Find how much of a substance remains after a given time, based on its half-life.
What this calculator does
Enter an initial amount, the half-life and the time elapsed, and this calculator returns how much remains and how much has decayed.
Half-life is the time for half of a quantity to decay — used in physics, chemistry and pharmacology.
The default scenario, worked through
As an example, with a initial amount of 100, the calculator returns a amount remaining of 12.5. Underneath, Half-lives passed comes out at 3 and Fraction remaining at 0.125. Change any field and every figure updates as you type.
Step by step
- Enter the initial amount.
- Enter the half-life and the time elapsed (same units).
- Read the amount remaining.
The calculation
remaining = initial × (½)^(time ÷ half-life)
FAQ
What is a half-life?
The time it takes for half of a decaying quantity to disappear. After two half-lives, a quarter remains; after three, an eighth.
Does it work for any decay?
Yes — any process with a constant half-life, from radioactivity to drug elimination, follows this formula.
What units should I use?
Any, as long as the half-life and elapsed time share the same unit (seconds, years, etc.).