For a round above ground pool, the calculation is:
Diameter x Diameter x Depth X 5.9 = Number of Gallons
Edit note : The above formula uses the conversion factor of 5.9 US gallons to the cu. ft.
From my high school days, a century ago, I believe this is incorrect.
I recall that 7.48 US gallons occupy one cu.ft. of water. I checked my memory of this at two different websites which confirmed my recollection.
The second site is [ http://www.math.com/students/converters/source/volume.htm ].
Thus the answer given below appears to be approx. 25 % too low.
Perhaps the 5.9 incorrectly used above, is the British or Imperial gallon conversion factor ?
Please note that the depth is the ACTUAL water depth NOT the height of the pool wall (the water is usually around 6 inches lower than the wall height).
For your example:
For a 18-foot round pool with an actual water depth of 3.5 ft: 18x18x3.5x5.9 = 6,690 gallons. NOT CORRECT !!!
Corrected answer : 18 x18 x3.5 x 7.48 = 8,482 US gallons
Visit this page: http://www.poolspa.com/calculator/ which uses the correct 7.48 conversion factor for US gallons
Depends on the shape of your pool.
Round or Oval pools:
Length * Width * Average Depth * 5.9 = Gallons
Rectangle, square, or free-form pools:
Length * Width * Average Depth * 7.5 = Gallons
1x72 2x36 3x24 4x18 6x12 8x9
4x18 Leap of faith
1x72, 72x1, 2x36, 36x2, 3x24, 24x3, 4x18, 18x4, 6x12, 12x6, 8x9, 9x8
Tables of 8 or 4 or 2 72 = 8x9 or 4x18 or 2x36 80 = 8x10 or 4x20 or 2x40
using only whole numbers there are six answers: 1x72 2x36 3x24 4x18 6x12 8x9
Chopin, Nocturne E Flat Major Op. 9 No. 2
12, 6x2, 4x3, 24, 4x6, 36,4x9, 6x6, 48, 6x8, 4x12, 60, 6x10, 4x15, 72 6x12, 4x18 i think