A footing is the block of concrete that carries a post down to soil that does not move. Getting the quantity right matters more here than on a slab, because a footing you run out of mix on has a seam through the middle of it — and that seam is where it will fail.
Round tubes and square forms
Most residential footings are poured into a cardboard tube — Sonotube is the common brand name, and the sizes run 8, 10, 12 and 16 inches in diameter. The volume is pi times the radius squared times the depth, which is why going from a 10 inch tube to a 12 inch tube adds 44 percent more concrete, not 20 percent.
Square forms built from scrap lumber are the other option, and they are usually cheaper for a handful of footings. The arithmetic is simpler — width times width times depth — but the form has to be braced or the wet concrete will push it out of square.
The calculator handles both. Switch the shape and the formula shown in the steps changes with it.
Depth comes from the frost line, not the load
This is the part people get wrong. The depth of a residential footing is almost never set by how much weight it carries. It is set by how deep the ground freezes where you live, because water in the soil expands when it freezes and lifts anything sitting above it.
That depth is a published local number. In much of the southern United States it is zero or twelve inches. Through the middle of the country it is thirty to forty-two inches. In the northern tier and at altitude it can be four feet or more.
Call your building department and ask for the frost depth. They will tell you in one sentence, and the footing has to reach below it. A deck built on footings that stop above the frost line will heave every winter and settle every spring until the ledger tears off the house.
Why the bag count looks so high
An 80 lb bag of ready-mix yields about 0.6 cubic feet of finished concrete. That is a small amount — a 12 inch tube four feet deep holds 3.1 cubic feet, so it takes five and a half bags for one footing.
Six footings like that is 33 bags, which is 2,640 pounds of material. That is over the payload of a half-ton pickup, so it becomes two trips or a delivery.
Once the total passes about a cubic yard, ready-mix delivered by truck usually costs less than the bags and takes a fraction of the labour. Most plants have a short-load minimum and a small delivery fee, and it is still generally the better deal.
The bell at the bottom
Some jurisdictions require a flared base — a wider pad at the bottom of the footing that spreads the load over more soil. A bell adds volume the tube calculation does not capture.
If you are digging a bell, measure its diameter and depth separately and add that volume yourself, or simply add another ten percent on top of what this calculator gives you. Concrete left over is cheap; concrete you run out of is not.
What to do with the extra
Ten percent waste is built into the default because the hole is never exactly the shape of the form, the tube never sits perfectly, and some mix stays in the wheelbarrow.
If you have leftovers, do not pour them back around the top of a tube — that creates a collar that frost can grab. Dump them somewhere flat and let them set as a block you can break up later.