Half-Life Calculator

Half-Life Calculator is a clean, fast tool for working with radioactive decay — completely offline. Give it any three of the four quantities (initial amount, remaining amount, elapsed time and half-life) and it solves for the fourth in an instant, then shows the decay constant and mean lifetime alongside.

WHAT YOU CAN DO
• Find the remaining amount after a given time
• Find how long it takes to decay to a target amount
• Find the half-life from two measurements over time
• See the decay constant (λ) and mean lifetime (τ) for every result
• Switch the time unit between seconds, minutes, hours, days and years

THE MATHS
Everything is built on the standard decay law N = N0 × (1/2)^(t / T½), with the decay constant λ = ln2 / T½ and the mean lifetime τ = T½ / ln2 = 1 / λ. Enter values such as N0 = 100, T½ = 10 and t = 30 to get 12.5 — three half-lives, one eighth left.

REAL ISOTOPES
The Examples tab loads true half-lives for common isotopes — Carbon-14 (5,730 years), Iodine-131 (8.02 days), Uranium-238 (4.468 billion years) and Technetium-99m (6.01 hours) — so you can explore decay across an enormous range of timescales with one tap.

LEARN AS YOU GO
A built-in Learn tab explains, in plain language, what a half-life is, how exponential decay works, and the difference between the decay constant and the mean lifetime.

IMPORTANT: EDUCATIONAL MODEL
This app uses the idealized half-life model. It assumes a constant half-life and a large number of atoms, and ignores measurement uncertainty, branching decay and other real-world effects. Results are educational estimates — do not rely on them for safety, medical or regulatory decisions.

PRIVACY
The app collects no personal data and works fully offline — your inputs never leave your device. Ads are served by Google AdMob, which may use an advertising identifier as described in Google's policies.

Light and dark themes, a tiny download, and no clutter. Download Half-Life Calculator and work through decay with confidence.