Construction
Concrete Bags Calculator
Enter your slab's length, width, and thickness to get the concrete volume in cubic yards and the exact number of 40, 60, and 80-pound bags to buy.
Quick answer: An 80-lb bag of concrete yields about 0.6 cubic feet; a 60-lb bag yields 0.45 and a 40-lb bag 0.30. There are 27 cubic feet in a cubic yard, so one cubic yard takes roughly 45 eighty-pound bags.
How it works
1. Find the volume
Multiply length × width × thickness (in feet) to get cubic feet. For a 4-inch slab, thickness is 4 ÷ 12 = 0.33 ft. A small spillage allowance is added automatically.
2. Convert to bag yield
Each bag size yields a set volume: 0.30 cu ft for 40 lb, 0.45 for 60 lb, and 0.60 for 80 lb. Divide your total cubic feet by the yield to get the bag count.
3. Compare with ready-mix
Bags make sense for small pours. Above about one cubic yard (roughly 45 eighty-pound bags), ready-mix truck delivery is usually cheaper and faster.
Frequently asked questions
How many 80 lb bags of concrete are in a cubic yard?
About 45 eighty-pound bags make one cubic yard, since each bag yields roughly 0.6 cubic feet and a cubic yard is 27 cubic feet. For 60 lb bags it's about 60 bags, and for 40 lb bags about 90.
How many bags of concrete for a 10x10 slab?
A 10×10 slab at 4 inches thick is about 1.23 cubic yards, or roughly 56 eighty-pound bags. That's near the break-even point where ready-mix delivery starts to win on price.
Is it cheaper to buy bags or ready-mix?
Bagged concrete is cheaper for small jobs under about a cubic yard. For larger pours, ready-mix delivered by truck costs less per yard and saves the labor of mixing dozens of bags.