Conflicting gallons help...

[DigiManTX]

Having a new pool installed starting next week. It's a Viking Caribbean "16x40"... my problem is what I am being told doesn't line up with what I find elsewhere. Using a basic formula of Length (40) x Width (16) x Average Depth (5) x Free Form Multiplier (7.5), I come up with 24k gallons... I am being told that this is a 17k gallon pool... what gives here? that is a drastic difference! I don't want to be given equipment sized for a 17k pool when I actually need larger stuff...

 

[Lskul60]

I believe 17k looks about right.

You mention free form multiplier, but you are using 7.5, which is the number of gallons per square foot.

You left out the free form multiplier, which from the looks of the sketch, seems to be approx 75%, which would make sense that the calculation is 17k.

 

[DigiManTX]

ok... can you give me the formula that you're using to get that? cause i'm just not understanding here...

 

[Lskul60]

ok... can you give me the formula that you're using to get that? cause i'm just not understanding here...

gallons is a measure of volume.

but, we are measuring volume in cubic feet (Length x width x depth), then multiplying by 7.5 gallons per cubic foot.

for your pool, the volume in cubic feet is length (40) times width (16) times average depth (5). But since you do not have a rectangular pool then length times width must be factored down to the surface area of the free form of your pool. I guessed that the irregular (sort of oval) shape of your pool is approx 75% of a 16 x 40 rectangle.

 

[MinerJason]

ok... can you give me the formula that you're using to get that? cause i'm just not understanding here...

Length x width x average depth = cubic feet if it were a rectangle

rectangular cubic feet x 0.75 free form multiplier = approximate free form cubic feet

free form cubic feet x 7.48 gallons per cubic foot = gallons

so 16ftx40ftx5ft = 3,200 cubic feet
3,200 cubic feet X 0.75 = 2,400 cubic feet
2,400 cubic feet x 7.48 gal/cubic foot = 17,952 gal

Edit: There are also a lot of steps and benches in there. The 0.75 free form multiplier is a pretty rough estimate, and should not be relied on. I'd go by the volume the manufacturer lists, as they likely know best, and it seems like a reasonable volume using rough back of the envelope calculations.

 

[jblizzle]

A fiberglass pool manufacturer will know the exact volume of their pools as they are likely drawn up in a CAD program. Certainly going to be more accurate than a generic calculation estimate.

 

[DigiManTX]

even doing that, i still come up with 18k... 40*16*5 * 7.5 - 25% =18k... i guess 1k is better than 7k

 

[Lskul60]

even doing that, i still come up with 18k... 40*16*5 * 7.5 - 25% =18k... i guess 1k is better than 7k

I just checked their website. they list the surface area as 468 square feet, which would suggest free form multiplier of 468/640=0.73125

so here we go.
468*5*7.48 = 17,503, which is just a smidgen over 17,500. But lets take out the volume of water that is displaced by the steps and the curved corner benches and we're now below 17,500, which rounds down to 17k.

I think you're good to go.
(Nice pool by the way)

 

[MinerJason]

even doing that, i still come up with 18k... 40*16*5 * 7.5 - 25% =18k... i guess 1k is better than 7k

Yes, but the 25% reduction for free form is a guesstimate at best, every free form pool is different, and there are steps and benches in that design that take up volume. Like jblizzle said, the manufacturer will know the exact volume with great precision. If they say it's 17k gal, I'd trust that way more than a generic guesstimate formula.