How this calc works

Concrete is ordered by the cubic yard, but you measure a slab in feet and inches — so every order starts with a unit problem. This calc takes your length and width in feet and your thickness in inches, converts the thickness to feet, multiplies the three together for cubic feet, then divides by 27 to get cubic yards (there are 27 cubic feet in a yard).

The number it shows already includes a 10% waste factor. That covers uneven sub-grade, spillage, and the truck not scraping perfectly clean. Tap Show the Math to see every step — the area, the thickness conversion, the raw volume, and the waste add-on — so you can hand the number to the supplier and defend it.

Common mistakes

The number-one error is leaving thickness in inches. A 4-inch slab is 0.33 feet, not 4 — multiply by 4 and you order twelve times too much. This calc handles that conversion for you, but if you do it by hand, that is where it goes wrong.

The second is ordering the exact yardage with no cushion. A ready-mix truck will not come back for a quarter yard, and you cannot pour half a slab and run to the store. Round up to the nearest quarter yard and keep the 10% factor on.

Code & regional notes

Slab thickness is code-driven: 4 inches is typical for patios and walkways, but driveways carrying vehicles often require 5-6 inches, and many jurisdictions want a compacted gravel base and rebar or wire mesh on anything structural. Check your local building department before you pour.

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