So you check your "tank level" regularly, right? That's good, but how do you know just how much oil is remaining in the tank? You might get a feel based on previous fill-ups, but that's still not really much of a reckoner. As long as you have a modern, cylindrical oil tank, then use the form below to check. All you need to know is the tank's quoted capacity, the diameter of the tank, and the oil level measured from the tank's centre line.