A standard residential oil tank holds 275 gallons. Based on that size, the following readings indicate that your tank contains approximately this many gallons: 1/8 = 40 gals Use a tank stick (can be purchased from a local plumbing supply or Amazon.com) and measure the inches of oil in your tank. Then match up the inches with the size of you oil tank that corresponds to the chart below. Sticking your tank to see how much oil you have is more accurate than a tank gauge. To measure the amount of fuel oil in under ground or above ground outdoor oil tank: Remove your oil fill cap and insert the stick into the tank until it reaches the bottom. Remove the stick and take note of the amount of oil in inches that is showing on the stick.