Half-Life Calculator
Calculate radioactive decay or any exponential half-life process. Enter initial quantity, half-life, and elapsed time.
REMAINING QUANTITY
250 units
HALF-LIVES ELAPSED
2
DECAYED
750
% REMAINING
25%
💡 How to Calculate Half-Life
What Is Half-Life?
Half-life is the time required for one-half of the atoms of a radioactive substance to disintegrate or decay. It is also used to describe any process that follows exponential decay, such as drug metabolism in the body or chemical reaction kinetics.
After one half-life, 50% of the original substance remains. After two half-lives, 25% remains. After three, 12.5%. After 10 half-lives, only about 0.1% of the original amount is left.
How to Calculate Half-Life Decay
Half-Life Formula
Where:
- N = remaining quantity
- N₀ = initial quantity
- t = elapsed time
- t½ = half-life period
For example, if you start with 100 grams of Carbon-14 (t½ = 5,730 years) and 11,460 years have passed (2 half-lives):
N = 100 × (½)² = 100 × 0.25 = 25 grams remaining
Common Radioactive Half-Lives
| Isotope | Half-Life | Use |
|---|---|---|
| Carbon-14 | 5,730 years | Radiocarbon dating |
| Uranium-238 | 4.5 billion years | Geological dating |
| Iodine-131 | 8 days | Medical imaging |
| Radon-222 | 3.8 days | Home testing |