Concrete Footing Calculator

The amount depends on the footing type and its dimensions. For a strip footing, use length times width times depth, all in feet, then divide by 27 to convert to cubic yards. A 20 ft strip footing that is 12 inches wide and 12 inches deep works out to 0.74 yd3 before waste. Adding a 10% overage brings the recommended order to about 0.81 yd3. The calculator above automates this and includes bag counts at the same time.

Footing depth depends mainly on frost protection and structural load. In warm regions with little or no frost, 12 inches may be enough for lighter residential work. In colder climates, the bottom of the footing usually has to extend below the local frost line, which may range from roughly 12 inches to 48 inches or more. Soil conditions, drainage, and code requirements can all push the final depth upward, so local code should always control.

A strip footing, sometimes called a continuous footing, runs in a long line beneath a wall and distributes the wall load across its full length. A pad footing is an isolated square or rectangular block placed beneath one column, pier, or post. Strip footings are usually used for continuous wall loads, while pad footings are used for concentrated point loads where the support locations are discrete rather than continuous.

Start by calculating one pad, then multiply by the number of identical pads. For example, if each footing is 24 by 24 inches and 12 inches deep, one pad is 2.0 x 2.0 x 1.0 / 27 = 0.148 yd3. If the project needs six pads, the total is 0.89 yd3 before overage, and about 0.98 yd3 after adding 10%. Using the quantity field in the calculator is the fastest way to handle repeated pads accurately.

One 80 lb bag of concrete mix yields about 0.022 cubic yards, so divide the final order volume, including the 10% overage, by 0.022 to estimate the bag count. For a footing order of 0.81 yd3, that works out to roughly 37 bags of 80 lb mix. Once the job becomes large enough that hauling and mixing many bags adds labor and consistency risk, ready-mix usually becomes the better option.

Formwork area estimates the total exposed footing side area that needs boards, stakes, or panels before the pour. It matters because concrete volume tells you how much mix to order, while formwork area helps you estimate forming materials and labor. Even on simple residential work, knowing the surface area of the footing sides makes it easier to plan lumber purchases, especially when the trench is not fully self-supporting and the footing must be fully formed.

For very small footing pours under about 1 cubic yard, bagged mix is often practical if labor is available and placement time is not critical. Between 1 and 3 cubic yards, some crews still use bags with a mixer, but labor starts to add up quickly. Once the pour gets much larger, ready-mix is generally more economical, more consistent, and better suited for completing the footing in one continuous placement without cold joints.

Most residential footings benefit from reinforcement, especially where soils are expansive, seismic demand exists, or structural loads are higher. A common baseline detail is two horizontal #4 bars near the lower third of the footing, but that is only a rule of thumb. Local code and engineered drawings control the final reinforcement requirement. Smaller post and fence footings may not always require rebar, but reinforcement often improves crack control and long-term durability.