Half-Life / Radioactive Decay Calculator
Estimate remaining quantity after elapsed time, percent decayed, and the time needed to reach a target safe level.
Last updated: 2026-03-26
Half-life decay calculator
Enter your values
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.
Calculation History(0)
Example calculations
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.
How the half-life decay estimate works
This calculator treats decay as an exponential process. It divides elapsed time by the half-life to find how many half-lives have passed, then multiplies the starting quantity by one-half raised to that count.
Time to a target safe level is solved by rearranging the same decay equation. That makes it easy to answer both common questions with the same set of inputs: how much is left now, and how much longer until it reaches a threshold.
Half-life calculator FAQs
Use the same quantity unit throughout, and remember the model is about exponential decay rather than linear decline.
How does the half-life formula work?
The calculator uses exponential decay: remaining quantity equals the starting quantity multiplied by one-half raised to elapsed time divided by half-life. Each full half-life cuts the remaining amount in half again.
Why is the safe-level time not linear?
Decay follows a curve, not a straight line. Moving from 100 to 50 takes one half-life, from 50 to 25 takes another full half-life, and so on, so the same percentage drop always takes the same amount of time.
Can I use grams, curies, or any other quantity unit?
Yes. The calculator is unit-agnostic for quantity. Just keep the starting quantity and safe quantity in the same unit so the comparison stays valid.
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