Half-Life Calculator (Radioactive Decay)
By Santaji GadepatilinUpdated October 27, 2026
Amount remaining vs Time elapsed (years) — other inputs held at your current values.
How to use this calculator
Enter the initial amount, half-life and time elapsed to get the amount remaining. Or enter the remaining amount to find the age (time elapsed) or the half-life. Time units can be years, days, hours, minutes or seconds.
Formula
N = N₀ × (½)t / T½
Worked example — carbon-14
Half-life 5,730 years. After 10,000 years, of 100 units:
N = 100 × 0.510,000/5,730 = 29.8 units remaining (29.8%)
Common half-lives
| Isotope | Half-life |
|---|---|
| Iodine-131 | 8.0 days |
| Cobalt-60 | 5.27 years |
| Caesium-137 | 30.1 years |
| Carbon-14 | 5,730 years |
| Uranium-238 | 4.47 billion years |
Frequently asked questions
What is half-life?
The time it takes for half of a radioactive substance to decay. After two half-lives a quarter remains, after three an eighth, and so on.
How does carbon dating work?
Living things keep a steady level of carbon-14. After death it decays with a half-life of about 5,730 years, so the remaining fraction gives the age.
Can this be used for medicines?
Yes — drug elimination half-lives follow the same maths. Enter hours instead of years.