The present invention is a non-intrusive method to determine the fluid level in a vessel. In a preferred embodiment, the vessel is a delayed coker drum in a refinery. Waves are generated in the vessel from an outside source. For wall vibration generated in the frequency range of 1-20,000 Hz, accelerometers on the exterior wall of the vessel measures the attenuation of the vibration modes of the vessel. The fluid level can related to the attenuation of the vibration mode. For wall vibration generated in the ultrasonic range, the wave traveling in the vessel wall are guided waves. The guided wave will leak energy into the fluid inside the vessel and will attenuate depending on the fluid level. Ultrasonic receivers on the outside of the vessel measure the attenuation. The fluid level can be related attenuation of the ultrasonic waves.