If the tank is rectangular, it's easy
L x W x H (in inches), divided by 231, which is the cubic inches in a gallon. If the tank isn't rectangular, you'll need the math skills to come out with of cu. in....then divide that by 231.You'll prob'ly come out with a weird number...like 16.24569. That's because you're measuring outside dimensions. The tank size would be 15 gallons...'cuz that's the useable capacity.Yes there are tank level monitors...this one tells you exactly how much is in the tank: http://ferriellosales.com/default.php And because the sender is on the outside of the tank, it never comes in contact with the contents, so it can never clog or corrode. Not the cheapest, but FAR from the most expensive...the best on the market IMO.