Half-Life Calculator
Enter the starting amount, the half-life and the time that has passed.
This result is an estimate only. Double-check important figures before relying on them.
How it is calculated
- Half-lives elapsed = time elapsed ÷ half-life
- Remaining = starting amount × 0.5^(half-lives)
- Decayed = starting amount − remaining
The same idea in finance is shown by the rule of 72 calculator.
Worked example
100 g of carbon-14 (half-life 5,730 years) after 11,460 years:
- Half-lives = 2
- Remaining = 100 × 0.25 = 25 g
Why your number may differ
Half-life values are measured and rounded. Decay is statistical, so a very small sample can differ from the expected figure.
Frequently asked questions
What is a half-life?
The time it takes for half of a substance to decay or disappear. After two half-lives, a quarter is left.
What is the formula?
Remaining = starting amount × (½)^(elapsed ÷ half-life).
Does it only apply to radioactivity?
No. It also describes anything that falls by a fixed share each period, such as a drug cleared from the body.