How Much Concrete Do I Need? Yards, Bags and Getting It Right

Published 2026 ? 4 min read

Concrete is sold in two ways ? by the cubic yard (delivered by truck) or in bags (for small jobs). Either way, the calculation is the same: length ? width ? depth, converted to cubic yards.

The calculation

Measure your project in feet, multiply length by width by depth (in feet ? 4 inches is one-third of a foot). Divide the result by 27 to get cubic yards. That's the number the ready-mix company needs.

Bag math

For small slabs, footings or posts, bags are more practical. A 60 lb bag makes about 0.45 cubic feet; an 80 lb bag makes about 0.6 cubic feet. Divide your total cubic feet by those numbers and round up.

Order a little extra

• For delivered concrete, add 5?10% ? trucks can't top up mid-pour.
• For bags, add a couple extra for waste and spills.
• Subgrade settlement can eat volume if the ground isn't level.

Common mistakes

The biggest error is mixing units ? depth in inches when everything else is in feet. Convert first, then multiply. The second is forgetting a 4-inch slab is 0.33 ft, not 0.4.

Run the numbers

Use the concrete calculator ? enter length, width and depth, and it gives you cubic yards plus the exact number of 60 lb and 80 lb bags.