How to Calculate Concrete for a Slab

Accurately calculating the amount of concrete needed for a project is crucial to avoid costly shortages or wasteful overages. Our concrete calculator simplifies this for you, but understanding the formula is key for any DIY or professional construction project.

The Concrete Volume Formula

The basic formula for the volume of a rectangular slab is simple: Volume = Length × Width × Thickness. However, the complexity comes from ensuring all your measurements are in the same unit before calculating the final volume, which is typically ordered in cubic yards or cubic meters.

For Imperial Measurements (feet, inches):

1. First, convert the thickness from inches to feet by dividing by 12. For a 4-inch slab, this is 4 / 12 = 0.333 feet.
2. Next, calculate the volume in cubic feet: Length (ft) × Width (ft) × Thickness (ft).
3. Finally, since concrete is ordered in cubic yards, convert cubic feet to cubic yards by dividing by 27 (since there are 27 cubic feet in one cubic yard).

For Metric Measurements (meters, cm):

1. Convert the thickness from centimeters to meters by dividing by 100. For a 10cm slab, this is 10 / 100 = 0.1 meters.
2. Calculate the volume in cubic meters: Length (m) × Width (m) × Thickness (m). The result is already in cubic meters, which is how concrete is ordered in the metric system.

Always consider ordering slightly more (about 5-10%) than your calculated amount to account for uneven subgrade, spillage, and variations in form-work.