Science & Engineering
Half-Life Decay Calculator
Calculate the quantity remaining after a number of half-lives and solve the elapsed half-life count from time and half-life. The calculator uses remaining amount = initial amount × (1/2)^(elapsed time ÷ half-life). It returns more than one result so you can check the main answer against a useful secondary measure. Each half-life multiplies the remaining amount by one half; it does not subtract a fixed amount. The simple model assumes one exponential decay process with a constant half-life and no production term.
Educational model only. Use applicable standards, calibrated measurements, and qualified review for safety-critical work.
Calculate and compare
Use the number box for precision or the slider for fast scenario testing.
Scenario results
Amount remaining
12.5
After 3 half-lives.
Fraction remaining
12.5%
Share of initial quantity.
Time to threshold
26.5754 time units
When the simple decay curve reaches the entered threshold.
How the calculation works
Use consistent units and retain full precision until the final display step.
Worked example
Reproduce the displayed scenario, then change one assumption at a time.
Assumptions behind the result
- • Inputs use the units shown beside each control.
- • The displayed formula is applied without hidden market or demographic data.
- • Rounding occurs only for display; calculations keep full numeric precision.
- • Each half-life multiplies the remaining amount by one half; it does not subtract a fixed amount.
- • The simple model assumes one exponential decay process with a constant half-life and no production term.
Mistakes that change the answer
- • Mixing percentages with decimals or mixing incompatible units.
- • Relying on a rounded intermediate value instead of the full result.
- • Changing several assumptions at once instead of testing initial amount separately.