Half-Life Calculator
How much of a substance remains after a given time of decay.
Result
Amount remaining
12.5000
Percent remaining (%)
12.50
How it works
A half-life is the time it takes for half of a quantity to decay away. The remaining amount follows initial × (1/2)^(time ÷ half-life), so the decay is exponential rather than linear. That distinction matters. After one half-life, 50% is left; after two, 25%; after three, 12.5%. The substance never reaches exactly zero — each interval halves what remains, so a small fraction persists long after the material is practically gone. Half-lives span an extraordinary range, from fractions of a second for some isotopes to billions of years for uranium-238. The concept extends well beyond radioactivity: pharmacologists use it for how long a drug stays active in the body, which is what sets dosing intervals.
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Frequently asked questions
What is a half-life?
The time for half of a substance to decay — constant regardless of how much you start with.
How much is left after 3 half-lives?
One eighth, or 12.5%, of the original amount.
How does carbon dating work?
Living things absorb carbon-14, which stops when they die and then decays with a half-life of about 5,730 years. Measuring how much is left dates the sample — reliably back to roughly 50,000 years, after which too little remains to measure.
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