# EPA On-line Tools for Site Assessment Calculation

Half lives and rate constants are related through the formula t_{1/2} = 0.6931/λ, where t_{1/2} is the half life and λ is the first order rate constant. The calculator below does this calculation using various time units.

##### Half Life/Rate Constant Converter

**Half Life** t
_{1/2}
= 0.6931 / λ

t
_{1/2}
= half life [T]

λ = rate constant [T
^{-1}
]

Note: The rate constant is positive because the transport equation has the term - λ c The minus sign denotes loss of the contaminant to degradation.