Using the Half-Life Calculator
Keep half-life and elapsed time in the same unit. If the half-life is in hours, enter elapsed hours rather than days. The initial amount can use any consistent quantity unit.
Worked example
Use the example inputs below to reproduce this result. These are the form's starting values, not recommended targets. Changing your inputs updates your result above; this worked example stays fixed for comparison. Results are rounded for display.
Example inputs
- Initial amount
- 100
- Half-life
- 10
- Elapsed time
- 20
Example result
- Remaining amount
- 25
- Remaining (%)
- 25
Method and assumptions
Remaining = initial × 2^(−elapsed / half-life). Time units must match.
Check signs and valid inputs
Powers, roots and logarithms have restrictions that ordinary addition does not. Zero, a negative base or a fractional exponent may change whether a real-number answer exists. Read the method's supported domain and keep parentheses around negative values where the input is an expression. A rejected input may indicate a mathematical restriction rather than a formatting problem.
Common question
Does two half-lives mean nothing remains?
No. Half remains after one half-life and half of that remains after the second, leaving one-quarter of the initial amount.
Guide updated .