A rain gauge is an instrument used by meteorologists and hydrologists to measure liquid precipitation (rain) in a certain amount of time. It is usually measured in millimetres and inches. Rain gauge is a meteorological instrument for determining the depth of precipitation (usually in mm) that occurs over a unit area (usually one metre square) and thus measuring rainfall amount. One millimetre of measured precipitation is the equivalent to one litre of rainfall per metre square.

Usually a tapering funnel of [mercury] or polyester of standard dimension allows the rain water to collect in an enclosed bottle or cylinder for subsequent measurement. The gauge is set in open ground with the funnel rim up to 30 cm above the ground surface. Some gauges are calibrated to allow the amount of rainfall to be read directly; with others it must be calculated from the depth of water in the container and the dimensions of the funnel.

The second type of rain gauge is the autographic gauge which can be either of the tilting type or the tipping type. The recording chart on an autographic rain gauge is mounted on a drum which is driven by clockwork and typically rotates round a vertical axis once per day. For a tilting-siphon rain gauge, the rainwater in a collector displaces a float so that a marking pen attached to the float makes a continuous trace on the paper. The two buckets in a tipping-bucket rain gauge rest on a pivot so that when one bucket has received 0.2 (or 0.5 mm) of rain it tips by gravity, empties the rainwater and allows the other bucket to start collection. During the tip, an electrical switch is closed and triggers a nearby autographic recorder to register each 'tilt', thus giving a fairly continuous record of precipitation and, in a more sophisticated form, even rainfall intensity. Rain gauges must be sited in as representative a location as possible, but the choice of location is difficult, since many precipitation events are highly aggregate. King Sejong the Great was the man who introduced rain gauge to the world. There are advantages and disadvantages of this instrument.