they reward care.

Estimating cost and method

For a handful of post footings, bagged concrete is practical and the calculator gives the bag count directly; once the total passes about a cubic yard — many piers or a continuous footing — ready-mix delivered by the truck is cheaper, more consistent and far less labour than mixing dozens of bags by hand. Beyond the concrete, budget for tube or timber forms, rebar (sized by the related calculator), and the anchor bolts or post bases set into the wet pour to tie the structure down. Footings are the first concrete of a project and everything above depends on them, so they reward care: size them to the load and soil, dig below the frost line to firm bearing soil, form them plumb and to the right height, position the steel with proper cover, and let the concrete cure before loading. An accurate volume estimate ensures you neither run short mid-pour — which leaves a cold joint — nor waste an expensive partial bag or yard of mix that cannot be returned.