The patented First flush valve for Stormwater metering is designed to control the “first flush” of a rainstorm. The first zone of the FFV is open to allow flow under normal conditions, as well as functioning as a clean-out port for accumulation on the bottom of the pipeline. The second zone is a holding area retaining the heavy “first flush” downpour. By creating a restriction, the pipeline then functions as a storage tank. As flow rates increase, zone three permits the increasing volume to discharge at a controlled rate. The fourth zone allows for full flow when the system is surcharged, and allows floatables to discharge. The FFV is available in sizes 6” to 120”, and can be installed externally or internally. This is a “passive device”, requiring no external control, power source or actuation.