El impulso específico es el período en segundos durante el cual 1 kg de masa de propergol (el combustible y oxidante juntos) producirá un empuje de 1 kg de fuerza. Aunque el impulso específico es una característica propia del sistema de propergoles, su valor exacto varía de acuerdo a algunas condiciones de operación y diseño del motor de un cohete. Por esta razón se asignan diferentes números a un determinado propergol o combinación de éstos. Como característica importante de los combustibles, el número indica la eficiencia de los propelentes. Cuanto más alto es el número, mayor es la eficiencia.

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Specific impulse (usually abbreviated Isp) is a measure of the efficiency of rocket and jet engines. By definition, it is the total impulse (or change in momentum) delivered per unit of propellant consumed and is dimensionally equivalent to the generated thrust divided by the propellant flow rate. If mass (kilogram or slug) is used as the unit of propellant, then specific impulse has units of velocity. If weight (newton or pound) is used instead, then specific impulse has units of time (seconds). Multiplying flow rate by the standard gravity (g0) before dividing it into the thrust, converts specific impulse from the mass basis to the weight basis. A propulsion system with a higher specific impulse uses the mass of the propellant more efficiently in creating forward thrust, and in the case of a rocket, less propellant needed for a given delta-v, per the Tsiolkovsky rocket equation. In rockets, this means the engine is more efficient at gaining altitude, distance, and velocity. This is because if an engine burns the propellant faster, the rocket has less mass for a longer period of time, which makes better use of the total force times time that was acquired from the propellant. This is much less of a consideration in jet engines that employ wings and outside air for combustion to carry payloads that are much heavier than the propellant. Specific impulse includes the contribution to impulse provided by external air that has been used for combustion and is exhausted with the spent propellant. Jet engines use outside air, and therefore have a much higher specific impulse than rocket engines. The specific impulse in terms of propellant mass spent (multiplying the weight-based Isp in seconds by g0) is in units of distance per time, which is an artificial velocity called the "effective exhaust velocity". This is higher than the actual exhaust velocity because the mass of the combustion air is not being accounted for. Actual and effective exhaust velocity are the same in rocket engines not utilizing air. Specific impulse is inversely proportional to specific fuel consumption (SFC) by the relationship Isp = 1/(go·SFC) for SFC in kg/(N·s) and Isp = 3600/SFC for SFC in lb/lbf-hr.

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