Tank capacity
There are several ways to determine how much is in the tank:1. shine a light on it (only works if the tank is plastic and a vertical tank wall is accessible)... 2. on average, each person continuously aboard will put approx 3 gals/day into the tank, so divide the tank capacity by 3 gal/person to estimate how many days it takes to fill up the tank...3. install a gauge. In fact, although few people know it because enforcement is lax, CG regs actually require a gauge in all holding tanks that are not readily accessible for visual inspection. The best "bang for the buck" is the Snake River Electronics AcuGage...not only does it give a true 1/4, 1/2, 3/4, full reading in any size tank for less than half the price of any other gauge that does more that let you know when the tank is just about full, but the tank contents never come in contact with the sender, so it can never clog up. The online chandlery here has it for a VERY good price. I just checked...the single tank "mini" doesn't show up in a search...ask for a quote.