Half-Life Calculator
Enter the starting amount, the half-life, and the elapsed time to get the remaining fraction and a decay curve
Plug in a substance's half-life (t½) and how much time has passed to model exponential decay, then see the remaining amount as a percentage along with a visualized decay curve.
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How to use
- Enter the starting amount (100% by default, or your own units) and the half-life of the substance.
- Enter the elapsed time — seconds, minutes, hours, days, or years are all supported, as long as the units match.
- Review the results from the exponential decay formula: the final remaining percentage, the percentage lost to decay, and the SVG decay curve.
FAQ
- What is the half-life formula?
- For an elapsed time T and a half-life H, the remaining amount N(T) equals the initial amount N(0) multiplied by (1/2) raised to the power of T/H. So after one half-life 50% is left, after two 25%, and after three 12.5%.
- Are half-life and mean lifetime the same thing?
- No. Half-life is the time it takes for a quantity to drop to exactly half its value, while mean lifetime (τ) is the average time an individual particle takes to decay. The two are related by 'mean lifetime = half-life ÷ ln(2)', which makes the mean lifetime about 1.44 times longer than the half-life.
- Are half-life and mean lifetime the same thing?
- No. Half-life is the time it takes for a quantity to drop to exactly half its value, while mean lifetime (τ) is the average time an individual particle takes to decay. The two are related by 'mean lifetime = half-life ÷ ln(2)', which makes the mean lifetime about 1.44 times longer than the half-life.