Mastering Concrete Slab Calculations: Formulas, Yields & Planning

At its most basic level, concrete is sold by volume, not weight. In the United States, the construction standard for ordering ready-mix concrete is the Cubic Yard.

To find this, you must convert all your dimensions (Length, Width, and Thickness) into feet, multiply them to get Cubic Feet, and then divide by 27 (since there are 27 cubic feet in one cubic yard).

You can skip the manual math and conversions by using our Concrete Calculator, or you can use the formula below to understand exactly how the numbers work.

For smaller projects (typically under 1 or 2 cubic yards), buying premixed bags from a home center is often more economical than paying the "short load" fees charged by ready-mix trucks.

However, a bag of concrete is not 100% solid volume; it is a mix of aggregate, sand, and cement that consolidates when water is added. Professional estimators use specific Yield Constants to determine how many bags are required.

According to manufacturer specifications compliant with ASTM C387 (such as this Quikrete Technical Data Sheet), the typical yields for standard weight concrete are:

• 80lb Bag: Yields approximately 0.60 cubic feet.
• 60lb Bag: Yields approximately 0.45 cubic feet.
• 50lb Bag: Yields approximately 0.375 cubic feet.

To find the number of bags needed, you divide your project's total Cubic Feet by the yield of the bag you intend to buy.

If you calculate the exact mathematical volume of a 10x10 slab and order exactly that amount of concrete, you will run short.

In the real world, "perfect" dimensions do not exist. Professional contractors and engineers always add a Waste Factor (typically 5% to 10%) to account for three variables:

• Subgrade Unevenness: The ground beneath your slab is rarely perfectly flat. A dip of just ½ inch across a large patio can absorb several cubic feet of concrete.
• Form Deflection: Wooden forms often bow slightly outward under the immense pressure of wet concrete (which weighs ~145 lbs per cubic foot).
• Spillage: Some material is inevitably lost during mixing, transport, or screeding.

For most residential slab projects, a 10% safety margin is the industry standard to ensure you don't run out halfway through the pour.

A common question for DIYers is: "Should I mix this by hand or call a truck?"

The threshold for most professionals is 1.5 to 2 Cubic Yards.

1 Cubic Yard requires roughly 45 bags (80lb). Mixing 45 bags by hand is physically demanding and time-sensitive; you must mix the last bag before the first bag begins to set.

Short Load Fees: Ready-mix suppliers often charge extra fees for orders under 4 cubic yards. However, for projects over 2 yards, the cost of a truck is often comparable to buying 90+ bags, with significantly less labor and a more consistent structural result.

For detailed standards on ready-mix specifications, you can also refer to the National Ready Mixed Concrete Association (NRMCA).