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Concrete Calculator

Estimate the volume of concrete needed for a slab, in cubic yards and in 80 lb bags.

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What Concrete Calculator does

Ordering concrete is unforgiving in both directions: too little means a cold joint in the middle of a pour, and too much is paid for and wasted.

Enter the slab dimensions to get the volume in cubic yards, which is how ready-mix is ordered, and an equivalent count of 80 lb bags for a small job you are mixing yourself.

  • Slab volume from length, width and thickness
  • Result in cubic yards
  • Equivalent number of 80 lb bags

How to use Concrete Calculator

  1. 1

    Measure the area

    Length and width in feet.

  2. 2

    Enter the thickness

    In inches. Four inches is typical for a patio or walkway; six for a driveway.

  3. 3

    Add a waste allowance

    Add 5 to 10 percent yourself. The calculation gives the exact volume, and real ground is never exactly level.

Limits and known behaviour

  • Rectangular slabs only. Footings, columns, steps and irregular shapes must be broken into rectangles and added.
  • No waste factor is included. Uneven subgrade, spillage and over-excavation mean the real requirement is always higher than the exact volume.
  • Bag counts assume 80 lb bags at about 0.6 cubic feet each. Check the yield printed on the bags you are buying.
  • Reinforcement, sub-base, formwork and finishing materials are not calculated.
  • Imperial units only: feet, inches, cubic yards.

Privacy and data handling

Runs entirely in your browser

  • The calculation runs in this page. Nothing you enter is transmitted or stored.
  • The tool works offline once the page has loaded.

Site-wide data handling, including analytics and advertising, is described in the privacy policy.

Frequently asked questions

How much extra should I order?

Most guidance suggests 5 to 10 percent above the calculated volume. Running out mid-pour creates a cold joint, which is a structural weakness and far more expensive to deal with than surplus concrete.

How many bags are in a cubic yard?

Roughly 45 at 80 lb each. Beyond about a cubic yard, ready-mix is usually cheaper and much faster than mixing bags by hand.

How thick should my slab be?

Four inches is common for patios and walkways, and six or more for driveways and anything carrying vehicles. Local building codes and ground conditions take precedence.