Planning a patio, driveway, or shed base? Determining the exact amount of concrete required is crucial to avoid running out mid-pour or overspending on materials. This Concrete Slab Calculator helps you determine the precise volume and number of premix bags needed for your project.
The Concrete Calculation Formula
To calculate the amount of concrete needed for a rectangular slab, we use the volume formula adjusted for density. The basic math involves three steps:
Calculate Volume in Cubic Meters: Multiply Length (m) × Width (m) × Thickness (m). Note that thickness is usually measured in centimeters, so divide by 100 to convert to meters.
Calculate Total Weight: Dry concrete mix typically has a density of approximately 2,100 to 2,400 kg per cubic meter once cured. We use a standard factor of roughly 2,130kg/m³ for dry premix yield.
Determine Bags Needed: Divide the total weight by the size of the bags you are purchasing (typically 20kg or 25kg).
Why Add a Waste Margin?
Professional concreters always add a safety margin, typically between 5% and 10%. This accounts for:
Spillage during mixing and pouring.
Uneven ground (sub-grade) depth which may require more material to level.
Material remaining inside the mixer or wheelbarrow.
Our calculator defaults to a 10% safety margin to ensure you don't run short.
Calculation Example
Imagine you are pouring a concrete slab for a garden shed with the following dimensions: