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Dimensionless ratio of thrust to weight of a jet or propeller engine
Thrust to weight ratio is a dimensionless ratio of thrust to weight of a rocket, jet engine, propeller engine, or a vehicle propelled by such an engine that is an indicator of the performance of the engine or vehicle. The instantaneous thrust to weight ratio of a vehicle varies continually during operation due to progressive consumption of fuel or propellant and in some cases a gravity gradient. The thrust to weight ratio based on initial thrust and weight is often published and used as a figure of merit for quantitative comparison of a vehicle's initial performance.

The thrust to weight ratio is calculated by dividing the thrust (in SI units – in newtons) by the weight (in newtons) of the engine or vehicle. The weight (N) is calculated by multiplying the mass in kilograms (kg) by the acceleration due to gravity (m/s^2). Note that the thrust can also be measured in pound force (lbf), provided the weight is measured in pounds (lb). Division using these two values still gives the numerically correct (dimensionless) thrust to weight ratio. For valid comparison of the initial thrust to weight ratio of two or more engines or vehicles, thrust must be measured under controlled conditions.

The thrust to weight ratio and lift to drag ratio are the two most important parameters in determining the performance of an aircraft. The thrust to weight ratio varies continually during a flight. Thrust varies with throttle setting, airspeed, altitude, air temperature, etc. Weight varies with fuel burn and payload changes. For aircraft, the quoted thrust to weight ratio is often the maximum static thrust at sea level divided by the maximum takeoff weight. Aircraft with thrust to weight ratio greater than 1:1 can pitch straight up and maintain airspeed until performance decreases at higher altitude. A plane can take off even if the thrust is less than its weight as, unlike a rocket, the lifting force is produced by lift from the wings, not directly by thrust from the engine. As long as the aircraft can produce enough thrust to travel at a horizontal speed above its stall speed, the wings will produce enough lift to counter the weight of the aircraft.

Engine Mass Thrust, vacuum Thrust to weight ratio (kN) (lbf) RD 0410 nuclear rocket engine 35.2 Pratt & Whitney J58 jet engine(Lockheed SR 71 Blackbird) 150 Rolls Royce/Snecma Olympus 593turbojet with reheat (Concorde) 169.2 Pratt & Whitney F119 RD 0750 rocket engine three propellant mode RD 0146 rocket engine Rocketdyne RS 25 rocket engine(Space Shuttle Main Engine) RD 180 rocket engine4,152 RD 170 rocket engine F 1(Saturn V first stage) NK 33 rocket engine SpaceX Raptor 2 rocket engine 2,256 Merlin 1D rocket engine, full thrust version 914 205,500 199.5