The calculator at this site, Concrete Volume Calculator , will take your measurements and tell you how many bags (various sizes), cu ft or cu yards the job will require.
I ran a calc to find that 1 cu ft requires 1.7 80'lb bags....so one bag is about .6 cu ft.
For the amount I wouldnt know so im assuming Ford_6 is correct for laying the slab. When buying 2x4s get the straightest ones possible. To prevent bowing of the boards (which berely ever happens on this small of a slab) put a stake every 2 feet or so on each board. Then get a draw string to go across the forms to put a stake or two in the central area of the slab for "grade stakes" so when you pour the concrete in the middle.
When you have reached the middle you will know what height (same height as 2x4s) to screet the mud to for being flat from those grade stakes. Screeting can be done with a NON bowed or warped board so you don't have dips in the concrete. If you already knew this I guess I just remided you of it but good luck.
I have not used this one way that I think you are using is it what they call premixed? or pre made?? We dad used to buy the concrete in the bags and get, me to mix it with navajack a ( 3/4" minus, sand and gravel) He told me 4 to 1 was a good mix for gravel/ cement. If I was making concrete pipes I went 3 to 1 used to drive dad nuts. ( I do miss him) Don't use to much water in it for it will slump more that way. I wants to be just wet enought to shovel and for it to stay on the shovel. As for the gravel ( navajack) there would be amounts listed in loose materials for gravels that should be close the to volume that you need. Have you go a gravel sales company around you that could advise. I didn't read the volume that you had. 4" = 1/4 cu ft x width x length.. Will give you cubic feet. Sorry you probably know this but you can almost never over support/ brace concrete. Hope it works out for you it has been 30 yrs since last worked much concrete slabs.
67 Bags? thats a lot of 80 lb bags to lug home from Home Depot. Check in theYellow Pages for concrete companies that will mix and deliver short loads on site.
If he goes to a ready-mix company it might be more expensive since there is a delivery fee most likely & the price of mud has gone up also. I think from what I remember on our last job, the company we used it was 300 bucks for 2 - 1/2 yards of mud. So if he has a big bin and two guys mixing with shovels I am sure it will go fine.
Like mitchntx said, the coverage is the same as the bag laid on the ground, you add water to it but it does not swell up, Naturally you can use the concrete calculator for large jobs, but I (at Lowe's) always tell folks it they are pouring a sidewalk or stepping stones the bag size is what it will be once mixed.
Also take a look in the phone book to see if there's a U-Cart company -- they have a small cart that carrys about 1 - 2 yards of concrete they've mixed, all you do is haul it to your house and dump it into your forms. The problem with mixing that much bag concrete is that the first bags you mix are drying by the time you're mixing the last bag unless you're fast.
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