We propose a direct method to calculate the decay width of a hadron decaying to two photons, which is conventionally obtained by an extrapolation for various off-shell form factors.
The new method provides an simple way to examine the finite-volune effects. As an example, we apply the method to $\eta_c\rightarrow 2\gamma$. Using three $N_f=2$ twisted-mass gauge ensembles with different lattice spacings, we obtain the final decay width $\Gamma_{\eta_c\gamma\gamma}=6.57(15)_{\textrm{stat}}(8)_{\textrm{syst}}$ keV, where the systematic error accounts for the fine-tuning of the charm quark mass.
This method can also be applied for other processes which involve the leptonic or radiative particles in the final states.