How to calculate the amount of concrete needed for a slab?

A comprehensive guide to measuring, calculating, and ordering concrete for your slab project.

Before pouring a concrete slab for a patio, driveway, or garage, it is crucial to order the right amount. Too little, and you risk a weak cold joint. Too much, and you pay for wasted concrete. Here is how to do the exact calculation.

1. Necessary Measurements

To determine the concrete volume, you need to measure three dimensions accurately:

• Length (L): the longest dimension of your space.
• Width (W): the dimension perpendicular to the length.
• Depth (D): the thickness of the slab. For a standard patio, 10 cm (4 inches) is generally used. For a driveway, aim for 15 cm (6 inches).

2. The Calculation Formula

Concrete is ordered in cubic meters (m³) or cubic yards (yd³). The basic formula is simple:

Make sure all your units are the same before multiplying. For example, if you measure in meters, a depth of 10 cm becomes 0.1 m.

3. The Golden Rule: Waste Factor

Professionals always recommend adding a margin of error, called a waste factor. Generally, add 10% to your total volume.

Why? Because the ground is never perfectly flat. A slight depression in the gravel base can require more concrete than expected.

4. Conversion to Bags

For small projects, you will likely buy concrete in bags. A 30 kg bag yields about 0.013 m³ of concrete. Simply divide your total volume by this yield to find the number of bags required.