RVs are usually constructed based on some sort of large truck chassis, if not a commercial bus or motorcoach. This means that their fuel tanks will usually be fairly rectangular or cylindrical, as opposed to the oddly-shaped tanks found under some small cars. Calculating tank size is a relatively simple matter, requiring little more than a tape measure, a calculator and a few basic mathematical formulas.
Measure the length, width and height of your tank in centimeters. Metric measures are best for this since the calculations required to convert cubic centimeters are simpler than those required to convert from cubic inches to gallons. Example: your rectangular RV tank is 152 centimeters (five feet) long, 72 centimeters (2.5 feet) wide and 61 centimeters (two feet) high. Multiply these figures together to get the tank's volume in cubic centimeters (cc), which in our example comes out to 667,854 cc.
Convert the cubic centimeter figure to liters. One liter equals 1,000 cc. Divide the cubic centimeter measurement by 1,000 to get the volume in liters. In our example, this comes out to 667.854 liters; round the figure off to 668 liters. To convert your liter measurement to gallons, divide it by 3.78. The 5-foot by 2.5-foot by 2-foot tank in our example has an interior volume of roughly 176.71 gallons.
Measure the distance from one side of a cylindrical tank to the other to get the tank's diameter in centimeters. Divide this measurement by two to get the radius. Apply the area-of-a-circle formula "Pi x radius squared (abbreviated π r 2)" and multiply that by the tank's length in centimeters to get the volume in cubic centimeters. Example: Your tank is 76 centimeters across (diameter) and 152 centimeters long. Divide the diameter by two to get the tank's 38 centimeter radius. Multiply the radius by itself (38 x 38 = 1,444) and multiply it by Pi (3.14) to get an area of 4,534 cc. Multiply that by the tank's 152 centimeter length and you'll wind up with a total volume of 689,192 cc, or 689 liters. Divide the liter measurement by 3.78 and you'll find that this tank has an interior volume of roughly 182.27 gallons.
