Make a dipstick. Fill it with, say, 5 gallons of water, take a wood dowel, and dip it in, and then put a notch in the dowel. then add water a gallon at a time and repeat the process until you can measure up to 13-14 gallons. then save the dowel of course!
I like to squeeze the nickle until the buffalo craps-mt rob
"Why don't we get drunk and screw?" Jimmy Buffett