Concrete Calculator
Skip the manual arithmetic and calculate your own numbers.
Concrete volume formula for a rectangular slab
For a rectangular slab, multiply length by width by thickness. Use consistent units before multiplying.
If length and width are measured in feet but thickness is measured in inches, divide the thickness by 12 first so all three dimensions are in feet.
Convert cubic feet to cubic yards
Ready-mix concrete is commonly discussed in cubic yards. There are 27 cubic feet in one cubic yard.
For example, a 10-foot by 10-foot slab that is 4 inches thick has about 33.33 cubic feet of volume. Dividing by 27 gives about 1.23 cubic yards before any waste allowance.
Why add a waste allowance?
Real projects are not always perfect rectangles. Excavation can vary, forms may not be exact, and some material remains in equipment or is lost during placement. A modest extra allowance can reduce the risk of running short.
The appropriate allowance depends on the project. A simple calculator can show both the mathematical volume and a volume after a selected waste percentage.
Irregular shapes and multiple sections
For a project made from several rectangular areas, calculate each section separately and then add the cubic yards together. For circles, cylinders, footings or steps, use the geometry that matches the actual shape rather than forcing everything into a rectangle formula.
Measurement tips before ordering
- Measure length and width in several places if the area is not perfectly square.
- Confirm the finished thickness rather than estimating by eye.
- Convert inches to feet before multiplying.
- Separate areas with different thicknesses.
- Discuss minimum-order amounts, delivery charges and short-load fees with the supplier.
Common concrete-calculation mistakes
- Forgetting to convert inches of thickness to feet.
- Stopping at cubic feet when the supplier quotes cubic yards.
- Using square footage without multiplying by thickness.
- Failing to add separate sections of a project.
- Ordering the exact mathematical volume with no allowance for real-world variation.
Frequently asked questions
How many cubic feet are in a cubic yard?
One cubic yard contains 27 cubic feet.
Do I need extra concrete beyond the exact calculated volume?
Often a modest allowance is useful because site dimensions and placement are rarely perfect. The amount depends on the project.
Can I calculate concrete from square footage alone?
Not completely. You also need the concrete thickness to convert surface area into volume.