Construction

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Construction

Concrete Footing Calculator.

Estimates the volume of concrete needed for a rectangular footing and the number of 60-lb or 80-lb bags required.

Results update live as you type.
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Your inputs

How it works

  1. 1

    Measure the length, width, and depth of the footing in feet.

  2. 2

    Multiply length × width × depth to get the volume in cubic feet.

  3. 3

    Convert cubic feet to gallons by multiplying by 7.48051948.

  4. 4

    Divide the gallons by the yield per bag (0.45 for 60 lb, 0.6 for 80 lb) and round up to get the number of bags.

length * width * depth * 7.48051948

Frequently asked questions

How much concrete does a 60 lb bag make?

A 60 lb bag of concrete mix yields about 0.45 cubic feet (3.36 gallons) when mixed with water.

How much concrete does an 80 lb bag make?

An 80 lb bag yields about 0.6 cubic feet (4.49 gallons).

Should I order extra concrete?

It's wise to add 5-10% extra to account for spillage, uneven subgrade, or variations in footing dimensions.

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Results

Formula checked

Concrete volume

0gallons

Cubic feet0ft³
Bags needed0bags
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Estimate for general guidance only — verify important decisions with an appropriate professional.

How it works

Estimates the volume of concrete needed for a rectangular footing and the number of 60-lb or 80-lb bags required.

  1. Measure the length, width, and depth of the footing in feet.
  2. Multiply length × width × depth to get the volume in cubic feet.
  3. Convert cubic feet to gallons by multiplying by 7.48051948.
  4. Divide the gallons by the yield per bag (0.45 for 60 lb, 0.6 for 80 lb) and round up to get the number of bags.

Formulas

The math behind this calculator, written out so you can verify the result.

Volume of rectangular footing

V = L × W × D

Multiply the length, width, and depth of the footing to get the volume in cubic feet.

Example:

Input: L = 4 ft, W = 2 ft, D = 1 ft

Calculation: 4 × 2 × 1 = 8 ft³

Result: 8 cubic feet

Bags of concrete

Bags = ceil(V / yield_per_bag)

Divide the total volume by the yield of one bag (0.45 ft³ for 60 lb, 0.6 ft³ for 80 lb) and round up.

Example:

Input: V = 8 ft³, 60 lb bags

Calculation: 8 / 0.45 = 17.78 → 18 bags

Result: 18 bags

Real-world use cases

Where this calculation shows up in everyday life.

Deck footings

Calculate concrete needed for deck support footings to ensure a stable base.

Example: A 4×2×1 ft footing for a deck post.

Fence posts

Determine concrete volume for setting fence posts in concrete footings.

Example: A 1×1×2 ft footing for a fence post.

Shed foundation

Estimate concrete for a shed foundation footing to avoid over-ordering.

Example: A 10×3×1.5 ft footing for a small shed.

Tips and common mistakes

Tips

  • Always measure in feet and keep consistent units.
  • Add 5-10% extra concrete to account for waste and spillage.
  • Check the bag yield on the product label; it may vary slightly by brand.
  • For irregular shapes, break the footing into rectangles and sum the volumes.

Common Mistakes to Avoid

  • Forgetting to convert inches to feet (divide by 12).
  • Using the bag yield in cubic feet but calculating volume in cubic yards.
  • Not rounding up the number of bags, leading to a shortage.

Assumptions and limitations

  • Use the stated inputs and units.
  • Results are estimates for planning and education.
  • Check measurements and source data before making an important decision.