Concrete Footing Calculator — Volume, Bags and Rebar

How much concrete for a 40 foot footing?

Forty feet of footing at 16 inches wide and 8 inches thick is 40 × 1.33 × 0.67 = 35.6 cubic feet. With an 8% over-dig allowance that becomes 38.4 cu ft, which is 1.42 cubic yards.

In 80 lb bags that is 64 bags — more than a ton, and a full day of mixing. At that volume ready-mix is both cheaper and faster; order 1.5 cubic yards and ask about the short-load fee.

Two continuous rebar runs over 40 feet, with an 8% allowance for lapping at the splices, is 86 linear feet, which is five 20 ft bars.

How wide and how thick should a footing be?

The traditional rule is that a footing is twice the width of the wall it carries and at least as thick as the wall, projecting equally on both sides. An 8 inch block wall therefore gets a 16 inch wide footing, at least 8 inches thick, with 4 inches of projection each side.

That rule is a starting point, not the answer. The real requirement depends on what the wall carries and what the soil underneath can bear — the IRC has prescriptive tables, and anything unusual wants an engineer. Soft clay, fill, or a high water table all change the answer, sometimes dramatically.

Minimum thickness in the code is generally 6 inches. Going deeper than 8 inches on a residential footing rarely helps; going wider is what spreads the load.

Why footings fail, and it is almost never the concrete

Footings crack and settle for three reasons, and the concrete mix is rarely one of them.

Frost heave. The footing was not below the frost line. Water in the soil freezes, expands, lifts the footing, and drops it unevenly in spring. This is the most common cause of a cracked foundation wall in cold climates.

Disturbed soil. The trench was dug too deep and backfilled to level, or dug in fill that was never compacted. Concrete poured on loose soil settles into it. If you overdig, fill the extra depth with compacted gravel, never loose dirt.

Water. Poor drainage against the footing softens the bearing soil over years. A perforated drain pipe in gravel alongside the footing, running to daylight, costs very little at the time and solves it permanently.

Rebar: how much and where it goes

Rebar in a footing resists the bending that happens when the soil underneath is not perfectly uniform. Two continuous #4 bars — half inch — running the length is standard for a residential strip footing. Wider footings take three or four.

Position matters as much as quantity. The bars sit in the lower third of the pour, about 3 inches up from the bottom and 3 inches in from each side, held there on plastic or concrete chairs. Bars laid on the dirt and left there do nothing at all: concrete has to surround them completely or they rust and split the footing from the inside.

Splices lap rather than butt. The usual figure is 40 bar diameters, so 20 inches for a #4 bar, wired together. That is where the 8% the calculator adds goes.

At corners, bend a bar round or add an L-shaped piece lapping into both runs. A corner where two straight bars simply stop is the weakest point in the whole footing.

Stepped footings on a slope

On sloping ground a footing does not follow the slope — it steps. Each level run sits flat, then the footing steps up or down to the next level, with a vertical face between them.

The code limits how steep those steps can be: generally the vertical step is no more than three quarters of the horizontal run between steps, and no step is taller than the footing is thick without extra reinforcement.

For the volume, treat each level run as a separate footing and add them, then add the vertical steps as small blocks. The calculator handles this if you total the lengths and pour the steps as part of the same order — just add a little more than 8%, because stepped footings need formwork and formwork leaks.

Pouring: what to have ready before the truck arrives

A ready-mix truck gives you a limited window before it starts charging waiting time, and concrete does not pause. Everything has to be ready first.

The trench inspected if your area requires it. Rebar tied and chaired. Forms braced — concrete is far heavier than people expect and an unbraced form will bow or burst. A route for the truck, or a plan for barrowing, with boards down over soft ground. Enough people: pouring 1.5 yards is not a one-person job.

Screed the top roughly level but do not trowel it smooth. A footing wants a slightly rough surface for the wall above to key into, and many block walls need a keyway formed down the middle while the concrete is still wet.