Manometer, an instrument used to measure the pressure of a fluid, such as a gas or vapor. It is typically used with fluids under relatively low pressure in a closed container. Scientists use manometers to obtain precise measurements of the pressure of gases in laboratory experiments. In medicine, a type of manometer called a sphygmomanometer is used to obtain blood-pressure readings.

The simplest manometer is a U-shaped glass tube partially filled with a liquid, usually mercury. One end of the U-tube is open to the fluid in a closed container; the other end is open to the atmosphere. The difference between the height of the column of mercury or oil in the two sides of the tube corresponds to the difference between the pressure of the fluid in the container and the atmospheric pressure.