Volume of liquid in a cylinder lying on its side?

A gas tank is in the shape of a cylinder but it's lying on its side. The trucker needs to determine if he has enough gas left to make the trip. His fuel gage his broken, so the only thing he can measure is the depth of the gas in the tank. He needs 1/4 of a tank to make the trip, so what depth of gas would be 1/4 of a tank?