C-14 dating method is based on the fact that

As long as a plant or animal is alive, a dynamic equilibrium exists in which an organism excretes or exhales the same amount of C-14 that it takes in and the ratio of C-14 to C-12 in the living organism is the same as that in the upper atmosphere. When the plant or animal dies, however, it no longer takes in more C-14 and ratio of C-14 to C-12 slowly decreases due to decay of C-14. 614C⟶714N+−10eThus, from the activity of C-14 in the dead organism and that of fresh sample (in the atmosphere), age can be decided. λ (decay constant) =2.303t (age) log⁡N0 (original activity) N (final activity)