The impulse per unit weight-on-Earth (typically designated by ) has units of seconds. Because the weight on Earth of the reaction mass is often unimportant when discussing vehicles in space, specific impulse can also be discussed in terms of impulse per unit mass, with the same units as velocity (e.g., meters per second). This measure is equivalent to the effective exhaust velocity of the engine, and is typically designated . Either the change in momentum per unit of propellant used by a spacecraft, or the velocity of the propellant exiting the spacecraft, can be used to measure its "specific impulse." The two values differ by a factor of the standard acceleration due to gravity, gn, 9.80665 m/s² ().
the impulse per unit weight-on-earth (typically designated by ) has units of seconds because the weight on earth of the reaction mass is often unimportant when discussing vehicles in space specific impulse can also be discussed in terms of impulse per unit mass with the same units as velocity (eg meters per second) this measure is equivalent to the effective exhaust velocity of the engine and is typically designated  either the change in momentum per unit of propellant used by a spacecraft or the velocity of the propellant exiting the spacecraft can be used to measure its "specific impulse" the two values differ by a factor of the standard acceleration due to gravity gn 980665 m/s² ()