Enter your isotope's half-life, initial activity, and the elapsed time to calculate the remaining activity after radioactive decay. Choose your preferred activity units (µCi, mCi, MBq, etc.) and time units — the calculator returns the remaining activity, decay factor, and number of half-lives elapsed so you can track isotope decay precisely.
Remaining Activity
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Half-Lives Elapsed
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Decay Factor
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Percent Remaining
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Percent Decayed
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Radioactive decay is the spontaneous process by which an unstable atomic nucleus loses energy by emitting radiation. Over time, the number of radioactive atoms — and therefore the activity — decreases at a predictable exponential rate determined by the isotope's half-life.
A half-life is the time required for exactly half of the radioactive atoms in a sample to decay. After one half-life, 50% of the original activity remains; after two half-lives, 25% remains; and so on. Each isotope has a unique, constant half-life ranging from fractions of a second to billions of years.
Remaining activity is calculated using the formula: A = A₀ × (0.5)^(t / t½), where A₀ is the initial activity, t is the elapsed time, and t½ is the half-life. This is an exponential decay equation. The decay factor (0.5)^(t/t½) tells you what fraction of the original activity remains.
This calculator supports a wide range of activity units including cpm (counts per minute), dpm (disintegrations per minute), pCi, nCi, µCi, mCi, Ci, kBq, MBq, and GBq. The activity unit is applied consistently to both the input and output — no unit conversion between input and output is performed.
Yes. This calculator automatically converts both the half-life and the elapsed time to a common unit (seconds) before performing the decay calculation, so you can freely choose different units for each — for example, a half-life in years and an elapsed time in days.
The decay factor is the fraction by which the initial activity is multiplied to get the remaining activity. It equals (0.5)^(number of half-lives elapsed). A decay factor of 0.25, for example, means 25% of the original activity remains after two half-lives.
After 7 half-lives, less than 1% of the original activity remains (~0.78%). After 10 half-lives, less than 0.1% remains. In practice, most radiation safety guidelines consider a material effectively decayed after 10 half-lives, though this varies by isotope and regulatory context.
This calculator works for any isotope when you know its half-life. Commonly used isotopes in medical and research settings include P-32 (14.28 days), I-131 (8.02 days), Tc-99m (6.01 hours), Co-60 (5.27 years), and H-3 (12.32 years). Simply enter the correct half-life and time elapsed for your specific isotope.