build your own Cistern, complete Howto

Cistern Howto
This article is based on a 16ft diameter, 4 ft tall tank, holding 6,000 gallons. However, you can alter the dimensions to fit your needs.
Round tank:
π (3.14) x radius x radius x height x 7.5 = gallons
(e.g. 3.14 x 8ft x 8ft x 4ft x 7.5 = 6028.8 gallons)
Square tank:
length x width x height x 7.5 = gallons
(e.g. a 18ft square that is 4ft tall will hold 9,720 gallons)
Liner Dimensions:
You want to make your liner a little larger than the tank's dimensions, so that it has some slack. Also make it 1ft taller than your tank's walls.
Even though a square tank is more efficient with space and thus your liner, we would unequivocally recommend going with a circular design. We have done both and the round one is far stronger and requires less work. Any money you might save on the liner for a square tank is negated by the extra strength you will have to add to the frame. If you decide to go square, bury the bottom 1/3 of the tank.

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1. Mark out the area where you wish to build your tank, and level it. You can dig down or fill in, though a combination of the two is often the least labor intensive.
2. Put a layer of sand, about 6" deep over the whole area and compact it well.
3. Place a rebar or post in the center of the area and attach a string to it.
4. Tie the other end of the string to a stick or piece of metal, so that the distance between the stick and the center post equals the radius of your tank, in this case 8 ft.
5. Keeping the string taut and the stick upright, mark the sand in a circle around the central post.
6. Center bricks over this line all the way around the circumference, leveling them with each other.
7. Fill your circle with sand, then compact it well, so that the sand is an inch or two below the top of the bricks.
8. Fill that inch or so with finely screened sand and compact again.