Two half-lives
100 g with a half-life of 5 days leaves 25 g after 10 days.
Half-Life Calculator
Solve radioactive decay problems: amount remaining, initial amount, time elapsed or half-life.
Result
Enter your values and select Calculate to see the result.
Runs in your browser — nothing you enter is sent to a server.
How the Half-Life Calculator works
A half-life is the time it takes for half of a radioactive substance — or anything that decays exponentially — to disappear. Choose which value to find and fill in the other three: initial amount, remaining amount, time and half-life. Use the same units for both amounts and for both times.
Formulas
Decay
N = N₀ × (½)t ÷ T
Time
t = T × log₂(N₀ ÷ N)
Examples
Carbon dating
Carbon-14 has a half-life of about 5,730 years. A sample with 12.5% of its carbon-14 left is about 17,190 years old (three half-lives).
Frequently asked questions
Does anything ever fully decay?
In theory the amount halves forever; in practice, after about 10 half-lives less than 0.1% remains.
Can I use it for medicines?
Drug elimination often follows the same curve, but dosing decisions belong to a doctor or pharmacist.
What units should I use?
Any, as long as they match: grams with grams, years with years.
Related tools
Logarithm Calculator
Calculate logarithms in base 10, e, 2 or any base, with ln, log₁₀ and log₂ side by side.
Math Tools
Molar Mass Calculator
Calculate the molar mass of a chemical formula and its composition by mass, with brackets and hydrates.
Science Tools
Scientific Notation Converter
Convert numbers to and from scientific, E and engineering notation, exactly.
Math Tools