The mass ratio is a useful quantity for back-of-the-envelope rocketry calculations: it is an easy number to derive from either numbers or from rocket and propellant mass numbers, and therefore serves as a handy bridge between the two. It is also a useful number to give an impression of the size of a rocket: while two rockets with mass fractions of, say, 92% and 95% may appear similar, the corresponding mass ratios of 12.5 and 20 clearly indicate that the latter system requires much more propellant.
The definition arises naturally from the Tsiolkovsky rocket equation:
This equation can be rewritten in the following equivalent form:
The fraction on the left-hand side of this equation is the rocket's mass ratio by definition.
This equation indicates that a Δv of times the exhaust velocity requires a mass ratio of . For instance, for a vehicle to achieve a of 2.5 times its exhaust velocity would require a mass ratio of (approximately 12.2). One could say that a "velocity ratio" of requires a mass ratio of .