The weight ratio of oxidizer to fuel in the combustion chamber of a rocket engine. For example, the mixture ratio for the space shuttle's main engine is 6:1, meaning six parts of oxidizer (liquid oxygen) to one part of fuel (liquid hydrogen).

In 1962, NASA's Marshall Space Flight Center developed control circuits and valves for a Saturn 1B launch vehicle that could change the mixture ratio during the engine's operation. Extra thrust could be created by changing the ratio from 4:5 to a maximum of 5:5.