How to Calculate Concrete Needed To Pour a Slab

An accurate determination of the amount of concrete you need to pour a slab is crucial -- you don't want a concrete company truck to arrive with less than you need, and you don't want to load more bags of ready-mix than necessary onto your truck. Measuring your slab is simple enough, but converting the measurements to the units necessary for ordering concrete can be confusing.

Common Units

When ordering concrete for a large project, such as the slab for a new garage, you need to quote the amount you need in cubic yards. For smaller projects, such as the floor for a garden shed, cubic feet is a more appropriate unit. If you're buying the concrete in bags, neither of these units is as relevant as the number of bags you need, and that depends on whether you're buying 80-, 60- or 40-pound bags. A simple conversion table comes in handy:

  • 1 cubic yard equals 27 cubic feet, which equals 46,656 cubic inches.
  • One 80-pound bag of concrete mix makes 0.6 cubic feet of concrete.
  • One 60-pound bag makes 0.45 cubic feet.
  • One 40-pound bag makes 0.3 cubic feet.

Measuring the Slab

To calculate the volume of concrete you need, you must measure the surface area of the slab and multiply this by the thickness of the slab. You usually measure the length and width of the slab in feet, thus giving you a value for area in square feet when you multiply them, but you usually measure thickness in inches. In most cases, it's best to convert the thickness measurement to feet to get the volume in cubic feet, which makes it easy to calculate the number of yards you need -- if it's a big project -- or the number of bags, if it's a small one.

Sample Calculation

A large slab for a building foundation may be 60 feet long, 40 feet wide, and have a thickness of 6 inches.

The area of the slab is 60 feet x 40 feet = 2,400 square feet. Converting the thickness measurement to 0.5 feet and multiplying the area by this yields a volume of 1,200 cubic feet.

This is a large project, so it's best to have the concrete delivered by a truck. Applying the proper conversion factor, you'll need to order 44.5 cubic yards. You wouldn't want to transport the concrete yourself, because you would need 2,000 80-pound bags.