How many bags of pre-mixed concrete

How many 60lb bags of pre-mixed concrete will I need to make a 4 foot by 4 foot sidewalk square at 4 inches thick? (16 sq.feet) I only found one web based calculator and it said I need almost 40 bags. That cant be right. Heck, the bags are close to 4 inches thick and they are larger than a square foot. Math is not my thing....
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This one says 12 bags;
http://www.packagepavement.com/qk_concrete_calculator.html
You may need an extra bag or two to allow for uneven ground.
I used about 43 bags of the 80 lb size for a hole 3x3 feet and about 3 feet deep.
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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
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On Fri, 21 Sep 2007 21:59:49 -0000, Big_Jake

I have found it is faster to do these calculations in cubic inches if you have one of those new fangled calculator things.
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4' x 4'  SF x .33333 = 5.33 CF x 155#/CF 6.66 lbs / 60# per bag = 13.77 bags by my calculations.
Buy 15 to be safe.
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wrote:

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.
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wrote:

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.
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12 according to this: http://www.quikrete.com/Calculator/Main.asp
steve

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On Sep 21, 7:24 am, snipped-for-privacy@hotmail.com wrote:

60lb=1/2cu ft
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No way. A cubic foot of concrete weighs between 140 and 155 pounds.
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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.
R
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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.
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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
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wrote:

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