Identify the starting amount
Measure the amount at later times
Find the time it takes for the amount to drop to half of the starting amount
Use the decay formula if needed: (N(t)=N_0 e^{-kt})
Solve for half life: (t_{1/2}=frac{ln 2}{k})
If using two measurements, find the decay constant first
Rearrange the formula to get (k=frac{1}{t}lnleft(frac{N_0}{N(t)}right))
Substitute (k) into (t_{1/2}=frac{ln 2}{k})
For repeated halving, count the time between each halved amount
Use a graph and locate the time when the value reaches half the initial value