The probability of an event that is certain to occur is 1, or 100%. This means that if an event is guaranteed to happen, there is no chance of it not happening.
In probability theory, the scale of probability ranges from 0 to 1. A probability of 0 means that an event will never happen, while a probability of 1 indicates that it will definitely happen. For example, consider the event of rolling a die and getting a number between 1 and 6. Since every roll will always result in one of those numbers, we say that the probability of this event is 1.
Understanding this concept is foundational in probability, as it helps to establish a baseline for comparing other events, which may have a probability of less than 1.