In a method for determining the free field thrust of a gas turbine engine the gas turbine engine is attached to a movable support means in an enclosed gas turbine engine test facility, and is operated at a selected engine operating point. The thrust applied by the engine to the thrust cradle is measured via thrust measurement means. Calculations are performed to determine the gas turbine engine intake momentum drag generated by airflow into the gas turbine engine intake, the thrust cradle drag force generated by airflow past the moveable support means of the thrust cradle, the base drag generated as a result of accelerating nozzle ejector airflow, a pre-stream tube force related to turning of stream lines in the intake air path, a pre-stream tube force related to a bell mouth pull-off force. Also forces due to forward anemometer plane and nozzle static pressures and a nozzle exit buoyancy force and determined, and all of these forces are summed according to their respective positive and negative values to determine the free field thrust of the engine at the selected engine operating point.