Concrete Footing Calculator

Start with the drawing dimensions, because those define what the footing is supposed to be. Length runs along the trench, width is across it, and depth is the vertical thickness of concrete, which is not the same as how deep the trench was excavated.

Then measure what was actually dug, at several points. Trenches widen at the top, walls slump, and machine-dug trenches are rarely uniform. A trench that averages two inches wider than specified along a long run holds a meaningful amount of extra concrete, and that is money and material you need to have on hand.

For a continuous footing that turns corners, measure along the centerline rather than the inside or outside face. Using the outside face double-counts the corners, and using the inside face undercounts them. On a rectangular layout the centerline length is what actually gets filled.

The size of that error is easy to underestimate. On a building 30 feet by 20 feet with a footing 2 feet wide, the centerline perimeter is 100 feet, the outside perimeter is 108 feet, and the inside perimeter is 92 feet. Measuring the wrong line changes the concrete by 8 percent in either direction before anything else has gone wrong, and on a footing 10 inches deep that is close to half a cubic yard.

Where a footing steps down to follow a slope, treat each level as its own section with its own depth rather than averaging the two. Averaging works only when the step is symmetrical, and it quietly hides the volume in the step itself, which is often deeper than either level beside it.

A continuous or strip footing runs beneath a wall and is calculated as a long, shallow rectangular prism: length along the run, width across, depth as the concrete thickness. The arithmetic is the same as a slab, just with different proportions.

A pad footing supports a single point load such as a column or post. It is calculated the same way when square or rectangular, and with the cylinder formula when round. Because pads usually repeat, the quantity field does most of the work: calculate one and multiply.

Round piers and drilled footings use the cylinder formula, pi times the radius squared times the depth. If you are filling a cardboard form tube rather than an open hole, the form tube calculator is set up for that case, and post holes for fence or deck posts are handled by the post hole calculator.

Slabs are poured inside forms that hold their shape, so the calculated volume is usually close to what is placed. Footings are often poured directly against earth, and earth does not hold a precise rectangle.

Trench walls slough, the base is uneven, and the bucket that dug it was probably wider than the specified footing. Every one of those adds volume that the drawing dimensions do not account for. On a footing poured against soil, running out partway through is particularly awkward because the pour is usually continuous around the building.

That is why the allowance field exists, and why footings are a case where estimators often use a larger percentage than they would for a slab. The right figure depends on your ground and how the trench was cut, so YardCalc does not set it. Look at the trench, compare it with the drawing, and choose.

Footings for a whole building are usually a ready-mix job, both for volume and because a continuous pour avoids joints in a structural element. Isolated pads, deck footings, and small piers are often done with bags.

The calculator prices either route from your own numbers: a quoted price per cubic yard for ready-mix, or a bag yield and price per bag for bags. As always, the yield comes from the bag label rather than from this page.

One scheduling point matters more for footings than for most pours. If the footing is structural and continuous, whether it can be placed in stages is a question for whoever specified it, not a matter of arithmetic. Settle that before deciding between a truck and bags.