Hi Christine.

The general formula for exponential decay given a half-life is

where N(t) is the amount at time t, N₀ is the initial amount (at time t=0), and t_1/2 is the half life of the substance.

Carbon-14 (¹⁴C) is a radioactive substance that has a half-life of about 5730 years. While a biological organism is alive and respiring, it absorbs ¹⁴C from the air and has a constant percentage in its tissues. When the organism dies, the amount of ¹⁴C gradually diminishes in accordance with the formula above. This makes it possible to measure the percentage of remaining ¹⁴C in dead organic matter and determine the approximate time of death.

You can read more about the science of radiocarbon dating here.

Stephen La Rocque.>