Link-budget calculations are a common way to assess system parameters, such as the required transmitter power and receiver sensitivity, in free-space optical (FSO) communication systems. One of the biggest challenges for long-range FSO deployment is its signal propagation under turbulent atmospheric conditions, which produce intensity fluctuations. Methods to estimate atmospheric-fading loss in radio-frequency systems cannot be adapted to the FSO channel. Until now no general closed-form methods have been developed to describe the fading loss in such a channel. A method to calculate the losses due to scintillation fading in the threshold approach, based on lognormal statistics of the received power, is presented.