Calculate the capacity of a heating oil tank before refilling to know the exact amount of fuel you need to purchase. Fuel oil tanks, found in the basements of homes, are cylindrical in shape. The capacity in gallons, or volume, of such a tank depends on its dimensions. The pertinent dimensions are its length and its radius. Radius is measured from the centre of one of the tank's circular ends to its edge. A larger volume means more storage capacity for the tank.

Measure the radius and length of the tank in inches. For example, the radius could be 20 inches while the length might be 96 inches.

Convert the radius and length measurements into feet by dividing by 12. Performing this step leads to a radius of 1.7 feet and a length of 8 feet.

Multiply the number pi times the square of the radius times the length to obtain the volume of the tank in cubic feet. Use 3.14 for the number pi. Completing this step, you arrive at 3.14 times 1.7 feet times 1.7 feet times 8 feet, or a volume of 72.6 cubic feet.

Divide the volume in cubic feet by 0.13368 to obtain the capacity of the oil tank in gallons. (See References 3) Now you have 72.6 cubic feet divided by 0.13368 cubic feet per gallon which equals 543.1 gallons.