How this calculation works
Each half-life leaves half of what was present at the beginning of that interval. Use the same time unit for half-life and elapsed time. This assumes a constant exponential decay rate.
Remaining amount = starting amount × (½)^(elapsed time ÷ half-life)A worked example
Starting with 100 units and a half-life of 10 time units leaves 25 units after 20 time units.
Good to know
An ideal exponential model. Not a medication dosing, exposure assessment or radioactive-material handling tool.
Each interval removes half of what is left
Starting from 100 units gives 50 after one half-life, 25 after two and 12.5 after three. It does not remove 50 units in every interval, because the half applies to the current remaining amount.
In the continuous mathematical model, the amount approaches zero without reaching exact zero in finite time. A displayed zero can instead reflect rounding or a very small numerical result.
Use one time unit and one consistent model
If half-life is in hours, elapsed time must also be in hours. A value quoted in days needs conversion before it can be compared with an hourly half-life.
The model assumes a fixed exponential decay rate. It does not establish how a real medicine, exposure or mixed system behaves and must not be used to choose a dose or handling procedure.
A few questions, answered
Can I enter a fraction of a half-life?
Yes. Elapsed time can be fractional while the half-life remains positive.
Does two half-lives mean nothing remains?
No. One quarter of the starting amount remains after two half-lives.