1. The problem statement, all variables and given/known data Finding the volume of a partially full cylindrical tank that lies on it's length. L=6', D=4" 2. Relevant equations pi r sq times L = cu ft when full. Convert to gallons 1 gal. = .1336 cu. ft. (I think) 3. The attempt at a solution (pi r sq times L)/2 = when half full How about when the fuel is one foot off the bottom? Is this simply (pi r sq times L)/4 ? How about when the fuel is 10" off the bottom? The volume of this tank is about 560 gal. which is way too much for the application. Diesel fuel goes bad after a year or two. Only 50 gallons are used each year. If you fill it then you throw out 500 gal. each year. The intake tube from the tank to the generator motor is 6" off the bottom of the tank. I want to know how many gallons to put in the tank that would allow 75 gallons or so to be available-so you would calculate how many gallons are there at 6" then add 75 gallons. That's why I ask is this just a straight line problem where one foot off the bottom equals one fourth of the tanks total capacity etc.? If so then 6" being 1/8 total would be... 560gal/8=70gallons + another 75 gallons=145gal. To put it another way does the area of a circle decrease linearly as a line touching two points on the circle gets shorter?