Gain-loss ratio is an essential parameter of gas lasers and has a substantial effect on the design and performance of ring laser gyroscopes. Small ring gas lasers have wide mode spacing; therefore, it is convenient to measure the gain-loss ratio. A simple formula for calculating the gain-loss ratio is derived theoretically. Then, an experimental method is proposed to measure this parameter, and experiments are conducted for a large number of lasers. Results show that by taking advantage of precise optical machining and cavity alignment, the average gain-loss ratio of a ring laser can reach up to 3.55.