Reaction Half-Life Calculator

For a first-order reaction the half-life equals the natural log of two divided by the rate constant, with no concentration term, so it stays the same no matter how much reactant you start with.

Zero-order half-life depends on initial concentration over twice the rate constant, first-order depends only on the rate constant, and second-order depends on one over the rate constant times the initial concentration.

The half-life is in whatever time unit your rate constant uses. If k is in per second, the half-life is in seconds. The rate constant units differ by order, so keep them consistent with your other values.

They share the definition (the time for a quantity to fall by half) and the same formula, but only for first order. Radioactive decay is always first order, with t½ = ln(2)/λ where λ is the decay constant, which is the exact analogue of the first-order chemical case t½ = ln(2)/k and gives a constant half-life. Zero- and second-order chemical reactions have half-lives that change as the concentration drops, something that never happens in radioactive decay. So the 'half-life formula' you need depends on whether the process is first order.