When an organism dies, it stops exchanging carbon with the environment, and the carbon-14 it contains begins to decay at a known rate, with a half-life of approximately 5,730 years.

By measuring the ratio of carbon-14 to carbon-12 in a sample, a radiocarbon laboratory can calculate how long ago the organism died — how much carbon-14 has decayed since death.

This calculation produces a “radiocarbon age” expressed in years before present, with “present” conventionally defined as 1950 CE.

The relationship between radiocarbon years and actual calendar years is not perfectly constant.

The amount of carbon-14 in the atmosphere has varied over time, due to changes in cosmic ray flux, ocean circulation, and other factors, and these variations mean that a given radiocarbon age does not always correspond to the same calendar age.