Half-Life Calculator
Radioactive decay: find the remaining amount, elapsed time or half-life, with a decay curve.
Results update as you type.
About this calculator
The half-life is the time it takes for half of a radioactive sample to decay, and it sets the pace of the whole process through N = N₀ × (½)^(t / t½), where N₀ is the starting amount, t is the elapsed time and t½ is the half-life. After one half-life half the material remains, after two a quarter, after three an eighth, and so on — the amount never quite reaches zero but halves with every t½ that passes. Choose what to solve for: the amount left after a given time, the time needed to fall to a given amount, or the half-life itself inferred from a measured decay.
As a worked example, carbon-14 has a half-life of 5,730 years, so a sample that starts with 100 units of C-14 falls to 100 × (½)^(2000 / 5730) ≈ 78.5 units after 2,000 years. The calculator also reports the decay constant λ = ln2 / t½ ≈ 0.693 / t½ (the probability a given atom decays per unit time) and the mean lifetime τ = 1 / λ = t½ / ln2, which is about 1.44 half-lives — the average time an individual atom survives — and plots the decay curve.
Because the maths is scale-free, the time and the half-life only need to share the same unit — years, days, seconds or hours — and the initial and remaining amounts can be in grams, atom counts, becquerels or any consistent measure. This makes the tool equally at home with radiocarbon and radiometric dating, medical isotope dosing, nuclear-waste decay planning and any first-order exponential decay, including some drug-elimination and discharge problems that follow the same half-life mathematics.
Frequently asked questions
What is the half-life decay formula?
The amount left after time t is N = N₀ × (½)^(t/t½). After one half-life half remains, after two a quarter, after three an eighth, and so on.
What is the decay constant and mean lifetime?
The decay constant is λ = ln2 / t½ ≈ 0.693 / t½, the probability of decay per unit time. The mean lifetime τ = 1 / λ = t½ / ln2 is the average time an atom survives — about 1.44 half-lives.
Can I find the half-life from two measurements?
Yes. Set "solve for" to half-life and enter the initial and remaining amounts plus the elapsed time; the tool solves t½ = t × ln2 / ln(N₀/N).
How is carbon-14 dating related to half-life?
Carbon-14 decays with a half-life of 5,730 years, so measuring how much C-14 is left in a once-living sample dates it. From 100 units, about 78.5 remain after 2,000 years, and roughly half after 5,730 — the basis of radiocarbon dating.
What units do the time and half-life use?
Any unit, as long as the elapsed time and the half-life share it. Enter both in years, or both in seconds; the ratio t / t½ is all that matters, and the results come back in that same unit.
How much is left after several half-lives?
Multiply by one-half for each half-life: 50% after one, 25% after two, 12.5% after three, 6.25% after four. After ten half-lives under 0.1% remains, which is why "about ten half-lives" is a common rule for effectively gone.
Does half-life depend on how much material I start with?
No. It is a fixed property of the isotope, independent of sample size, temperature or chemical state. Whether you start with a gram or a tonne, half decays in exactly one half-life — which is what makes it such a reliable clock.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/half-life/curl
curl "https://calculator.free/api/v1/half-life/?solve=remaining&initial=100&half_life=5730&time=2000"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/half-life/?" + new URLSearchParams({
"solve": "remaining",
"initial": "100",
"half_life": "5730",
"time": "2000"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.