How much concrete do I need for a slab?

The basic calculation

Concrete volume is length times width times depth. The one catch: slab thickness is almost always given in inches, while length and width are in feet, so you convert the thickness first.

Divide the thickness in inches by 12 to get feet. A 4-inch slab is 4 / 12 = 0.333 ft; a 6-inch slab is 6 / 12 = 0.5 ft. With all three dimensions in feet, multiply them together for the volume in cubic feet.

Ready-mix is ordered and priced by the cubic yard, not the cubic foot. There are 27 cubic feet in a cubic yard, so divide your cubic-foot total by 27 to get yards.

The formula

Volume (cubic feet) = length (ft) x width (ft) x thickness (ft), where thickness (ft) = thickness in inches / 12.

Cubic yards = cubic feet / 27.

Knowing the two steps lets you sanity-check the calculator's output before you commit to an order.

Worked example: a 10x10 slab at 4 inches

Pouring a 10 ft by 10 ft slab, 4 inches thick:

Convert the thickness: 4 / 12 = 0.333 ft. Volume: 10 x 10 x 0.333 = 33.3 cu ft. Cubic yards: 33.3 / 27 = 1.23. Add 10 percent for waste and plan for about 1.36 cubic yards.

Mixing from bags instead of ordering ready-mix? Work from the cubic-foot figure. An 80-lb bag yields about 0.6 cu ft, so 33.3 / 0.6 is about 56 bags.

• Thickness: 4 / 12 = 0.333 ft
• Volume: 10 x 10 x 0.333 = 33.3 cu ft
• Cubic yards: 33.3 / 27 = 1.23
• With 10 percent waste: about 1.36 cubic yards
• In 80-lb bags: 33.3 / 0.6 = about 56 bags

Bagged concrete vs. ready-mix

For small jobs, bagged concrete you mix yourself is convenient. Yield is roughly 0.6 cu ft per 80-lb bag and about 0.45 cu ft per 60-lb bag. Divide your cubic-foot volume by the bag yield to get a count. The 33.3 cu ft example works out to about 56 of the 80-lb bags or about 74 of the 60-lb bags.

Past a small pad, mixing dozens of bags by hand becomes impractical, and ready-mix by the yard is usually easier and often cheaper. A shortfall hurts most with ready-mix: if a second batch arrives after the first has started to set, you get a cold joint. That is the main reason to round up rather than down.

These are planning and budgeting estimates, not engineering figures. Actual yield shifts with how wet you mix, how much you spill, and how level your subgrade is.

Why order about 10 percent extra

Forms are never perfect. The ground under the slab is rarely dead level, so low spots pull extra concrete. Spillage, material left in the mixer or wheelbarrow, and slightly over-dug edges add up. Ordering roughly 10 percent more than the calculated volume covers it.

On the 10x10 example, that turns 1.23 cubic yards into about 1.36. It feels like a lot until you are 10 minutes from finishing and come up short. Running out mid-pour costs far more than a little leftover, in both dollars and the quality of the finished slab.