10x20x4in concrete slab
Here’s the concrete needed for a 10 ft by 20 ft slab at 4 inches thick.
Different dimensions? Use the concrete calculator above.
About the concrete calculator
The volume formula here is straightforward — length times width times thickness — but the units need care. Slab length and width are measured in feet, while thickness is almost always specified in inches, so this calculator converts thickness to feet (dividing by 12) before multiplying. Skipping that conversion is the most common way people end up with a volume estimate that's off by a factor of 12.
Once you have cubic feet, dividing by 27 converts to cubic yards, which is how ready-mix concrete is actually ordered and priced once you get past small bagged-mix jobs. Ready-mix trucks and most supplier price sheets quote by the cubic yard, so this is the number you'd give a concrete supplier over the phone.
The 0.45 and 0.6 cubic-feet-per-bag yield figures for 60lb and 80lb bags are the standard published yields printed on typical bagged concrete mix. Notice that an 80lb bag doesn't yield proportionally more than a 60lb bag per pound of dry mix — that's because bag yield accounts for the water and other components added during mixing, not just the dry weight of the bag itself.
Bagged concrete makes sense for small pours — fence post footings, a small patio patch, or anything roughly a cubic yard or less. Beyond that, mixing by hand or with a small mixer becomes labor-prohibitive, and ordering ready-mix from a truck ends up cheaper per unit of concrete and far less physically demanding for anything larger.
This calculator assumes a uniform rectangular slab poured at a constant thickness. It doesn't add extra for sub-base prep, formwork, spillage, or an over-pour buffer — many contractors add 5-10% on top of the calculated volume specifically to avoid running short mid-pour, since a concrete truck won't wait around for a second batch once it starts setting.
Concrete calculator questions
How to estimate concrete volume and bag counts.
How many bags of concrete do I need for a slab?
Divide the slab's cubic feet by the bag's yield — about 0.45 cu ft for a 60lb bag or 0.6 cu ft for an 80lb bag — and round up. For example, a 10x10 ft slab at 4 inches thick is 33.3 cubic feet, which needs about 74 bags of 60lb mix or 56 bags of 80lb mix.
How do I calculate cubic yards of concrete?
Multiply length by width by thickness (converted from inches to feet) to get cubic feet, then divide by 27 to get cubic yards. A 10x10 ft slab at 4 inches thick is 33.3 cubic feet, or about 1.23 cubic yards.
How thick should a concrete slab be?
A standard concrete patio or walkway slab is typically 4 inches thick, while driveways that carry vehicle loads are often poured 5-6 inches thick. Check your local building code for load-bearing applications.
How many 80lb bags of concrete are in a cubic yard?
One cubic yard is 27 cubic feet, and each 80lb bag yields about 0.6 cubic feet, so it takes about 45 bags of 80lb mix to make one cubic yard of concrete.
How do I calculate concrete for a patio?
Measure the patio's length and width in feet and its thickness in inches (commonly 4 inches), then enter those into the calculator to get the cubic yards and bag count needed.
What is the formula for concrete volume?
Volume in cubic feet equals length (ft) × width (ft) × thickness (ft), where thickness is converted from inches by dividing by 12. Divide cubic feet by 27 to convert to cubic yards.