I filled the tank with water one gallon at a time, and noted a total capacity of 18 gallons. I drained the tank through the fuel outlet until flow stopped. I then drained all remaining water out through the drain sump, collected and measured it. Sitting still on a table in the straight and level flight attitude, the unusable portion measured 15 and one third cups. I will round the unusable portion to one gallon and use that as the basis for calibrating the fuel gauge. I found there to be no leaks.