The tool the U.S. Army uses to assess risk levels is a chart called a risk-assessment matrix. The side of the chart lists degrees of severity of possible outcomes, from negligible to catastrophic. The top of the chart lists the likelihood of an event, from unlikely to frequent.
From the categories on the sides and top of the chart, a user can rank the risk of various types of threats. Risk refers to the amount of harm that is expected due to a given event during a certain period of time. The level of risk is derived from the multiplication of the chances that harm happens times the level of seriousness of the harm. Because neither calculation is precise, risk is typically categorized into only a few levels.