How this calc works

Here is the math a contractor does in his head before he tells you to buy bagged mix instead of calling a truck. The calc finds the volume of your pour in cubic feet, then divides by the yield of one bag: an 80-lb bag makes about 0.6 cubic feet, a 60-lb bag about 0.45, a 50-lb bag about 0.375. It rounds up and adds 10% so you are not one bag short at the end.

Bagged mix makes sense for small jobs — post holes, a small pad, steps. Once you cross roughly 1 cubic yard (about 45 of the 80-lb bags), a ready-mix truck is usually cheaper and far less work than mixing 45 bags by hand.

Common mistakes

People grab the yield off the front of the bag and forget it assumes the mix is at the right water ratio. Add too much water to make it easier to pour and you lose strength; the bag count does not change but the slab does.

The other miss is buying by weight without checking your back. Forty-five 80-lb bags is 3,600 pounds of mixing. For anything past a few bags, rent a mixer or split to 60-lb bags so the lifting does not wreck you halfway through.

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