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.
For a poured slab, order in quarter-yard increments from the ready-mix company and confirm their minimum charge - some charge for a full yard even if you use less. If you are mixing bags instead, keep the math simple: an 80 lb bag yields about 0.6 cubic feet, so divide your cubic feet by 0.6 to get the bag count, then add 10% and round up to whole bags.