Footing Calculator: How Much Concrete for a Strip Footing

Enter the total length of the footing in feet, then its width and depth in inches. That is all you need. The footing calculator multiplies the three dimensions, converts the result to cubic yards (the unit ready-mix suppliers sell by), adds a 10% allowance for over-dig and spillage, and tells you how many 80-lb bags you would need if you mix it yourself.

For a house or garage perimeter, the total length is the sum of all sides, so a 24 x 30 ft garage has 24 + 30 + 24 + 30 = 108 ft of footing. Open More options to switch to metric, change the bag size, use a different waste allowance, calculate several identical pad footings at once, or enter a price per yard or per bag to estimate cost.

A continuous footing (also called a strip or wall footing) is the wide, shallow ribbon of concrete that runs under a foundation wall, a garden wall, or a block retaining wall. It spreads the wall's weight over more soil so the wall doesn't sink. Unlike a slab, which is wide in both directions and only a few inches thick, a footing is long and narrow and usually 8-12 inches deep.

That shape is why a slab calculator often misleads people: slabs are measured in square feet and thickness, while footings are best measured as a running length times a cross-section. A common rule of thumb for light residential work is a footing twice as wide as the wall it carries and at least as deep as the wall is thick, which is where the familiar 16 x 8 in, 20 x 10 in and 24 x 12 in sizes come from. Your local building code and frost depth have the final say on size and how deep the bottom must sit, so check them before you dig.

Volume (cu ft) = Length (ft) x Width (in) / 12 x Depth (in) / 12 x Number of footings

Cubic yards = Cubic feet / 27

Order volume = Volume x (1 + Waste % / 100)

Bags = Order volume (cu ft) / Bag yield, rounded up

Bag yields used: 80 lb = 0.60 cu ft, 60 lb = 0.45 cu ft, 50 lb = 0.375 cu ft, 40 lb = 0.30 cu ft. In metric mode, length is in meters and width and depth are in centimeters; 1 cubic meter = 35.3147 cubic feet = 1.30795 cubic yards.

How much concrete for a 100 ft footing that is 20 x 10 inches? 100 x (20/12) x (10/12) = 138.9 cu ft = 5.14 cu yd. Add 10% and you would order about 5.66 cubic yards; most suppliers round up to 5.75 or 6.

Garage footing, 24 x 30 ft perimeter, 16 x 8 in: 108 ft of footing x 1.333 x 0.667 = 96 cu ft = 3.56 cu yd, or 3.91 cu yd with waste. In 80-lb bags that is 176 bags, which is a strong case for ordering ready-mix instead.

How many bags of concrete for a 10 ft footing, 12 x 8 in? 10 x 1 x 0.667 = 6.67 cu ft, 7.33 cu ft with waste, so 13 bags of 80-lb mix.

- Measure the trench, not the plan. Trenches are usually dug wider and deeper than drawn; measuring the real trench at several points gives a far better order.
- Order at least 10% extra. Running short mid-pour leaves a cold joint in the footing. A small surplus can fill a few post holes or a pad.
- Over about 1 cubic yard, price out ready-mix. One yard is 45 bags of 80-lb mix. Many suppliers charge a short-load fee under 3-4 yards, which is still often cheaper than the labor of hand mixing.
- Keep the bottom below frost depth and on undisturbed soil; never pour onto loose backfill.
- Plan the rebar first. Most footings take two continuous runs of #4 bar; use the rebar calculator to size the order.