Concrete Bag Calculator
How many bags of concrete mix your pour needs, using the yield printed on the bag you are actually buying.
Switching units keeps your measurements. Nothing is reset.
What are you pouring?
A 4 in slab is 0 ft 4 in, not 4 ft.
Your bags
Printed on the bag as the volume of mixed concrete it makes. Weight alone cannot tell you this.
Waste allowance
10% covers spillage and mixing losses, which are higher when mixing by hand.
Offcuts, breakages, pattern matching and the occasional damaged piece all mean you use more material than the bare area. Keeping a few spare pieces for future repairs is also normal.
This is a planning allowance, not a rule. Adjust it for your layout, your material and your own tolerance for a second trip to the store.
Estimated cost
Cost uses the price you enter. This product never shows invented market prices.
Your result appears here
Enter your measurements to see how much material to buy.
How to use this calculator
- 1
Describe the pour exactly as you would for ready-mix: slab, footing, wall or round column, with the thickness in inches.
- 2
Find the yield on the bag. It is the volume of mixed concrete one bag makes, usually printed as cubic feet, and it is not the same as the bag weight.
- 3
Enter that yield. The bag count is then your total volume divided by the yield, rounded up.
- 4
Add your price per bag if you want a cost, and remember that a large pour is usually cheaper and far easier as ready-mix.
A worked example
Four post footings, each 12 in across and 30 in deep, using bags that yield 0.45 ft³, with a 10% overage:
Add the sections together
1.96 ft³ + 1.96 ft³ + 1.96 ft³ + 1.96 ft³7.85 ft³
Add the waste allowance
7.85 ft³ × 1.18.64 ft³
Divide by the coverage of one package
8.64 ft³ ÷ 0.45 ft³19.2
Round up to whole packages
round up 19.220
Coverage you are buying
20 × 0.45 ft³9 ft³
You will need
20 bags
This example is produced by running the calculator above, so it always matches what the tool does.
The formula
Volume is calculated from the pour geometry exactly as it is for ready-mix, then the overage is applied. The bag count is that volume divided by the yield of one bag, rounded up. There is no formula from bag weight to yield: yield depends on the mix design, so it has to come from the bag.
Working with bagged mix
Yield is not weight
Two bags of the same weight from different manufacturers can make different volumes of concrete. That is why this calculator asks for the yield rather than converting from pounds, and why any calculator that does convert from weight alone is guessing.
There is a point where bags stop making sense
Mixing forty or fifty bags by hand for a single slab is a long, hard day and risks an uneven pour. Past a certain size, ready-mix is both cheaper and better concrete.
Mix losses are real
Hand mixing leaves material in the barrow, the bag and the ground. The overage covers that, and it is worth keeping when you are mixing on site.
Common questions
- How many bags of concrete do I need?
- Divide the volume of your pour by the yield printed on the bag, then round up. Enter the pour above and the calculator does it, including an adjustable overage for spillage.
- How much concrete does an 80 lb bag make?
- It depends on the product. The yield is printed on the bag and differs between manufacturers and mixes, so this calculator asks for it rather than assuming a figure that might be wrong for the bag in front of you.
- Is bagged mix the same strength as ready-mix?
- Bagged mixes come in different strengths, stated on the bag. The bigger practical difference is consistency: a hand-mixed pour varies more between batches than a delivered load.