Formula and calculation method
Remaining amount = initial amount × (1/2)^(elapsed time / half-life).
Use consistent units for all comparable inputs. The displayed result is rounded; arithmetic is performed with the full available numeric precision. Check your source measurements before relying on the result.
How to use half-life calculator
- Set Initial quantity. The example below uses 100.
- Set Half-life in chosen time units. The example below uses 10.
- Set Elapsed time in same units. The example below uses 20.
- Select Calculate to update the result. Reset restores the illustrated inputs. Copy, print or download your own result if you need a record.
Worked example
Example inputs
- Initial quantity
- 100
- Half-life in chosen time units
- 10
- Elapsed time in same units
- 20
- Remaining quantity
- 25
- Decayed quantity
- 75
Factors affecting your result
Use consistent units for all comparable inputs. The displayed result is rounded; arithmetic is performed with the full available numeric precision. Check your source measurements before relying on the result.
Changing initial quantity changes the scenario being evaluated. Adjust one input at a time when comparing results, keep a copy of the assumptions, and compare values using the same unit and period. A precise arithmetic result does not make an uncertain assumption precise.
Frequently asked questions
How does the half-life calculator work?
Remaining amount = initial amount × (1/2)^(elapsed time / half-life).
What inputs does Half-Life Calculator need?
Enter initial quantity, half-life in chosen time units, elapsed time in same units. Choose values that correspond to the same situation and measurement convention.
How should I interpret the half-life calculator result?
Use consistent units for all comparable inputs. The displayed result is rounded; arithmetic is performed with the full available numeric precision. Check your source measurements before relying on the result.
Are my half-life calculator inputs uploaded?
No. The calculation runs in this browser. Favorites, recent tool names, theme and formatting preferences may be saved locally, but calculation inputs are not sent to a calculation server or stored by this website. Shared links do not include entered values.
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