We propose an online app (bit.ly/2KQKNmd) which calculates the risks of COVID-19 infection for a person coming into contact with a group of individuals characterized by a specified prevalence rate. The user provides the size of the group, the number (and duration) of contacts and the level of precautions. For a well-documented September 2020 White House event the app predicts that with an assumed 3% prevalence rate one’s risk of infection was almost 5 % and seven of the 150 guests would become infected - as actually happened. The tool, destined to the general public, can thus quantify the risks of infection in special populations (social gatherings, prisons, etc), but also in general ones (stores, stadiums, etc.).