The authors obtain a new equation to estimate the forward component of a photon dose generated through the interaction between a target and a short pulse high power laser. As the equation is quite simple, it is useful for calculating the photon dose. The equation shows that the photon dose is proportional to the electron temperature in the range>3 MeV and proportional to the square of the electron temperature in the range<3 MeV. The dose estimated with this method is roughly consistent with the result of Monte Carlo simulation. With some assumptions and corrections, it can reproduce experimental results obtained and the dose result calculated at other laboratories.