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Thrust-to-weight ratio is, as its name suggests, the ratio of instantaneous thrust to weight (where weight means weight at the Earth’s surface) is a dimensionless parameter characteristic of rockets and jet engines, and of vehicles propelled by such engines (typically space launch vehicles and jet aircraft). It is used as a figure of merit for quantitative comparison of engine or vehicle design.

The value is larger for an engine than for a whole launch vehicle; the engine thrust-weight is of use since it determines the maximum acceleration that any vehicle using that engine could theoretically achieve with minimum propellant and structure attached.

For a takeoff using pure thrust and no wings, the thrust-weight ratio for the vehicle has to be more than one (for launch from the Earth's surface, for launch from the Moon it needs to be more than 0.1654). In general, the thrust-to-weight ratio is numerically equal to the g-force that the vehicle can pull, provided the g-force exceeds local gravity then takeoff can occur.

Many factors affect a thrust-to-weight ratio, and it typically varies slightly over the flight. For valid comparison, thrust should be measured under controlled conditions. The main factors that affect thrust include freestream air temperature, pressure, density, and composition. Depending on the engine or vehicle under consideration, the actual performance will often be affected by progressive fuel consumption (causing a rise in thrust-weight ratio), buoyancy, and local gravitational field strength.

The Russian-made RD-180 rocket engine (which powers Lockheed Martin’s Atlas V) produces 3,820 kN of sea-level thrust and has a dry mass of 5,307 kg. Using the Earth surface gravitational field strength of 9.80665 m/s², the sea-level thrust-to-weight ratio is computed as follows: (1 kN = 1000 N = 1000 kg⋅m/s²)