Calculate how much concrete you need for footings in cubic yards, cubic feet, bags, weight, and cost for strip, pad, square, and round pier footings.
Concrete Footing Formula
The amount of concrete for a footing starts with the volume of the footing. The volume formula depends on the footing shape.
For a strip, rectangular, or square footing the volume is the cross section multiplied by the length:
V = L * W * D
For a round pier or sonotube footing the volume uses the circle area times the depth:
V = pi * (Dia / 2)^2 * H
Convert cubic feet to cubic yards by dividing by 27:
CY = V(ft3) / 27
Estimate the number of bags from the volume, a waste allowance, and the yield of each bag:
Bags = V(ft3) * (1 + W%) / Yield
Estimate the weight of the concrete from the volume and the mix density:
Weight = V(ft3) * Density
- V = volume of concrete (cubic feet before unit conversion)
- L = footing length or total run
- W = footing width
- D = footing thickness or depth
- Dia = diameter of a round pier footing
- H = depth or height of a round footing
- CY = volume in cubic yards
- W% = waste or overage allowance as a decimal
- Yield = volume one bag of mix produces (cubic feet)
- Density = unit weight of the concrete, near 150 lb per cubic foot
The shape selector controls which volume formula is used and which inputs you enter. The solve-for selector switches between sizing concrete for a footing you describe and pricing or weighing a known amount of concrete. The number of footings multiplies a single footing volume when you pour several identical piers or pads. The waste percent adds overage so you order a little extra for spillage and uneven subgrade. The bag size sets the yield used to convert volume into bags, and the price field turns the volume into a material cost.
Concrete Bag Yields and Footing Reference
Bagged concrete is sold by weight, but you order by volume, so the yield per bag matters. The table below lists the cubic feet each common bag size produces.
| Bag size | Yield (cubic feet) | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 | 45 |
| 60 lb | 0.45 | 60 |
| 50 lb | 0.375 | 72 |
| 40 lb | 0.30 | 90 |
Once a footing reaches roughly one cubic yard, ready mix delivery is usually cheaper and faster than mixing bags by hand.
| Footing type | Typical size | Common use |
|---|---|---|
| Strip / wall | 16 in to 24 in wide, 8 in to 12 in thick | Foundation and load bearing walls |
| Square pad | 24 in to 36 in side, 12 in thick | Posts and columns |
| Round pier | 8 in to 16 in diameter | Decks and sonotube piers |
Always confirm dimensions and depth against your local building code and frost line.
Example Problems
Example 1: Square pad footing. You pour one square pad footing that is 3 ft on each side and 12 in (1 ft) thick. The volume is 3 * 3 * 1 = 9 cubic feet. In cubic yards that is 9 / 27 = 0.33 cubic yards. Using 80 lb bags at 0.60 cubic feet each with no waste, you need 9 / 0.60 = 15 bags.
Example 2: Strip footing with waste. A continuous wall footing runs 40 ft long, is 20 in (1.667 ft) wide, and 10 in (0.833 ft) thick. The volume is 40 * 1.667 * 0.833 = 55.5 cubic feet, or 55.5 / 27 = 2.06 cubic yards. Adding 10 percent waste gives 2.06 * 1.10 = 2.26 cubic yards to order from a ready mix supplier.
FAQ
How much extra concrete should I order for footings? A waste allowance of 5 to 10 percent is normal. Footing trenches and form bottoms are rarely perfectly even, so a little overage keeps you from running short during the pour, which is hard to fix once concrete starts setting.
Should I use bags or ready mix concrete? Bags are convenient for small piers and a few pads totaling well under a cubic yard. Once your total reaches about one cubic yard, ordering ready mix is usually cheaper, faster, and gives a more consistent mix than mixing many bags by hand.
How much does concrete for a footing weigh? Standard concrete weighs about 150 lb per cubic foot, or roughly 4,050 lb per cubic yard. Multiply your footing volume in cubic feet by 150 to estimate the poured weight, which is useful for planning delivery and support.
