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Concrete Footing Calculator

Updated How we build and test

Enter the length and cross-section of your footings to see how much concrete to order in cubic yards, or how many bags to mix for a small job.

Total run of continuous footing. For a rectangle, add all four sides.

in
in
pads
in
in
%
$ per yd³

Concrete to order

Enter the footing length, width and thickness.

How to use the concrete footing calculator

  1. Enter the total length of continuous footing. For a 30 × 30 ft foundation that is 4 × 30 = 120 ft.
  2. Enter the width and thickness from your plans.
  3. Add any square pad footings under posts or columns.
  4. Keep some extra: trenches are never cut to exact size and the ground soaks up some of the pour.

Formulas

Strip footing (ft³) = length (ft) × width (in) ÷ 12 × thickness (in) ÷ 12
Pad footings (ft³) = number × (size ÷ 12)² × thickness ÷ 12
Cubic yards = cubic feet ÷ 27
To order = volume × (1 + extra % ÷ 100)
80 lb bags = cubic feet ÷ 0.60, rounded up

The International Residential Code sets a minimum footing width of 12 in and a minimum thickness of 6 in for light-frame houses, and requires wider footings as the number of stories, the snow load or the wall weight goes up or the soil gets weaker. Use the size on your approved plans.

Worked example: 120 ft of 16 × 8 in footing

  1. Cross-section: 16 in ÷ 12 = 1.333 ft wide and 8 in ÷ 12 = 0.667 ft thick.
  2. Volume: 120 × 1.333 × 0.667 = 106.7 ft³.
  3. Cubic yards: 106.7 ÷ 27 = 3.95 yd³.
  4. With 10% extra: 4.35 yd³. Order 4.5 yards from a ready-mix plant.
  5. In bags that would be 117.3 ÷ 0.60 = 196 bags of 80 lb mix, which is why footings this size are poured from a truck.

Questions

How wide should a footing be?

A common rule is twice the wall thickness: 16 in for an 8 in block or poured wall. The code minimum is 12 in, and weak soil or extra stories call for more. The building department or your plans have the final word.

How deep do footings need to be?

The bottom of the footing must sit below the local frost line and at least 12 in below undisturbed ground. Frost depth ranges from nothing in the far South to 4 ft or more in northern states.

Should I use bags or ready-mix?

Under about 1 yd³ (45 bags of 80 lb mix), bags are practical. Above that, ready-mix is faster, more consistent and usually cheaper per yard.

Sources

  1. 2021 International Residential Code, Chapter 4, Section R403 (Footings), International Code Council. Used for: minimum footing width, thickness and depth.
  2. QUIKRETE Concrete Mix (No. 1101) product data sheet. Used for: 0.60 ft³ yield per 80 lb bag.

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