How to calculate the size of a heating oil fuel tank

Updated February 21, 2017

Knowing the actual size of the heating fuel oil tank can come in handy when determining the installation location and will also let you know how much fuel the tank can hold. As fuel oil costs fluctuate, determining how much fuel to purchase for the heating season can be daunting. By following a basic process you can calculate the size of a fuel oil tank.

Use the tape measure and record the measurements on paper. In the case of a square tank you will need the height, width and length of the tank. A round tank will only require the diameter and length. In the case of a fuel tank with a round top and bottom with straight sides, measure the length of the tank, the full width and then the height of the straight sides before the tank begins to curve.

Calculate the capacity of a rectangular tank with the following dimensions: 3 feet high by 2 feet wide and 6 feet long. Find the volume of the tank by using this formula: volume = height times width times length, or V = H x W x L. Plug in the numbers and you have V = 3 x 2 x 6. The answer is 36 cubic feet. Multiply 36 cubic feet by 7.48 gallons per cubic foot of space. The capacity of the tank is 269.28 gallons.

Determine the capacity of a round tank that is 3 feet in diameter and 6 feet long. First find the area of the 3-foot diameter circle by using this formula: area = pi times radius squared, or A = Pi x R^2. Pi is equal to 3.1416 and the radius of 3 feet is 18 inches, or 1.5 feet. Plugging that into the formula, you have A = 3.1416 x 1.5^2. The answer is 7.06 square feet. Multiply 7.06 by 6 feet (the length of the tank) and the volume is equal to 42.4 cubic feet. The capacity of the round tank is 42.4 cubic feet x 7.48 gallons per cubic foot. That's 317.2 gallons.

Find the gallon capacity of a tank with a round top and bottom that has a dimension of 6 feet long, a width of 2 feet, an overall height of 4 feet and a straight-side length of 2 feet. Combine steps 2 and 3 to find the capacity. First find the rectangular portion to the tank by using the straight-side height of 2 feet times the width of 2 feet and the overall length of 6 feet. By plugging in the numbers from step 2, you will find the volume of the tank is equal to 24 cubic feet. The gallon capacity of this portion is 179.5 gallons.

After calculating the rectangular volume, you will need to determine the capacity of round portion of the tank. The dimensions are 6 feet long with a diameter of 2 feet. Remember, the overall height of the tank is 4 feet, and since the straight sides of the tank are 2 feet long, there are two round portions, top and bottom, of 1 foot each. If you combine the top and bottom round portions of the tank you will have a 2-foot diameter circle. Using the formula in step 3, you will find the area of the 2-foot diameter circle to equal 3.14 square feet. Multiply the area by the length of the round portion and the volume is equal to 18.9 cubic feet. The capacity of the round portion of the tank is 141.8 gallons.

Add the gallon capacity from step 4 to the capacity found in step 5 and the total tank size is 321.3 gallons.


For your knowledge 1 cubic foot equals 7.48 gallons. You can mark the outside of the tank with equally spaced lines to understand just how much volume can be in the tank during different levels of the liquid.

Things You'll Need

  • Tape measure
  • Pencil
  • Paper
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