The volume of the holding tank is somewhere between 15 & 18 gallons. I don't think anyone knows for sure. I have never seen any definitive documentation on it. We always assumed it was 18 gallons. But we've had pump out guys tell us they pumped 20 or more gallons out. Maybe the thought they would be paid by the gallon?? When we are cruising we worked on a 3 day cycle for a pump out with just two of us on board. We now have a 36 gallon tank in the V-bunk which, we hope, will last a week without calling the pump out guy.