Independent guide. Not a contractor, and we don't sell leads.
Plain HomeCosts
How we make money

Concrete Footing Calculator

Enter your footing's total length, width and thickness to get cubic yards and bags of concrete, with the formula and a worked example.

Updated
Reading time2 min
Sources verified

Key takeaways

  • Footing volume = length × width × thickness, with inches converted to feet.
  • This tool gives volume, not the footing size your project requires.
  • The 2021 IRC requires exterior footings at least 12 inches below undisturbed ground, and frost rules can require more.

Enter the total length of footing, then its width and thickness in inches. For several runs, add their lengths together.

  • Concrete needed1.38 cubic yards37.3 cubic feet, including your extra allowance
  • 80 lb bags63 bagsAbout 5,040 lb to move and mix
  • 60 lb bags83 bags
  • 40 lb bags125 bags

Nothing you enter leaves your browser. Gives volume only. Footing size and depth come from your local building code or an engineer.

Show the math with your numbers
  1. Volume: 40 × (16 ÷ 12) × (8 ÷ 12) = 35.56 cu ft
  2. With extra: 37.33 cu ft
  3. Yards: 37.33 ÷ 27 = 1.38
What if: footing width
Footing widthConcrete needed
12 in1.04 cubic yards
16 in (yours)1.38 cubic yards
20 in1.73 cubic yards
24 in2.07 cubic yards

How the calculator works

  1. Cubic feet: length × (width ÷ 12) × (thickness ÷ 12)
  2. With extra: cubic feet × (1 + allowance %)
  3. Cubic yards: cubic feet ÷ 27
  4. Bags: cubic feet ÷ yield per bag, rounded up

Worked example

A footing 40 feet long, 16 inches wide and 8 inches thick, with a 5% allowance:

  • 40 × 1.333 × 0.667 = 35.6 cubic feet
  • 35.6 × 1.05 = 37.3 cubic feet
  • 37.3 ÷ 27 = about 1.38 cubic yards
  • 63 bags of 80 lb, 83 bags of 60 lb, or 125 bags of 40 lb

Measuring the length

For a rectangular building, measure along the center of the footing. If you add up outside dimensions instead, you count each corner twice, which slightly overstates the volume.

Footing size comes from the code, not this tool

Section R403 of the 2021 International Residential Code (IRC) covers footings. It requires exterior footings to sit at least 12 inches below undisturbed ground, and depth must also meet its frost protection rules. Required width depends on your soil, loads and building.

Your local building department can tell you what applies and whether you need a permit. Ask an engineer for anything unusual or heavily loaded. Deck footings have their own IRC section, R507.3.

Before digging, contact 811 a few business days ahead to have buried utilities marked.

Worked example: a rectangular building

A 24 × 30 foot building, measured outside, with a 16-inch-wide footing:

  • Outside perimeter: 2 × (24 + 30) = 108 feet
  • Each corner overlaps. Subtract 4 × the width: 4 × 1.33 = 5.33 feet
  • Length to enter: about 102.67 feet

This assumes the footing's outer edge lines up with those dimensions. If your plans show it differently, follow your plans.

Frequently asked questions

How do I calculate concrete for a footing?

Multiply length, width and thickness in feet, then divide by 27 for cubic yards. The calculator also converts that into bags.

How deep does a footing need to be?

Under the 2021 IRC, exterior footings must be at least 12 inches below undisturbed ground, and frost rules can require more. Your local building department sets what applies to you.

Should I use bags or ready-mix for a footing?

Compare the bag count and weight against a quote. In our example, 63 bags of 80 lb means about 5,040 lb to move and mix. See bags or ready-mix for more.

Sources

We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.