How to Estimate a Floor Slab

Pouring a floor slab requires a steady flow of concrete for the foundation to fill the slab form in one session to avoid different portions of the slab curing at differing rates. To make certain you can do this, you’ll need to estimate the amount of concrete needed for the slab beforehand to ensure that enough concrete is available. This requires a simple volume calculation to determine the size of the foundation hole in cubic feet. Once you’ve done this, you can purchase the number of concrete bags needed to fill the space, preventing the need for any mid-pour runs to the hardware store for more.

Tape measure
Calculator

Instructions

- 1
  Measure the length and width of the foundation site with a tape measure. Round up the measurements to the nearest foot then multiply them together to get the surface area of the slab. For example, a hole 10 feet by 20 feet would have a surface area of 200 square feet.
- 2
  Lower the end of the tape measure to the bottom of the foundation site and measure the depth of the hole from the base to the rim, rounding this up to the nearest 1/4-foot. A depth of 5 inches, for example, would round up to 1/2-foot. Multiply this by the square feet to get the estimated volume of the slab. For a 200-square-foot hole with a rounded depth of 1/2-feet, the estimated volume would be 100 cubic feet.
- 3
  Examine a package of the concrete you intend to use for your slab to determine the coverage per bag. Many 80lb. bags of concrete have a coverage rate of 0.60-cubic foot per bag. Divide the volume in feet by the coverage rate in feet to determine the number of bags needed. For a hole with a volume of 100 cubic feet, the number of 0.60-cubic-foot bags needed for coverage is 167. Always round a partial bag result upward.

Can You Paint Over Marblecrete?

DIY Ideas for Widening Driveways

Paving