In the present research, amplified spontaneous emission (ASE) emerging from the lateral surface of a thin-disk laser is investigated. Through the method presented, an equation was derived to estimate the intensity of the ASE from the side surface of the disk. The equation results were compared with the findings of numerical approaches to validate the derived equation. Using this equation, the impact of the ASE on the reduction of the output power of a Yb:YAG disk laser was also explored. A comparison of the computational results with experimental findings indicated that the obtained equation could substantially enhance the estimation accuracy of the output power of the disk laser.