How many bags of pre-mixed concrete

This one says 12 bags;

I used about 43 bags of the 80 lb size for a hole 3x3 feet and about 3 feet deep.

Buy 15 to be safe.

No, it's not. An 80# bag will make about 2/3 cubic foot of concrete when mixed. You need 4' x 4' x 1/3' = 5-1/3 cu ft; divide that by 2/3, and you get eight **80** pound bags, or eleven 60-pound bags. I'd get one more, to be on the safe side.

Measure one, and you'll find it's a lot closer to 2-1/2".

And it takes up *less* space after you mix in the water.

That 155 pounds per cubic foot includes the water, thus leading you to overestimate the amount of dry mix needed. I put it at 11 bags.

steve

I get the same number. Maybe the OP calculated based on 16 CUBIC feet instead of 16 SQUARE feet at 4" thick. The math seems about right.

JK

I have found it is faster to do these calculations in cubic inches if you have one of those new fangled calculator things.

No way. A cubic foot of concrete weighs between 140 and 155 pounds.

I think they're saying that a 60 pound bag makes 1/2 CF of concrete. The bound water makes up the difference in weight.

Like I pointed out to you before... you're forgetting about the water.

I think "they" will not have enough concrete to fill the forms. I hate making bagged concrete pours, but occasionally they are a necessary evil in the business. I make an accurate take off on required volume and figure 80 pound bags at 1/2 CF. 40 and 60 pound bags are easier to carry and make the cost per bag look good, but weight is weight. 60 plus 60 just won't make 150.

call a local concrete guy, they may have a larger job in the area, bag concrete costs a fortune.

a add on job may be cheaper than buying and mixing bags