Home heating fuel tanks come in many different sizes but only a few shapes. Most have rectangular or round ends, but some, particularly those found at older homes, have oval ends. All of these tank shapes are basic geometric shapes, from the prism  an elongated cube  to the cylinder, whether the end of the cylinder is round or oval. All it takes to discover the size of a home heating fuel tank is some simple arithmetic.
Things You'll Need
 Measuring tape
The Square or Rectangular End

Measure the height and width of the end of the tank. Multiply the height of the end of the tank by its width to calculate the area of the end of the tank.

Measure the length of the tank.

Multiply the area of the end of the tank by the length of the tank to determine the volume of the tank: if your measurements of the circumference and length were in inches, the result is the number of cubic inches in the tank. Divide the number of cubic inches by 231, the number of cubic inches in a gallon, to determine the tank's capacity in gallons, or by 1,728, the number of cubic inches in a cubic foot, to determine its capacity in cubic feet.
The Round End

Measure the length of the tank.

Measure the widest distance across the end of the tank, its diameter, and divide that measurement by two. The result is the radius of the end of the tank.

Multiply the radius by 3.14159  "Pi." Multiply the answer by itself to determine the area of the end of the tank.

Multiply the area of the end of the tank by the length of the tank to determine the volume of the tank: if your measurements of the circumference and length were in inches, the result is the number of cubic inches in the tank. Divide the number of cubic inches by 231, the number of cubic inches in a gallon, to determine the tank's capacity in gallons, or by 1,728, the number of cubic inches in a cubic foot, to determine its capacity in cubic feet.
The Oval End

Measure the length of the tank.

Measure the circumference  the distance around  of the tank with a tape measure.

Divide the circumference of the tank by 3.14159  "Pi." Pi expresses the relationship between a circle's diameter and its circumference: Pi times the diameter of a circle equals its area. Dividing the circumference of the oval tank by Pi makes it possible to treat the oval shape as if it is a circle or, in the case of a tank, treat the tank with an oval end as if it's a cylinder.

Divide the result by two to learn the radius of the tank. Multiply the radius by Pi, then multiply that result by itself: this gives the area of the end of the tank.

Multiply the area of the end of the tank by the length of the tank to determine the volume of the tank: if your measurements of the circumference and length were in inches, the result is the number of cubic inches in the tank. Divide the number of cubic inches by 231, the number of cubic inches in a gallon, to determine the tank's capacity in gallons, or by 1,728, the number of cubic inches in a cubic foot, to determine its capacity in cubic feet.
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