An 80# bag of premix concrete yields 2/3 cubic foot of concrete.
Your "hole", once you tamp it and add an inch of gravel is 12' x 5' x 4" (1/3 foot)
4 x 5 = 20
(add 10% for waste)
You need 22 cubic feet of concrete.
If 80# of premix yields 2/3 cubic foot, then it takes 3 80# bags to yield 2 cubic feet.
You would need 33 80# bags of premix in your basement.
33x80=2640 lbs, or two trips to home depot in your half ton pickup. (by the way, they usually have a rental truck, or you can have it delivered. Call at least a week in advance of your project)
Did I mention you get to carry 33 80# bags?
You could also get 44 60# bags, for the same volume, but less weight per bag. You have to make more trips down those stairs, though.
I'm getting excited thinking about all this, are you?
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