The ratio of the fuel mass flow of an aircraft engine to its output power, in specified units. Specific fuel consumption (abbreviated sfc or SFC) is a widely used measure of atmospheric engine performance. For reciprocating engines it is usually given in U.S. Customary units of pound-mass per hour per horsepower ((lbm/h)/hp or lbm/(hp·h)), and International System (SI) units of kilograms per hour per kilowatt ((kg/h)/kW).