The Thrust Specific Fuel Consummption (TSFC) of an engine is the ratio of fuel mass consumed to thrust produced. For a simple turbojet engine:
TSFC = f / [(1+f) * Ue - U]
where f is the mass ratio of fuel consumed to airflow, Ue is the jet exit velocity, and U is the flight speed.
For a turbofan engine, the equation is as follows:
TSFC = f / [(1+f)*Ue + B*Uef - (1+B)*U]
where B is the bypass ratio and Uef if the bypass flow exit velocity.