Concrete Calculator

For slabs, footings and posts — in cubic yards and bags.

How to use this calculator

Enter the length, width and depth in inches, then press Calculate. It returns volume in cubic feet, cubic yards, and the number of 60 lb and 80 lb bags.

What the result means

Cubic yards is what ready-mix companies quote; bags are for smaller jobs. Adding a little extra covers subgrade settlement and waste.

How concrete is measured

Concrete is sold by the cubic yard in the US. A cubic yard is 27 cubic feet, so the calculator converts your length, width and depth into cubic feet, then divides by 27 to tell you how many yards to order. Depth is usually entered in inches, which is why the conversion matters - 4 inches is one third of a foot.

Worked example

A 10 ft by 10 ft slab at 4 inches deep is 10 x 10 x 0.333 = 33.3 cubic feet, or about 1.23 cubic yards. Most ready-mix trucks deliver in quarter-yard steps, so order 1.25 yards plus a little extra for spillage and grade.

The waste rule

Add 5-10% on top of the math. Forms move, ground settles, and finishers always use more than the plan says. For bagged mix, a 60 lb bag makes about 0.45 cubic feet and an 80 lb bag about 0.6 - divide your cubic feet by those numbers to get bag counts.

Frequently asked questions

How many bags are in a cubic yard?

Roughly 45 sixty-pound bags or 34 eighty-pound bags per cubic yard.

Should I order extra concrete?

For delivered concrete, add 5-10% — trucks can't top up mid-pour. For bags, add a couple extra.

How thick does a slab need to be?

Patios and walkways: 4 inches. Driveways and heavy loads: 5-6 inches. Footings and posts go deeper still - deeper always beats thicker for load-bearing work.