Concrete Slab Calculator Formula

Understand the math behind the concrete slab calculator. Each variable explained with a worked example.

Use the Concrete Slab Calculator

Formulas Used

Slab Area

area = length * width

Volume (Cubic Feet)

cubic_feet = length * width * thickness_ft

Volume (Cubic Yards)

cubic_yards = length * width * thickness_ft / 27

With 10% Waste

cubic_yards_with_waste = length * width * thickness_ft / 27 * 1.10

Variables

VariableDescriptionDefault
lengthSlab Length(feet)20
widthSlab Width(feet)12
thicknessSlab Thickness(inches)4
thickness_ftDerived value= thickness / 12calculated

How It Works

How to Calculate Concrete for a Slab

Formula

Volume (cu ft) = Length (ft) x Width (ft) x Thickness (ft)

Volume (cu yd) = Volume (cu ft) / 27

First convert thickness from inches to feet by dividing by 12. Standard slab thicknesses are 4 inches for patios and walkways, 4-6 inches for driveways, and 6-8 inches for heavy-duty applications.

Worked Example

A patio slab measuring 20 ft by 12 ft with a 4-inch thickness.

length = 20width = 12thickness = 4
  1. 01Convert thickness: 4 inches / 12 = 0.333 ft
  2. 02Volume = 20 × 12 × 0.333 = 80 cu ft
  3. 03Cubic yards = 80 / 27 = 2.96 cu yd
  4. 04With 10% waste = 2.96 × 1.10 = 3.26 cu yd
  5. 05Order 3.5 cubic yards from the concrete plant.

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