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Construction

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Construction

Concrete Foundation Calculator.

Estimates the volume of concrete needed for a rectangular foundation slab, along with the estimated weight and number of bags required.

Results update live as you type.
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Your inputs

How it works

  1. 1

    Convert thickness from inches to feet by dividing by 12.

  2. 2

    Multiply length by width to get the area in square feet.

  3. 3

    Multiply the area by the thickness in feet to get the volume in cubic feet.

  4. 4

    Use the volume to estimate weight and number of bags.

length * width * (thickness / 12)

Frequently asked questions

How do I convert cubic feet to cubic yards?

Divide the volume in cubic feet by 27, since there are 27 cubic feet in a cubic yard.

What is the density of concrete used for weight estimation?

We use 145 pounds per cubic foot, which is a typical density for reinforced concrete.

How many 60-lb bags of concrete mix are needed per cubic foot?

A 60-lb bag yields about 0.45 cubic feet, so divide the volume by 0.45 and round up.

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Results

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Concrete Volume

100cu ft

Estimated Weight14 500lbs
Bags Needed (60 lb)223bags
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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 foundation slab, along with the estimated weight and number of bags required.

  1. Convert thickness from inches to feet by dividing by 12.
  2. Multiply length by width to get the area in square feet.
  3. Multiply the area by the thickness in feet to get the volume in cubic feet.
  4. Use the volume to estimate weight and number of bags.

Formulas

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

Volume of a rectangular slab

V = L × W × T

Multiply length, width, and thickness (all in feet) to get the volume in cubic feet.

Example:

Input: L=20 ft, W=10 ft, T=6 in (0.5 ft)

Calculation: 20 × 10 × 0.5 = 100

Result: 100 cubic feet

Real-world use cases

Where this calculation shows up in everyday life.

Planning a concrete pour

Determine how much concrete to order for a foundation slab.

Example: A 20x10 ft slab at 6 inches thick needs 100 cu ft.

Estimating material costs

Calculate the number of bags needed to budget for a DIY project.

Example: 100 cu ft ÷ 0.45 = 223 bags (60-lb).

Checking weight limits

Estimate the weight of the concrete to ensure structural support.

Example: 100 cu ft × 145 lbs = 14,500 lbs.

Tips and common mistakes

Tips

  • Always add 5-10% extra for waste and spillage.
  • For large projects, order ready-mix concrete instead of bags.
  • Check local building codes for minimum thickness requirements.
  • Use a laser level to ensure the form is level before pouring.

Common Mistakes to Avoid

  • Forgetting to convert thickness from inches to feet.
  • Not accounting for the volume of rebar or gravel.
  • Underestimating the number of bags needed due to rounding down.

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.