A metering end cap provided to the neck of a flexible container containing a fluid and intended to transfer, at will, an amount of fluid from an upstream space towards a downstream space, this end cap including a chamber in which is disposed a movable valve normally biased towards a rest position and operationally moved towards an end position. The valve isolates the upstream space and the downstream space from each other in its end position and only in this position, and the outlet of the chamber communicates with the upstream space for any position of the valve other that its end position.