Ultrasonic flowmeters send sound waves through the flowing stream. They can measure either the Doppler shift as ultrasonic waves are bounced off particles in the flow stream, or the time differential of ultrasonic waves with the flow stream compared to against the flow stream. Either method gives a signal which is proportional to flow velocity. The Doppler method works with liquids with suspended solids, and the Transit time method works with liquids and gases. In both methods, the signal is proportional to flow velocity.

Ultrasonic meters are non-invasive but are relatively low accuracy. Because clamp-on ultrasonic meters are easy to install, they can be used temporarily to verify another flowmeter permanently installed in the pipe. Since the same meter can do a variety of sizes, they are particularly cost effective in large sizes.