Half-life is the period of time it takes for a substance undergoing decay to decrease by half. It is usually used to describe quantities undergoing exponential decay (for example, radioactive decay) where the half-life is constant over the whole life of the decay, and is a characteristic unit (a natural unit of scale) for the exponential decay equation. However, a half-life can also be defined for non-exponential decay processes, although in these cases the half-life varies throughout the decay process. The calculation of half-life used in this tool is based on the exponential decay equation.