The airport terminal is a complex building composed of many functional subsystems dimensioned to host passenger traffic, considering an appropriate level of service (LoS). The most widely known method to assess the LoS and design the terminal areas is the Airport Development Reference Manual by the International Air Transport Association (IATA). Based on this, a calculation tool in Microsoft Excel® was implemented to assess the capacity and dimension of the facilities for each functional subsystem of the terminal. The tool, composed of nine correlated spreadsheets, is a useful model to design new structures, evaluate the LoS currently offered, plan interventions, and face the new rules of passenger distancing due to the COVID-19 emergency. An international airport terminal with 20 million/year including Schengen and extra-Schengen passengers was studied. The LoS of the terminal subsystems was assessed and the areas needed for each subsystem were calculated. In the analyzed case study, most subsystems (departure hall, check-in, boarding gates, baggage claim, and arrival halls) were over-designed, according to the definition of the IATA LoS. This means that available spaces for queues and holding are sufficiently large to easily face the new rules of social distancing for passengers due to the recent COVID-19 emergency.