⏳ Half-Life Calculator
Half-life is the time it takes for half of something to decay or disappear — a radioactive isotope, a drug in the bloodstream, or any quantity fading exponentially. After each half-life the remainder halves again: 100 becomes 50, then 25, then 12.5.
Enter the starting quantity, one half-life period, and how much time has passed. The calculator applies the decay formula and also shows how many half-lives have elapsed.
How to use this calculator
Enter the starting quantity, the half-life period and the elapsed time, then press Calculate to see what remains.
- Type the starting amount into the Initial quantity field.
- Type one half-life period into the Half-life field, in whatever time unit you are using.
- Type how much time has passed into the Elapsed time field, using the same time unit.
- Press Calculate and read the remaining quantity, the percent remaining, and how many half-lives have elapsed.
Frequently asked questions
What is half-life?
Half-life is the time needed for half of a quantity to decay or disappear. After one half-life 50 percent remains, after two 25 percent, after three 12.5 percent, and so on, following an exponential curve.
What is the half-life formula?
Remaining equals the initial quantity times one-half raised to the power of elapsed time divided by half-life. The calculator applies this formula directly, so fractional half-lives work fine.
What can this be used for?
Radioactive isotopes are the classic case, but the same math describes medicines leaving the bloodstream, and anything else that fades by a fixed fraction per time period.
Can you show a worked example?
Start with 100 units and a half-life of 5 hours. After 10 hours, exactly two half-lives have passed, so 25 units remain, which is 25 percent of the start.