Half-Life Calculator
Calculate radioactive decay half-life, remaining quantity, or elapsed time.
Result
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A half-life calculator determines how much of a substance remains after a given time using the exponential decay formula.
Examples
Radioactive decay
100g with half-life 5 years after 15 years: 100 × (0.5)^3 = 12.5g
Finding half-life
200g decays to 50g in 10 hours: t₁/₂ = -10 × ln(2) / ln(50/200) = 5 hours
Medicine
Drug with 4hr half-life: after 12hrs, (0.5)^3 = 12.5% remains
Frequently Asked Questions
What is half-life?
Half-life is the time required for a quantity to reduce to half its initial value. It applies to radioactive decay, pharmacology, chemical reactions, and more.
How many half-lives until something is gone?
After 7 half-lives, less than 1% remains (0.78%). After 10 half-lives, about 0.1% remains. Technically, it never reaches exactly zero.
What is the decay constant?
The decay constant λ = ln(2)/t₁/₂ ≈ 0.693/t₁/₂. It represents the probability per unit time that a given particle will decay.
Result
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Quick Tips
- •Double-check your inputs — small errors lead to incorrect results.
- •Use consistent time units for half-life and elapsed time.
- •After 10 half-lives, less than 0.1% of the original amount remains.