How deep do footings need to be?

Deep enough that the bottom sits below the local frost line, which your code sets. The footing itself is usually 8–12 inches thick, but the trench may be several feet deep in cold climates to clear frost.

What is the 2x rule for footings?

A common rule of thumb that a footing should be about twice as wide as the wall it supports and at least as deep as the wall is thick. It is a starting point for typical soil, not a substitute for engineering on bad ground.

How much waste should I add on footings?

Use 10–15%. Hand-dug or backhoe trenches never come out to exact dimensions — the walls bulge and the bottom is uneven, so you reliably pour more than the calculated volume.

Concrete Footing Size Guide

Footings carry the whole building down to soil that can hold it. Here is how the standard residential sizes are set, and why frost depth and soil dictate the rest.

A footing has one job: spread the load of the structure over enough soil that the ground can carry it without settling. Size it too small and the building settles or cracks; oversize it and you are burying money in the trench. Residential footings follow well-established rules of thumb tied to wall thickness, frost depth, and soil bearing capacity. Here is how the standard sizes are set and when you need to deviate from them.

Use the Concrete Footing CalculatorContinuous and spread footings, frost depth aware.

The 2x wall thickness rule

The classic starting point is that a footing should be twice as wide as the wall it supports, and at least as deep as the wall is thick. An 8-inch foundation wall typically sits on a 16-inch-wide footing that is 8 inches deep. This spreads the wall load and gives the wall a stable, level base to start from.

Standard residential sizes

Most light residential construction lands in a narrow band of footing sizes. These are the common defaults before any engineering adjustment.

16" wide x 8" deep for a typical 8" wall
20" wide x 10" deep for heavier loads or 10" walls
24" wide x 12" deep for two-story or masonry loads
Spread/pier footings sized to the column load

Frost depth controls the bottom

In cold climates the bottom of the footing must sit below the frost line, or freeze-thaw cycles will heave it. Frost depth varies by region from a foot to several feet, and your local code sets the minimum. This is about depth of the trench, not the thickness of the footing — you may dig four feet down but still pour an 8-inch-thick footing at the bottom.

Soil bearing capacity

Everything above assumes average soil. Soft clay, fill, or organic ground has lower bearing capacity and needs a wider footing to spread the same load. When soil is questionable, an engineer specifies the size — do not eyeball it. The footing calculator handles the volume once the width and depth are set.

Footings always over-pour

Trench walls bulge and the bottom is never perfectly flat, so footings consistently take more than the print says. Add 10–15% waste on footings specifically.

Keep the bottom flat and level

A stepped or sloped trench bottom changes your real depth and your volume. Keep it flat, and step it cleanly where grade forces a change rather than ramping it.

Defer to the engineer on bad soil

Rules of thumb assume decent ground. On fill, clay, or anything questionable, the stamped drawing wins — and it protects you if anything settles later.

Frequently asked questions

The standard is twice the wall width, so 16 inches wide for an 8-inch wall, typically 8 inches deep. Heavier loads or poor soil push that wider, and an engineer should size it on questionable ground.

Put this into practice

Open the concrete footing calculator and run your numbers. Save the job and keep every area together.

Open the App