Which to use?? Using the Pool Calculator I have been using the Gallon size that Intex/Coleman printed on box for our 18ftx48 pool. But when I used the equation to figure gallon size. There seems to be a 500gallon difference. I have been using the Pool Calculator to log & follow what chemicals to use to balance pool. Having a wee-little issue getting pH and TA to stay at target/goal. But I think it may be because I have put in the manufacturer figures for Gallons . The goal/target are based on those figures? Which should I use? And maybe I did the math wrong. Pool box say 6,092 gallon at 90% full. ( as far as I understand) My calculation- 6,660gallons