Tank Capacities

How can I figure the size of my gas tank in my 1989 Dolphin 34-foot motorhome?
Von, (Alpine, CA) 

Von, for any rectangular container, be it a fuel tank, holding tank or fresh water tank, simply multiply the height (in inches) times the length (in inches) times the width (in inches) and divide by 231. The result will be the approximate gallons that tank will hold.

Formula: height X length X width, divided by 231

Example: A tank measures, 12.0 inches by 36.0 inches by 18.0 inches. So: 12 X 36 X 18 = 7,776 cubic inches

Divided by: 231 = 33.6 gallon capacity (approximately)

Not all RV tanks (holding tanks, fresh water tanks, fuel containers) are completely rectangular. In those instances of odd-shaped tanks or tanks with off-sets, it will be necessary to visualize the tank in rectangular forms and then simply add the total cubic inches of each section before dividing by 231. Also, remember that some RV containers cannot be filled to the fullest measured capacity. Take-off tubes, inlet and outlet plumbing fittings and fixtures, drain components, etc., may reduce the total capacity of that tank somewhat. Use this formula as an approximation only. Cylindrical LP containers require a completely different formula. This formula is for square or rectangular tanks only.