What instrument collects and measures the amount of rain that falls?

The instrument that collects and measures the amount of rain that falls is a rain gauge.

A basic rain gauge has a funnel that directs precipitation into a calibrated container. The collected water is read as a depth, usually in millimetres or inches. Because the measurement represents an amount spread over a horizontal area, 1 millimetre of rainfall equals 1 litre of water per square metre.

Modern observing stations may use tipping-bucket gauges. In these devices, a small container tips after receiving a fixed amount of water, allowing an electronic system to count tips and calculate rainfall totals and rates. Weighing gauges instead measure the increasing mass of collected precipitation.

Rain gauges must be positioned carefully. Buildings, trees, and strong wind can disturb the airflow and cause undercatch or uneven collection. A rain gauge measures precipitation at one location, so a single reading may not represent rainfall across an entire city or storm. Snow often requires a different setup or later melting before measurement.