Half-Life Calculator
Estimate radioactive, chemical, or biological decay using a half-life and elapsed time.
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Guide
How this half-life calculator works
This tool models exponential decay from an initial amount, a half-life, and either elapsed time or a target remaining amount.
Set the initial amount
Use any consistent unit, such as grams, milligrams, atoms, or activity units.
Enter the half-life
The half-life is the time required for half of the current amount to remain.
Choose a mode
Calculate the remaining amount after time passes, or estimate the time needed to reach a target amount.
FAQs
What is half-life?
Half-life is the amount of time it takes for a decaying substance or quantity to drop to half its current amount.
How is remaining amount calculated?
The calculator uses exponential decay: remaining amount equals initial amount multiplied by 0.5 raised to the number of half-lives elapsed.
What does percent remaining mean?
Percent remaining compares the remaining amount to the initial amount, so 25% remaining means three-fourths has decayed.
Where is half-life used?
Half-life appears in chemistry, physics, medicine, biology, and any setting where a quantity decays at a proportional rate.
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