Concrete volume question

[Fiveohhh]

I need to pour some footings at my house for my deck and I have 6 footing 12" round by 48" deep. Using v=pi*h*(r*r) I came up that each footing is 5,425cubic inches there are 1728 cubic inches in a cubic foot, so that is apprx 3.1 cubic feet per footing. By guessing an 80# bag of ready-mix concrete I should be able to get 2 cubic feet per bag. from experience I know for a fact I'll need more than 2 bags to fill the footing. Where in my measurments am I wrong.... On the preform tube it says I should need about 4 bags for each footing(seems about right)

 

[Viper GTS]

Your math looks OK to me (with the exception of the typo in the number of in³ per ft³), the only place I can see that you might have gone wrong is how much cement is in an 80 lb bag.

Viper GTS

 

[Fiveohhh]

Hmmm, I got 2 cubic feet from some site I looked at, gonna double check and see if I can find out how much concrete is in an 80# bag

 

[conjur]

sq. inches? You mean cubic, right?

BTW, it's 1728 cu. inches in a cubic foot.

 

[Fiveohhh]

Originally posted by: Fiveohhh
I need to pour some footings at my house for my deck and I have 6 footing 12" round by 48" deep. Using v=pi*h*(r*r) I came up that each footing is 5,425sq inches there are 1726 sq inches in a sq foot, so that is apprx 3.1 cubic feet per footing. By guessing an 80# bag of ready-mix concrete I should be able to get 2 cubic feet per bag. from experience I know for a fact I'll need more than 2 bags to fill the footing. Where in my measurments am I wrong.... On the preform tube it says I should need about 4 bags for each footing(seems about right)

edit: found a site that says .6 cubic feet per 80# bag. seems low but makes the numbers work...

fixed cubic - squared error