Concrete Slab Calculator

What the concrete slab calculation means

A slab is usually estimated from length × width × thickness. Because thickness is often measured in inches while length and width are measured in feet, convert the thickness to feet before multiplying. Divide cubic feet by 27 to convert the result to cubic yards.

Example

Suppose a patio is 12 ft long, 10 ft wide and 4 in thick. The area is 120 square feet. Four inches is one-third of a foot, so the basic volume is about 40 cubic feet, or about 1.48 cubic yards. A project may require additional material depending on site conditions and the supplier’s ordering increments.

How to measure a slab

Measure the finished length and width of the area to be poured. For thickness, use the planned slab thickness rather than the depth of an excavation. If the subgrade or forms vary, take several measurements and use the project design or professional guidance to determine the appropriate thickness.

How much extra concrete should you order?

An allowance can help account for small measuring errors, uneven subgrade, spillage and other practical losses. The right amount varies by project. More is not automatically better: confirm the supplier’s minimum order and delivery increments, and avoid treating a generic percentage as a substitute for a site-specific estimate.

Related planning tools

For the underlying area calculation, use the Square Footage Calculator. For a deeper explanation of volume and cubic yards, read How Concrete Calculators Work. If the project is structural or subject to local requirements, use the applicable plans and professional guidance.

Use the calculator

Ready to run the numbers? Use the BuildPlanTools calculator, then compare the estimate with the product label, supplier information and project requirements.

These estimates are for general planning. Product specifications, site conditions, local requirements and professional recommendations should take priority where applicable.