Half-Life Calculator
Calculate remaining quantity after radioactive decay.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Enter the initial quantity, the half-life of the substance, and the elapsed time. Make sure the half-life and elapsed time use the same units.
- Enter initial quantity. This can be mass, number of atoms, or activity.
- Enter half-life. For Carbon-14, this is 5,730 years.
- Enter elapsed time. Use the same time unit as the half-life.
- Read the result. See remaining quantity, number of half-lives, and percentage remaining.
About the Half-Life Calculator
Half-life is the time required for half of a radioactive substance to decay. After one half-life, 50% remains. After two, 25%. After three, 12.5%. The formula N = N0 * (1/2)^(t/t_half) calculates the remaining quantity after any elapsed time. Half-life is constant for a given isotope regardless of the initial amount. Carbon-14 has a half-life of 5,730 years and is used in radiocarbon dating. Uranium-238 has a half-life of 4.5 billion years. Some isotopes decay in fractions of a second.
Frequently Asked Questions
What is Carbon-14's half-life?
Carbon-14 has a half-life of approximately 5,730 years. This property makes it useful for dating organic materials up to about 50,000 years old.
Does half-life change with conditions?
For radioactive decay, the half-life is constant regardless of temperature, pressure, or chemical state. It is a property of the nucleus itself.
How many half-lives until a substance is gone?
A substance never fully disappears mathematically, but after about 10 half-lives, less than 0.1% remains. After 20 half-lives, less than one millionth remains, which is effectively zero for most purposes.