There is disclosed a fluid warming system (300) for warming intravenous fluid, comprising: a heater unit (304), including a heating element (314) and at least one temperature sensor a heat exchanger unit (302), removably attachable to the heater unit (304), which includes an inlet (306) and an outlet (308) through which the fluid can pass, and a controller (316), programmed: to receive at least one temperature measurement (TCI, TCO) from said at least one temperature sensor in the heater unit (304) to compute a fluid flow rate (q), corresponding to the rate of flow of fluid from the inlet (306) to the outlet (308) of the heat exchanger unit (302), in dependence on the amount of electrical power (PH) supplied to the heating element and said at least one temperature measurement (TCI, TCO) and to control the electrical power (PH) supplied to the heating element in dependence on the computed fluid flow rate (q).
