Footing Calculator

How to Use the Footing Calculator

Enter the total length of your footing run in feet, and the footing's width and depth in inches (footings are usually specified in inches since they're narrower and shallower than a slab). Pick your bag size to see how many bags of pre-mix concrete you'll need for the whole run.

How the Calculation Works

Because footing width and depth are entered in inches, the calculator first converts both to feet (dividing by 12), then multiplies length × width × depth to get the volume in cubic feet. That's converted to cubic yards (÷27) and divided by your bag's yield, then rounded up to a whole number of bags — the same underlying math as the Concrete Bags calculator, just tuned for footing-shaped dimensions (long and narrow rather than a wide slab).

Example Calculation

An 80 ft footing run, 16 inches wide and 8 inches deep: volume = 80 × (16÷12) × (8÷12) = 80 × 1.33 × 0.67 = 71.1 cubic feet, or 2.63 cubic yards. At 60 lb bags (0.017 yd³ each), that's 2.63 ÷ 0.017 ≈ 155 bags.

Frequently Asked Questions

What's the difference between a footing and a slab?

A footing is a continuous strip of concrete below grade that spreads a structure's load into the soil — under a foundation wall, deck posts, or fence posts. A slab is a flat, load-bearing surface poured on top of the ground, like a patio or garage floor.

How deep should a footing be?

Footing depth depends on your frost line and local code — in colder climates, footings often need to extend below the frost depth to avoid heaving. A common residential foundation footing is 8 inches deep and twice the wall width, but always confirm against your local building code.

Is pre-mix bag concrete strong enough for footings?

Standard 60 lb and 80 lb pre-mix bags (typically rated around 4,000 PSI at 28 days) are suitable for most residential footings like fence posts, deck supports, and small structures. Larger structural footings usually call for ready-mix truck-delivered concrete instead.