Free interactive tool
Pick a radioactive isotope, enter how much you start with and how much time passes, and see how much is left — plus how long it takes to fall to a tenth, a hundredth or a thousandth, and a one-line note on what each isotope is and why it matters. Everything runs here on the page — no sign-up, nothing stored.
Radioactivity falls by half every half-life: after one half-life half is left, after two a quarter, after ten about a thousandth. The amount remaining is A = A₀ × ½(t ÷ half-life) — a clean exponential curve for any single isotope.
Volatile fission product that concentrates in the thyroid — the isotope potassium-iodide (KI) tablets are meant to block; the dominant radioiodine hazard in the first weeks after a release.
Activity remaining
Time to decay to…
Decay is exponential, so the activity approaches zero but never mathematically reaches it — ten half-lives (to ~0.1%) is the usual practical "all gone" mark.
Decay timeline
| Half-lives | Time elapsed | Remaining | Activity |
|---|---|---|---|
| 1× | 8.02 days | 50% | 0.5 GBq |
| 2× | 16 days | 25% | 0.25 GBq |
| 3× | 24.1 days | 12.5% | 0.125 GBq |
| 4× | 32.1 days | 6.25% | 0.0625 GBq |
| 5× | 40.1 days | 3.125% | 0.03125 GBq |
| 7× | 56.1 days | 0.7813% | 0.007813 GBq |
| 10× | 80.2 days | 0.09766% | 0.0009766 GBq |
Other figures
Like the dose accumulated in a decaying fallout field, the total number of decays still to come is finite — it converges to the current activity × the mean lifetime, even integrated over all future time.
Activity (becquerels) counts decays per second — not the dose you receive. Whether those decays harm you depends on the type and energy of the radiation, distance, shielding, and above all whether the material is outside or inside your body. A pure alpha emitter like plutonium-239 or polonium-210 is almost harmless across the room but extremely dangerous if inhaled or swallowed; a strong gamma emitter like cobalt-60 or caesium-137 is a serious external hazard.
To work in sieverts once you know a dose rate, use the radiation dose calculator.
This calculator models one nuclide decaying by its own fixed half-life — clean exponential decay. Fresh nuclear fallout behaves very differently: it is a mixture of hundreds of fission products with half-lives from seconds to decades, and their combined dose rate falls along the empirical t−1.2 "7-10 rule" (every sevenfold increase in time ≈ a tenfold drop), not a single exponential.
So use this page when you know the specific isotope — a medical or industrial source, a single contaminant, a research sample. For the mixed field after a detonation, use the fallout-decay and acute-exposure sections of the radiation dose calculator, which already build in the 7-10 rule and its key consequence: because the rate collapses, the dose you can accumulate converges to a finite total.
Half-life, what it emits, and why each one matters. Half-lives are standard reference values shown at the precision usually quoted.
Volatile fission product that concentrates in the thyroid — the isotope potassium-iodide (KI) tablets are meant to block; the dominant radioiodine hazard in the first weeks after a release.
Short-lived radioiodine that adds to early thyroid dose right after a release but is largely gone within days.
Long-lived fission product and the main driver of lasting land contamination after Chernobyl and Fukushima; spreads through the body like potassium, with gamma from its Ba-137m daughter.
Reactor-produced caesium; its ratio to longer-lived Cs-137 helps date and fingerprint a release.
Pure beta emitter that mimics calcium and lodges in bone and teeth, irradiating marrow for years — a key internal hazard in fallout and reactor waste.
Inert noble-gas fission product; an early marker of reactor fuel damage that disperses as a cloud rather than contaminating surfaces.
Weapons-grade fissile material and an alpha emitter — barely a hazard across the room, but extremely dangerous if inhaled as fine particles.
Intense alpha emitter used to power spacecraft RTGs; its steady heat output, not its penetrating radiation, is the point.
The fissile uranium isotope enriched for reactors and weapons; only weakly radioactive thanks to its immense half-life.
The common uranium isotope, only weakly radioactive; fertile material that breeds plutonium and heads a long natural decay chain.
Alpha (plus low-energy gamma) source found in household smoke detectors; a contamination concern only if dispersed.
Naturally radioactive potassium in soil, food and your own body — a baseline source of background dose (the "banana" isotope).
Radioactive gas from the uranium chain that seeps from rock into buildings; the largest natural radiation dose for most people and a leading cause of lung cancer.
Cosmic-ray-produced carbon in all living things; the basis of radiocarbon dating and a minor natural internal dose.
Alpha emitter once painted onto luminous dials; decays to radon gas and remains a legacy contamination hazard.
Intense dual-gamma source used in radiotherapy, sterilisation and radiography; a prime "dirty bomb" and lost-source concern because the gammas are a serious external hazard.
The workhorse of medical imaging — a short-lived gamma emitter used in tens of millions of scans a year and gone within a day or two.
Compact, intense gamma source used for industrial radiography; frequently involved in lost-source radiation accidents.
Extremely toxic alpha emitter (the Litvinenko poison); also present in tobacco smoke, and dangerous only if taken internally.
Low-energy beta emitter used in self-powered exit signs and gun sights and as fusion fuel; a hazard mainly as tritiated water if ingested in quantity.
1 half-life → 50% left · 2 → 25% · 3 → 12.5% · 4 → 6.25%
5 → 3.13% · 7 → 0.78% · 10 → 0.098% (≈ gone) · 20 → 0.0001%