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Half-Life calculator

Solves decay both ways: amount left after a time, or time to reach a target, with the decay constant and a halving schedule.

Enter a starting amount and a half-life.

The math is the pure exponential law A = A₀·2^(−t/T), the same curve for radioactive nuclides, drug clearance, and any process that sheds a fixed fraction per interval. The model assumes a single well-defined half-life and first-order kinetics: it does not model dosing schedules, biological uptake and release, chains of decay products, or saturation. The halving schedule lists whole half-lives only, so the last row is not a prediction of when the substance is gone.
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