Thrust-to-weight ratio (where weight means weight at the Earth's surface) is a dimensionless parameter characteristic of rocket 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.
Of course the value is larger for an engine than for a whole launch vehicle; the latter has to be more than one, of course, and is e.g. 1.4, at launch (for a launch from e.g. the Moon it can be less than 1).
Many factors affect a thrust-to-weight ratio, and for valid comparison, thrust should be measured under controlled conditions (STP). Factors that affect thrust include freestream air temperature, pressure, density, and composition. Depending on the engine or vehicle under consideration, effective weight may be affected by progressive fuel consumption, buoyancy, and local gravitational field strength.
Example
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/sē, the sea-level thrust-to-weight ratio is computed as follows: (1 kN = 1000 N = 1000 kg⋅m/sē)
