Half Life Calculator
The following tools can generate any one of the values from the other three in the halflife formula for a substance undergoing decay to decrease by half.
Half Life Calculator
Please provide any three of the following to calculate the fourth value.

Half Life, Mean Lifetime, and Decay Constant Conversion
Please provide anyone of the following to get the other two.

Definition and Formula
By definition, halflife is the period of time it takes for a substance undergoing decay to decrease by half. An exponential decay process can be described by any of the following three equivalent formulas:
N_{t} = N_{0}(1/2)^{t/t1/2}
N_{t} = N_{0}e^{t/τ}
N_{t} = N_{0}e^{λt}
where
N_{0} is the initial quantity
N_{t} is the quantity that still remains after a time t,
t_{1/2} is the halflife
τ is the mean lifetime
λ is the decay constant
t_{1/2} = ln(2)τ = ln(2)/λ