[BlindHandyMan] water calculation

[Spiro]

Hi,
a niece just got a 4 ft x 18 round pool.
She and her downstairs neighbor share a water meter.
at 3.25 ft deep I've got this at 826.99987 cubic feet area.
Question, how many gallons to a sq foot?
or any similar ratio.
She's trying to figure what it will cost her to fill it.
I know it's more than I'd be affording right now.
Thanks