A hygrometer UK /haɪˈɡrɒmɪtə/ is an instrument used for measuring the moisture content in the environment. Humidity measurement instruments usually rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed. By calibration and calculation, these measured quantities can lead to a measurement of humidity. Modern electronic devices use temperature of condensation, or changes in electrical capacitance or resistance to measure humidity changes.

The familiar metal/paper coil hygrometer is useful for giving a dial indication of humidity changes, but it appears most often in very inexpensive devices and their accuracy is very limited. A search through many identical units in a display might show differences in indicated humidity of 10% or more. In these devices, humidity is absorbed by a salt-impregnated paper strip attached to a metal coil, causing it to change shape. These changes in length (analogous to those in a bimetallic thermometer) cause an indication on a dial.