Half-Life Calculator

How to use this calculator

1. Enter the initial amount, the substance’s half-life (any time unit), and elapsed time.
2. We compute how many half-lives have passed and apply exponential decay.
3. See remaining amount, percent remaining, half-lives elapsed, and the decay constant.

Inputs explained

Initial amount

Starting quantity of the substance (grams, mg, counts—any unit).

Half-life

Time it takes for the amount to halve. Keep units consistent with elapsed time.

Elapsed time

Time that has passed since start.

When to use it

• Radioactive decay calculations in nuclear physics or medicine dosing.
• Drug elimination half-life problems in pharmacokinetics.
• Any exponential decay scenario such as capacitor discharge or population decline.

Tips & cautions

• Keep half-life and elapsed time in the same units to avoid incorrect results.
• Use scientific notation for very large or small initial amounts if needed.
• The decay constant (λ) is handy for differential equation models beyond simple half-life math.
• Assumes pure exponential decay; real-world processes can deviate.
• Does not account for production terms or multi-compartment pharmacokinetics.
• User must supply correct half-life; we don’t look up nuclide or drug data.

Worked examples

Deep dive

FAQs

Is time unit-agnostic?

Yes. Just keep half-life and elapsed time in the same units (hours, days, years).

Can it handle growth?

Half-life implies decay. For growth, use doubling-time math (replace 1/2 with 2).

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