Quote Originally Posted by SnoopysGirl View Post
I am still very confused as to why it says that I have a 21,000 gallon pool in my signature. I have an in ground 18x38 pool 3ft shallow 8ft deep, roman shaped with bumped out step.
Here's why:

the top 3' level of pool is LESS than a full 18 x 38 rectangle, assuming your measurements are greatest length and width measurements. In other words, the top part of your pool will fit into a rectangular box, 18 x 38 x 3. But your water depth is going to be LESS than the maximum pool depth, which is normally stated to overflow level, rather than actual water level. So the box is 18 x 38 x 2.8 cft or about 1850 cft OR 13,800 gallons.

Most vinyl pools have nearly flat shallow ends, and the break begins at the midway point. That in turn means that the bottom or deeper section of your pool will fit inside a rectangular box that is 18 x 38/2 x (7.8 - 2.8) or 18 x 19 x 5. That means the absolute maximum volume in the deeper section of your pool is 1710 cft or 12,800.

So, even if your pool was shaped as 2 stacked rectangular boxes, with right angles and no slopes, the upper box (18x38x2.8) resting on the lower box (18x19x5), your absolute maximum pool volume would be 26,600 gallons. If you add 500 gallons for the steps, you'd still only be 27,100 gallons.

BUT, the shape of almost all vinyl pool 'hoppers' -- deeper sections -- is highly sloped, with only a SMALL flat rectangular area at the full depth. Most hoppers are LESS than 1/2 the volume of a rectangular box with the same dimensions. In that case, your pool volume (if we use a hopper at 1/2 rather than something less would be 13,800 (top) + 500 (steps) + 12,800/2 (bottom) or 20,700 or ~21,000 gallons.

If you want an more accurate measure, if you can find the "DIG DRAWING" for your liner, it will usually have a very accurate CAD calculated volume for that pool & liner . . . at the OVERFLOW level of your pool. So, if the normal level is 6" below the OVERFLOW level, you need to take the CAD volume, and subtract 18x38x0.5x 7.48 gal/cft, or 2550 gallons from the CAD measurement.

Pool builders and pool stores almost ALWAYS overstate pool volumes!

Plus I can tell you -- having taught numerous Certified Pool Operator classes, a majority of pool builders can not calculate pool volumes accurately. Trying to get them through that part of the material was always a struggle.