Most home-heating oil tanks do not have any sort of mechanism to tell you how much oil is left. It has to be judged by how many gallons of oil were delivered and how many days have gone by since then. It also depends on how cold the weather has been. Going through a cold spell and waking up one morning to a cold house can be very frustrating, but there is a way to prevent that from happening.
Purchase a 6-foot wood dowel or stick from a home improvement store. Lumberyards will also sell you a stick to use to measure the oil level in the tank. Some home heating oil suppliers as well as hardware stores have marked sticks for sale to measure the oil.
Remove the cap of the oil tank immediately after it has been filled. Insert the rod or stick into the opening, keeping it in a perfectly upright position. Remove the rod from the tank.
Mark the top end of the rod where it is does not contain any oil.
Wipe the rod with an old rag. Mark a small notch at the top of the rod with a knife. Make a notch every 12 inches, down the entire length of the rod. Whenever you need to know how much oil is left simply insert the rod in the top of the tank and notice where the oil level shows on the rod when you take it out of the tank.