Measure to find the exact height in feet of your 210-barrel oil tank. Depending on your situation, you may not need to measure at all --- the exact height will be on the documentation you received when you purchased or installed the tank, if you still have that documentation.
Measure to find the exact radius in feet of your 210 barrel oil tank. You can find this by measuring the diameter of the top portion of your oil tank, and dividing that number by 2. You can also measure from the center of the top or bottom of the tank to the outer radius of the circle (if your oil tank is on its side).
Multiply the radius of your 210 barrel oil tank by itself. If your radius was 10 feet, for example, you would multiply 10 by 10. Write this number down.
Multiply the height of your 210 barrel oil tank by the number you obtained in Step 3.
Multiply the number from Step 4 by 3.14. The number you are left with is the total volume of your 210-barrel oil tank in cubic feet.