16 foot round pool by 4 feet deep How many gallons of water is there?

I am on my sixth beer so I don't know if you talking about a 16 foot perimeter or a 16 food diameter...

if its a 16 foot diameter then 22773.83 gallons... That's to the very top.

if its a 16 foot perimeter which would be totally illogical but oh well, it would be 2306.69 gallons... that means the diameter would of your pool would only be 5.1ish feet.