Identify the initial amount, (N_0)
Identify the amount remaining, (N), after time (t)
Use the decay formula (N = N_0 left(tfrac{1}{2}right)^{t/T_{1/2}})
Rearrange to solve for half-life: (T_{1/2} = dfrac{t}{log_2(N_0/N)})
If using the decay constant (lambda), use (T_{1/2} = dfrac{ln 2}{lambda})
If the amount halves in a measured time interval, that interval is the half-life
If multiple halvings occur, divide the total time by the number of halvings to get the half-life