How to Calculate How Many Cubic Yards of Concrete You Need

 

To calculate how many cubic yards of concrete you need for a rectangular slab, multiply the length and width in feet by the thickness in inches, then divide by 324.

Cubic yards of concrete = length (ft) x width (ft) x thickness (in) / 324

For example, a 9-foot by 9-foot slab with a thickness of 4 inches has a geometric volume of 1 cubic yard. The thickness is an arithmetic example, not a design recommendation.

Use our Concrete Calculator to estimate volume in cubic yards, cubic feet, and cubic meters. You can also enter a manufacturer's yield per bag or a material price per volume unit.

Measure a rectangular concrete slab

1. Measure the planned length and width in feet.
2. Obtain the specified slab thickness in inches.
3. Multiply the three measurements.
4. Divide by 324 to convert the result to cubic yards.

Check whether the planned thickness is uniform. This formula does not account automatically for thickened edges, beams, holes, or changes in level.

Why divide by 324?

First convert thickness from inches to feet by dividing by 12. Then multiply length, width, and thickness to get cubic feet. Divide cubic feet by 27 to get cubic yards.

Since 12 x 27 = 324, you can combine those steps into one division.

For the example slab, 4 inches / 12 = one-third of a foot. Then 9 x 9 x one-third = 27 cubic feet. Finally, 27 / 27 = 1 cubic yard.

Example: a 12-by-10-foot slab

At an illustrative thickness of 4 inches:

12 x 10 x 4 / 324 = approximately 1.4815 cubic yards.

That equals 40 cubic feet. Keep the unrounded volume for any subsequent bag or cost calculation; round only the displayed result or final purchasing quantity as appropriate.

Square yards and cubic yards are different

Square yards describe area. Cubic yards describe volume. Knowing the surface area alone is not enough to order concrete without a thickness.

A 9-foot by 9-foot area is 81 square feet, or 9 square yards. At 4 inches thick, it requires 1 cubic yard geometrically, not 9 cubic yards.

For area measurements only, use our Square Footage Calculator.

How to calculate concrete bags

Bag equivalents = required volume / mixed concrete yield per bag.

Check the product's stated volume of mixed concrete produced per bag. Bag weight alone does not tell you that yield. Use matching volume units, then round the required bag count up to a whole bag.

If a product states a yield of 0.6 cubic feet per bag, one cubic yard requires 27 / 0.6 = 45 bags before any additional allowance. A different product yield changes the result.

For the 40-cubic-foot example above, 40 / 0.6 is approximately 66.67 bag equivalents, so the purchase estimate is 67 whole bags, before any additional allowance.

Metric concrete volume

Cubic meters = length (m) x width (m) x thickness (cm) / 100.

A 5-meter by 4-meter slab with a 10-centimeter thickness has a volume of 2 cubic meters. That equals 2,000 liters. At a stated yield of 20 liters per bag, it requires 100 bags. These are calculation examples; use your specified dimensions and the yield of your chosen product.

Estimate material cost

Multiply the unrounded volume by a price for the same volume unit. One cubic yard at an illustrative material price of $150 per cubic yard equals $150.

This does not include delivery, minimum-order charges, labor, formwork, reinforcement, site preparation, or taxes. It is not a current market quote. A cost based on bulk volume pricing is also different from the cost of buying a whole number of premixed bags.

Should you add an allowance?

The geometric formula includes no extra allowance for over-excavation, an uneven base, spills, or handling losses. Ask your supplier or contractor how site conditions and ordering increments affect the final quantity. Whole-bag rounding is separate from an extra percentage allowance.

Frequently asked questions

How many cubic feet are in a cubic yard of concrete?

One cubic yard equals 27 cubic feet. This is a volume conversion, not a weight conversion.

Can I use a slab calculator for hollow concrete blocks?

Not by entering the blocks' external dimensions. The fill volume depends on the internal voids and product design. Use the manufacturer's fill requirements.

Does the calculator tell me the right thickness?

No. Thickness, reinforcement, concrete specification, and base requirements depend on the project and applicable design requirements.

Calculate your concrete volume

Enter your planned measurements in the Concrete Calculator. Confirm the final order with your supplier or contractor.

This guide provides general calculation examples, not structural engineering, construction specifications, or installation advice.