# How Much Concrete Do You Need for a Footing?

_Published October 5, 2026_

**In short:** Concrete for a footing is length × width × depth, converted to cubic yards by dividing cubic feet by 27. A 40 foot footing that is 16 inches wide and 8 inches deep needs 1.32 cubic yards before waste, about 1.45 with a 10 percent allowance, or 60 bags of 80-lb mix.

## The footing concrete formula

A strip footing is a long rectangular block, so the volume is simple: length times width times depth. Work in feet, convert the width and depth from inches, then divide by 27 to get cubic yards.

```
cubic feet = length (ft) × width (in ÷ 12) × depth (in ÷ 12)
cubic yards = cubic feet ÷ 27
```

The Engineering ToolBox publishes the same relationship in one step: cubic yards = 0.0031 × length (ft) × width (ft) × depth (in). The 0.0031 is just 1 ÷ (12 × 27), so both methods give the same answer. The [concrete calculator](../../concrete-calculator/) has a footing mode that does this for you and also converts to 40, 60 and 80-lb bags.

## Common footing sizes and what they need

Footing size is not a number to guess. The Concrete Network notes that footing width runs from as little as 6 inches to 40 inches depending on soil bearing capacity, the type of construction and the number of stories, and that thickness is typically 8 to 12 inches. Below is the concrete for a 40 foot run at four sizes, before any waste allowance:

| Footing (width × depth) | Cubic feet | Cubic yards | 80-lb bags |
| --- | --- | --- | --- |
| 12 × 8 in | 26.7 | 0.99 | 45 |
| 16 × 8 in | 35.6 | 1.32 | 60 |
| 20 × 10 in | 55.6 | 2.06 | 93 |
| 24 × 12 in | 80.0 | 2.96 | 134 |

Doubling the width or the depth doubles the concrete, so a small change in the plans moves the order a lot. Your required size comes from the plans, an engineer or the building department, not from this table.

## Worked example: a 40 foot footing

Say the plan calls for a footing 16 inches wide and 8 inches deep along a 40 foot run, for example under a small addition or a garden wall.

- Width 16 in ÷ 12 = 1.333 ft, depth 8 in ÷ 12 = 0.667 ft.

- Volume = 40 × 1.333 × 0.667 = 35.6 cubic feet.

- 35.6 ÷ 27 = 1.32 cubic yards.

- Add 10 percent for waste: 1.45 cubic yards.

At 1.45 yards you are in the range where a ready-mix truck usually makes more sense than bags. The same pour in 80-lb bags is 60 bags before waste, which is about 2.4 tons of bagged material to mix by hand, and a pour has to be placed continuously. For a short footing of a few yards or less, the [cubic yard calculator](../../cubic-yard-calculator/) helps you compare quotes by the yard.

## Why trench footings need extra

Hand-dug trenches are never perfectly square. The bottom is uneven, the sides cave in slightly and the width creeps wider where the soil crumbles. All of that fills with concrete. Allow at least 10 percent on top of your calculated volume, and up to 15 percent for rough or sandy ground. Running out mid-pour leaves a cold joint in a structural element, which is far costlier than a few extra bags.

## Depth, frost and soil are local questions

The Concrete Network points out that footing depth varies with the frost line and soil strength: some footings can be shallow while others must be deep. Frost depth differs enormously across the country, and the bearing capacity of your soil changes the width. Ask your local building department for the required footing width, depth and frost depth before you dig, and call 811 first so buried utilities are marked. Any footing that carries a house, addition, deck or retaining wall should follow the approved plans.

## Common mistakes

- **Mixing inches and feet.** Divide both width and depth by 12 before multiplying by the length in feet.

- **Forgetting corners and returns.** Add the length of every leg of the footing, including the portions that overlap at corners, to your total run.

- **Using the wrong size.** A footing sized for a one-story wood frame is not suitable for a masonry wall or a two-story house on weak soil.

- **Ordering exactly what the math says.** Add the waste allowance; short loads are expensive.

- **Digging before checking utilities.** Contact 811 a few days ahead.

## What else to order or consider

Footings usually need rebar, chairs or spacers to hold it off the soil, forms if the trench walls will not hold, and sometimes a gravel layer under the concrete. If a foundation wall or slab will sit on the footing, calculate those separately with the concrete calculator, using its wall and slab modes, and add the yardage together. If you are also digging out the trench, the soil you remove becomes a hauling question, not a concrete one.

## Frequently asked questions

### How do you calculate concrete for a footing?

Multiply length in feet by width in feet by depth in feet to get cubic feet, then divide by 27 for cubic yards. A 40 foot footing 16 inches wide and 8 inches deep is 35.6 cubic feet, or 1.32 cubic yards.

### How many bags of concrete for a footing?

An 80-lb bag yields about 0.60 cubic feet. A 40 foot footing 16 inches wide and 8 inches deep needs 60 bags before waste, so most people order a truck instead.

### How wide and deep should a footing be?

It depends on soil bearing capacity, the structure and frost depth. Widths range from 6 to 40 inches and thickness is typically 8 to 12 inches, but your building department sets the requirement for your project.

## Sources and further reading

- [Concrete Footings 101: Footing Dimensions](https://www.concretenetwork.com/concrete/footing_fundamentals/footing_dimensions.htm) — The Concrete Network
- [Concrete Footing: Volume in Cubic Yards](https://www.engineeringtoolbox.com/concrete-footing-cy-d_1708.html) — The Engineering ToolBox
- [Call 811 before you dig](https://call811.com/) — Common Ground Alliance