Bags equal your volume divided by the yield on the bag
Yield is the finished concrete each bag makes, not its dry weight. Standard premixed mix gives about 0.45 ft³ from a 60 lb bag and 0.60 ft³ from an 80 lb bag. Find your pour volume (length × width × thickness, all in feet), divide, and round up — a 9 ft³ slab needs 20 bags of 60 lb or 15 bags of 80 lb.
Bags by pour volume
| Volume | 60 lb bags | 80 lb bags |
|---|---|---|
| 1 ft³ | 3 | 2 |
| 5 ft³ | 12 | 9 |
| 10 ft³ | 23 | 17 |
| 1 yd³ (27 ft³) | 60 | 45 |
Figures are rounded up to whole bags. Once you pass about half a cubic yard, ready-mix delivery usually beats bagging it by hand on both cost and effort.
Two things people get wrong
- Weight is not volume. An 80 lb bag is not "twice" a 40 lb bag in coverage terms — work from the yield printed on the bag, not the weight.
- Add a margin. Uneven subgrade and spillage eat into your total, so order about 10% extra. Running one bag short mid-pour leaves a cold joint.
Common questions
How many bags of concrete do I need per cubic foot?
About 2.2 bags of 60 lb mix or 1.7 bags of 80 lb mix per cubic foot, since a 60 lb bag yields roughly 0.45 cubic feet and an 80 lb bag about 0.60. Always round up, because you cannot buy a partial bag.
How many 80 lb bags make a cubic yard?
A cubic yard is 27 cubic feet, so at 0.60 cubic feet a bag you need about 45 bags of 80 lb mix, or roughly 60 bags of 60 lb mix. For that much volume, ready-mix delivery is usually cheaper.
What is the yield of a bag of concrete mix?
Yield is the finished volume of mixed concrete, not the dry weight. A standard premixed bag yields about 0.0075 cubic feet per pound: 0.45 ft3 for 60 lb and 0.60 ft3 for 80 lb.
