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Deck Footing Concrete Calculator

Calculate how much concrete you need for deck post footings.

Result

80lb Bags Needed
24
Total Concrete Volume
14.14 ft³ (0.4 m³)
Volume per Footing
2.36 ft³

Check your local frost line depth - footings generally need to extend below it to prevent frost heave, which can be much deeper than 36" in cold climates.

About the Deck Footings

Deck post footings are cylindrical holes filled with concrete, and their volume grows with the square of the radius, so a modest increase in footing diameter uses noticeably more concrete than the same increase in depth.

How It Works

The calculator finds the volume of a single cylindrical footing using its radius and depth, multiplies by the number of footings, and converts the total to 80lb bags of pre-mixed concrete.

volume per footing = π × radius² × depth; total ft³ = volume per footing × number of footings; bags = total ft³ ÷ 0.6

Examples

A typical deck

Six footings at 12" diameter and 36" deep need about 16.96 cubic feet total, roughly 29 bags of 80lb pre-mixed concrete.

A wider footing

Increasing diameter to 16" on the same 6 footings and depth needs about 30.2 cubic feet, nearly double, since footing volume scales with the square of the radius.

Advantages

  • Correctly applies the cylinder volume formula rather than a flat estimate
  • Converts directly to 80lb bag count for a DIY pour
  • Scales automatically for any number of footings

Common Mistakes

  • Not digging footings below the local frost line in cold climates
  • Underestimating how much diameter (versus depth) affects total volume
  • Forgetting that post base hardware and drainage gravel are separate from the concrete estimate

Edge Cases to Watch For

  • Footings generally need to extend below the local frost line to prevent frost heave from cracking or shifting them - this can be far deeper than the 36" default in cold climates, so check with your local building department.
  • Bell-shaped footings (wider at the bottom than the top, sometimes required by code for extra uplift resistance) hold more concrete than a straight cylinder of the same top diameter and depth.
  • This calculates concrete volume only - post base hardware, gravel drainage at the bottom of the hole, and rebar (if required by local code) are separate materials.

Common Use Cases

  • Estimating concrete for deck post footings before a pour
  • Comparing material needs across different footing diameters and depths
  • Budgeting a deck-building project's foundation work
Written & fact-checked by the Calculateus TeamLast updated August 5, 2026How we verify our formulas

Frequently asked questions

Why does footing depth matter so much in cold climates?

When soil freezes, it expands and can push shallow footings upward ("frost heave"), cracking the concrete or shifting the deck over time - footings need to extend below your area's frost line, which your local building department can specify.

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Conclusion

Cylindrical volume means diameter matters more than depth for footing concrete, since it's squared in the formula while depth is linear. Always confirm your local frost line depth and footing size requirements with your building department before digging.