Hi Var
And welcome to Koi Bito
you can get your exact gallons by measuring width, length depth multiply all by 7.5 should give you a close enough number. Our clubs web site has a calculator to give water volume..
www.koikichi.org
Also do some research on filtration and pumps, most swimming pool pumps are very high PSI, and are not recommended for koi pond use. They are very expensive to run 7/24.. Also sand filters are not recommended either, they get like concrete inside from the fish food and waste. ( sand filters do polish water very nicely) but not good filtration for ki ponds. You can try Genki koi or Champion Nishikigoi they are both in Northern California. Both are very reputable dealers and a pleasure to do business with.