The larger the mass of a moving object, the less readily it moves. According to Newton's second law of motion, the acceleration that the object experiences is inversely proportional to its mass, and you can calculate this acceleration from the object's change in speed over a set amount of time. When the object approaches the speed of light, its mass changes as it moves, but you can ignore this tendency at normal speeds.
Subtract the object's initial speed from its final speed. If, for instance, it accelerates from 20 m/s to 50 m/s: 50 - 20 = 30 m/s.
Divide this answer by the time it spends accelerating. For instance, if the object accelerates over the course of 5 seconds: 30 ÷ 5 = 6 m/s².
Divide the force acting on the body by this acceleration. If, for instance, a force of 12,000 Newtons acts on it: 12,000 ÷ 6 = 2,000. This is the object's mass, measured in kilograms.