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.
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)
length x width x height x 7.5 = gallons
(e.g. a 18ft square that is 4ft tall will hold 9,720 gallons)
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.
- 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.
- Put a layer of sand, about 6" deep over the whole area and compact it well.
- Place a rebar or post in the center of the area and attach a string to it.
- 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.
- Keeping the string taut and the stick upright, mark the sand in a circle around the central post.
- Center bricks over this line all the way around the circumference, leveling them with each other.
- Fill your circle with sand, then compact it well, so that the sand is an inch or two below the top of the bricks.
- Fill that inch or so with finely screened sand and compact again.