Concrete Bag Calculator
Bagged concrete is priced by the bag, not the yard, so the useful number is how many bags a pour takes. This converts your dimensions into counts for every common bag size.
Bags needed
11bags
- 40lb bags
- 22
- 50lb bags
- 18
- 60lb bags
- 15
- 80lb bags
- 11
- Cubic yards
- 0.24
- Cubic feet
- 6.6
- Includes a 10% waste allowance. The bare figure is 0.22 cubic yards.
- Bag yields are the standard figures printed on pre-mixed concrete: 0.6 cubic feet for an 80lb bag, 0.45 for a 60lb, 0.375 for a 50lb and 0.30 for a 40lb.
About the concrete bag calculator
Every bag of pre-mixed concrete states the volume it yields once mixed, and that yield is what a bag count is built on. An 80lb bag makes 0.60 cubic feet, a 60lb makes 0.45, a 50lb makes 0.375 and a 40lb makes 0.30.
The yields are not proportional to the weight in the way you might expect. Two 40lb bags give exactly the same 0.60 cubic feet as one 80lb bag, so buying smaller costs you nothing in volume — only in trips from the car.
This is the tool for small pours: a shed base, a set of footings, a few posts. Past about half a cubic yard the mixing becomes the whole job and ready-mix is usually both cheaper and easier.
What it works out
- Bag counts for 40, 50, 60 and 80lb pre-mix
- The same pour in cubic yards and cubic feet
- A waste allowance so you are not one bag short
The formula
Bags = (Length × Width × Thickness) ÷ Yield per bag
Every bag of pre-mixed concrete states the volume it yields once mixed, and that yield is what the count is built on. An 80lb bag makes 0.60 cubic feet, a 60lb makes 0.45, a 50lb makes 0.375 and a 40lb makes 0.30.
Note that the yield is not proportional to the weight in the way you might expect — two 40lb bags give you 0.60 cubic feet, exactly the same as one 80lb bag, so buying smaller bags costs you nothing in volume, only in trips to the car.
Bag counts always round up, because you cannot buy part of a bag, and it is worth adding a waste allowance on top of that. Running out with half a footing poured means a cold joint where the two pours meet, which is a permanent weakness rather than an inconvenience.
- Length, Width, Thickness
- The dimensions of the pour. Thickness is usually in inches while the other two are in feet.
- Yield per bag
- Cubic feet of mixed concrete one bag makes, printed on the bag.
A worked example
A six by three foot shed base, four inches thick, in 80lb bags.
That works out to 11 bags.
- 40lb bags
- 22
- 50lb bags
- 18
- 60lb bags
- 15
- 80lb bags
- 11
- Cubic yards
- 0.24
- Cubic feet
- 6.6
Questions
How many 80lb bags of concrete are in a cubic yard?
Forty-five. A cubic yard is 27 cubic feet and an 80lb bag yields 0.60, so 27 ÷ 0.60 = 45. For 60lb bags it is 60, and for 40lb bags it is 90.
At what point should I stop using bags?
Around half a cubic yard, which is roughly 23 80lb bags. Past that the mixing becomes the entire job, and most ready-mix suppliers have a minimum load of about a yard anyway.
How much water does a bag of concrete need?
Roughly three quarts for an 80lb bag, but go by what the bag says and add it gradually. Too much water is the most common cause of weak concrete — it raises the workability and lowers the final strength.
How many bags do I need for a fence post?
About four 50lb bags, or three 80lb bags, for a 4x4 post in a 10-inch hole three feet deep — the hole holds roughly 1.6 cubic feet and the post displaces a quarter of that. Use a column calculator for a precise figure, since a post hole is a cylinder rather than a slab.
Can I mix bags of concrete by hand?
Up to about ten bags, yes, in a wheelbarrow or mixing tub. Beyond that hire a mixer — hand-mixing gets inconsistent as you tire, and inconsistent mixing shows up as weak spots later.
Does this work for footings and post holes?
For rectangular footings, yes — enter the trench dimensions. Round post holes are cylinders, so the volume is different; the slab shape here will overestimate them.