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Construction

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Construction

Concrete Slab Calculator.

Estimates the volume of concrete needed for a rectangular slab and its approximate weight.

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 slab area in square feet.

  3. 3

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

  4. 4

    Optionally, divide by 27 to get cubic yards or multiply by 145 to estimate weight.

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 one cubic yard.

What is the weight of concrete per cubic foot?

Standard concrete weighs about 145 pounds per cubic foot, but this can vary with mix and moisture.

Should I order extra concrete?

It's common to order 5-10% extra to account for spillage, uneven subgrade, or slight over-excavation.

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Results

Formula checked

Concrete Volume

33.333cu ft

Cubic Yards1.23cu yd
Estimated Weight4,833lbs
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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 slab and its approximate weight.

  1. Convert thickness from inches to feet by dividing by 12.
  2. Multiply length by width to get the slab area in square feet.
  3. Multiply the area by the thickness in feet to get the volume in cubic feet.
  4. Optionally, divide by 27 to get cubic yards or multiply by 145 to estimate weight.

Formulas

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

Volume of a rectangular slab

V = L × W × T

Multiply the length, width, and thickness (all in the same units) to get the volume.

Example:

Input: L = 10 ft, W = 10 ft, T = 4 in (0.333 ft)

Calculation: 10 × 10 × 0.333

Result: ≈ 33.3 cu ft

Weight of concrete

Weight = Volume × 145 lb/ft³

Multiply the volume in cubic feet by the average density of concrete.

Example:

Input: Volume = 33.3 cu ft

Calculation: 33.3 × 145

Result: ≈ 4,829 lbs

Real-world use cases

Where this calculation shows up in everyday life.

Patio or walkway

Determine how much concrete to order for a new patio or garden path.

Example: A 10×10 ft patio at 4 inches thick needs about 1.23 cubic yards.

Footing or foundation

Estimate concrete volume for footings or small foundation pads.

Example: A 20×12 ft footing at 6 inches thick requires about 4.44 cubic yards.

Cost estimation

Get a rough volume to compare quotes from ready-mix suppliers.

Example: Knowing the cubic yards helps you estimate delivery and material costs.

Tips and common mistakes

Tips

  • Measure in feet and inches, then convert thickness to feet for consistent units.
  • Add 5-10% extra to your order to cover waste and uneven surfaces.
  • For large slabs, consider ordering ready-mix concrete rather than mixing on site.
  • Check local building codes for minimum thickness requirements.

Common Mistakes to Avoid

  • Forgetting to convert thickness from inches to feet, leading to a volume 12 times too large.
  • Not accounting for waste, resulting in a short pour.
  • Using the wrong density for concrete (e.g., using 150 lb/ft³ for lightweight mixes).

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.