Half-Life Calculator
Calculate radioactive or exponential decay using half-life. Enter the initial amount, half-life and elapsed time to get remaining amount, fraction left, number of half-lives and a chart.
How to use this tool
- Enter initial amount, half-life (time units) and elapsed time (same units) in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your remaining amount and the full breakdown beneath it.
The half-life t½ is the time needed for a quantity to reduce to half its value. Using A(t) = A₀ · e^(−λt) where λ = ln(2)/t½, this calculator finds the remaining and decayed amounts at any elapsed time.
Frequently asked questions
- What substances have known half-lives?
- Carbon-14 has a half-life of about 5,730 years (used in carbon dating). Uranium-238 has ~4.5 billion years. Iodine-131 used in medicine has ~8 days.
- Can I use this for non-radioactive decay?
- Yes — any process that halves on a fixed schedule (drug concentration, population loss, etc.) can be modelled this way.