How to calculate half life in chemistry

For a first-order reaction:

$$t_{1/2} = \frac{\ln(2)}{k}$$

where $k$ is the rate constant.

For a second-order reaction (where the rate depends on the square of the concentration of one reactant, or the product of the concentrations of two reactants):

$$t_{1/2} = \frac{1}{k[A]_0}$$

where $k$ is the rate constant and $[A]_0$ is the initial concentration of the reactant.

For a zero-order reaction:

$$t_{1/2} = \frac{[A]_0}{2k}$$

where $k$ is the rate constant and $[A]_0$ is the initial concentration of the reactant.