A system for monitoring the volume of fluid in a hydration pack comprises a monitoring unit which is attached to an output of the hydration pack and to a feed tube. The monitoring unit comprises a pressure sensor and a tilt sensor. The pressure sensor obtains a measure of pressure of fluid within the hydration pack that may be used to estimate remaining fluid in the hydration pack. The tilt sensor is able to provide adjustment parameters tor adjusting for changes in pressure due to tilt of the hydration pack. An application on a mobile telephone provides a user with an indication of remaining liquid in the hydration pack. The application provides a visual indication of remaining fluid and may also provide an indication of rate of consumption. The application may also calibrate the system, enabling the system to be fitted to existing hydration pack assemblies.