the amount of fuel used to find specific impulse can be measured in different ways. it is sometimes measured in mass and sometimes in weight. when the amount of fuel is measured in mass, the specific impulse is expressed as a speed. this is usually in meters per second. when specific impulse is measured as a speed, it has another name. it is also called the "effective exhaust velocity". the other way to measure the amount of fuel is weight. if weight is used, the specific impulse is in units of time, usually in seconds. these two ways are both common. they both compare the performance of engines.