I am planning to build a concrete slab on a soil surface to hold a 400 gallon water tank (Shown Below). I want to make it 7' x 7'. It should hold about 3,500 lbs.

Would I need to put in rebar? I've done much smaller slabs with 80 lbs bags of Quikrete Concrete ready mix. How many bags would I need?

Water Tank

  • You're asking for how many bags to use, but you don't tell us how thick you want the pad. Is the thickness a part of the question?
    – JPhi1618
    Jan 21, 2016 at 13:25
  • Would be helpful if you can mention the layers and thickness too.
    – Rick
    Jan 21, 2016 at 13:26

2 Answers 2


My typical answer to these questions is talk to a Licensed Engineer because there are a lot of considerations that are needed. I firmly stand behind that.

On this project make sure you're using the correct information.

Water ~3,300 pounds

Tank ~ 180 pounds,

Concrete (4in thick without reinforcement) ~2,400 pounds

Piping ? other accessories?

Thats only part of the weight that the ground below your tank will support; how sure are you that the ground will hold that? Typically those tanks don't overflow at their rated capacity, they have 5-15% excess capacity, this needs to be designed to passively let the excess overflow or designed for the tank to be filled the overflow level.


Each bag is .6 cu ft. You have 49 square feet divide by 3 for a 4” slab by 2 for a 6” slab. 49 divided by 3 is 16 cubic feet so it will take just over 27 bags. Expect to use a bit more as you always use more than expected, I would use a wire mesh reinforcement on a hand pour of this size.

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