Concrete Footing Calculator

What this calculator does

It handles the two footing types that appear on nearly every residential drawing, together: continuous strip footings under walls, and isolated piers for posts, columns and deck supports. It returns the volume of each, the combined total in cubic feet and cubic yards, the equivalent in 80- and 60-pound bags, linear feet of form tube for the piers, and whether the total has crossed the threshold where ready-mix is the obvious answer.

Most footing calculators do one or the other. Most real footings are both.

Strip footings: length times two small numbers

A strip footing is a rectangular prism: length × width × depth. The only trick is that length is in feet while width and depth are in inches, so both need dividing by twelve before anything is multiplied.

A 120-foot perimeter footing, 16 inches wide, 8 inches deep: 120 × (16/12) × (8/12) = 106.7 cubic feet = 3.95 cubic yards.

Order four and a half. Which brings us to waste.

Waste on footings is not padding

Ten percent is the default and it is genuinely needed, because footings are cast against earth rather than into forms.

Every inch of over-dig fills with concrete. A trench that is an inch wider than the mark along its whole length adds six percent to the volume by itself. Soft spots, a bucket that took a little more than intended, a corner that got squared up generously — all of it becomes concrete, and all of it is invisible until the truck is running low.

In soft or uneven ground, fifteen percent is more realistic. The failure modes are asymmetric: a leftover wheelbarrow of concrete is an annoyance, while running short mid-pour on a continuous footing gives you a cold joint you did not design and cannot easily remove.

Piers: the circle formula, and the diameter trap

A pier is a cylinder: π r² h. The trap is that piers are specified by diameter, and the formula wants radius — in feet.

So a 12-inch pier: radius is 6 inches, or 0.5 feet. π × 0.5² × depth. At 42 inches (3.5 ft) deep that is 2.75 cubic feet each. Six of them is 16.5 cubic feet, or 0.61 cubic yards.

Note how quickly diameter dominates. Going from a 12-inch to a 16-inch pier — which sounds like a modest change — increases volume by 78 percent, because the term is squared. If your drawing calls for 16-inch piers and you priced 12-inch, you are not slightly short.

Frost depth is the input that is not negotiable

Every other dimension on a footing is a design decision balancing bearing capacity against cost. Depth is not, in any climate that freezes.

Water in soil expands when it freezes, and it lifts whatever is above it — with enough force to raise a house corner, and reliably enough to do it every single winter. A footing bearing below the frost line sits in ground that never freezes, so it never moves.

Frost depth ranges from essentially zero along the Gulf Coast to five feet or more across the northern plains and interior Alaska. Your building department publishes the number for your jurisdiction and it is the number that governs, not the general figure for your state.

The related trap: a deck pier that meets frost depth but has a bell or flare near the surface gives frost something to grab, which is why smooth-sided tubes are used and why backfill around a pier matters.

Bags versus ready-mix

Each 80-pound bag yields about 0.6 cubic feet. A cubic yard is 27 cubic feet. So 45 bags to the yard — about 3,600 pounds of material to buy, transport, carry, open and mix.

Some honest arithmetic about where the line sits:

• Under about half a yard — bags are clearly right. A few piers, a small pad.
• Half a yard to a yard — bags are possible with a mixer and help. A long day.
• Over a yard — order ready-mix. Beyond the labour, hand-mixing a continuous footing means dozens of separate batches with slightly different water content, poured over hours, with cold joints between them. That is a quality problem, not just an effort problem.

Ready-mix suppliers usually have a minimum load charge, so the economics have a step in them — often the price for 2 yards and 3 yards is not very different. Ask before you decide.

What this leaves out

• Footing size. Width, depth and reinforcement come from the soil’s bearing capacity and the load above. This calculates volume for a given size.
• Reinforcement. The rebar calculator covers bar counts, laps and weight.
• Excavation. The excavation calculator turns the trench into truckloads, including swell.
• Forms and stakes for the strip footing, where the ground will not hold a clean trench.
• Anchor bolts, dowels, straps and hold-downs.
• Inspection. Almost everywhere, footings are inspected after excavation and before the pour. Do not fill the trench before that happens.

For slabs rather than footings, the concrete calculator handles flatwork volume and the same bag-versus-truck decision.