Estimate remaining quantity after elapsed time and calculate how long it takes to decay to a target threshold.
Remaining quantity
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Enter a starting quantity, half-life, elapsed time, and safe threshold to model the decay curve.
Tap an example to prefill the calculator with sample values.
Medical isotope window
Eight-day half-life over 24 days
Useful for seeing how quickly a short-lived isotope drops after a few half-lives and how long it takes to reach a lower handling threshold.
Result: Three half-lives leave 12.5% of the starting quantity, with roughly 10.6 more days to hit the target threshold.
Carbon-dating scale
Long half-life over multiple millennia
Long-lived isotopes decay much more slowly, so even two half-lives still leave one quarter of the original sample.
Result: Two half-lives leave 25% remaining, and reaching 10% takes much longer than the first 11,460 years.
Short tracer check
Rapid six-hour half-life
Short half-lives collapse quickly, so time-to-threshold can arrive much sooner than intuition suggests.
Result: Eighteen hours is three half-lives, leaving only 6.25 units from the original 50.