Half-Life Calculator
Half-life is the time it takes for half of something to decay or be used up. It describes radioactive atoms, and it also fits other things that fade by the same share each period.
Enter the starting amount, the half-life and the time that has passed.
- Free
- Works offline after your first visit
- Saves on this device
- No sign-up
- Updated
How it works
Each half-life, the amount left is cut in half. The number of half-lives that have passed is the time passed divided by the half-life. The amount left is the starting amount times one half, raised to that number.
The method assumes the amount falls by the same share in every equal period. Some real processes speed up or slow down, and then the simple model is only a guide.
Worked example
Starting with 1,000, after 24 day with a half-life of 8 day, 125 is left. That is 3 half-lives.
Questions
Does the half-life depend on how much I start with?
No. The half-life is a fixed time for a given substance, so a bigger start takes just as long to halve.
Does the amount ever reach zero?
In this model it gets smaller and smaller without quite reaching zero, since each period halves what is left.
Does this work outside of atoms?
Yes, for anything that falls by the same share each period. Some chemicals and materials break down this way too, but look up the figure for your substance.
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