The radioactive decay calculator computes the remaining activity in Becquerels and Curies after a given time, plus the percentage remaining and number of half-lives elapsed. Uses A(t) = A₀ × e^(−λt).
Radioactive Decay Activity Calculator
How to Use the Radioactive Decay Calculator
This calculator computes the remaining radioactive activity after a given time using A(t) = A₀ × e^(−λt), where λ = ln(2) / t½ is the decay constant.
Medical Isotope Example
Tc-99m (technetium-99m) is used in nuclear medicine scans. Half-life = 6 hours. If you receive 400 MBq at 8 AM, how much remains at 8 PM (12 hours = 2 half-lives)? A = 400 × (0.5)² = 100 MBq. After 24 hours (4 half-lives): A = 400 × 0.0625 = 25 MBq.
Carbon Dating
C-14 has a half-life of 5730 years. A living organism has an initial C-14 activity proportional to atmospheric concentration. After death, C-14 decays. If a sample has 25% of original activity remaining: 25% = (½)^n → n = 2 half-lives → age = 2 × 5730 = 11,460 years.
Units: Bq vs Ci
1 Bq = 1 disintegration/second. 1 Ci = 3.7 × 10¹⁰ Bq. Medical doses are typically in MBq (megabecquerel) or mCi (millicurie). Environmental measurements use Bq/m³ or Bq/kg. The Ci is an older unit still widely used in the US nuclear industry.
Frequently Asked Questions
What is radioactive activity?
Activity is the rate of radioactive decay — the number of disintegrations per second. It's measured in Becquerels (Bq), where 1 Bq = 1 disintegration/second, or Curies (Ci), where 1 Ci = 3.7 × 10¹⁰ Bq (activity of 1 gram of Ra-226). Activity A = λ × N, where λ is the decay constant and N is the number of radioactive atoms.
How do Becquerels and Curies relate?
1 Curie (Ci) = 3.7 × 10¹⁰ Becquerels. 1 mCi (millicurie) = 37 × 10⁶ Bq = 37 MBq. Medical doses for thyroid scans use ~100-400 MBq (2.7-10.8 mCi). The Curie is named after Marie and Pierre Curie; the Becquerel is the SI unit named after Henri Becquerel.
Is this calculator free?
Yes, completely free with no signup required. All calculations run in your browser.
Is my data private?
Yes. All calculations run locally. Nothing is transmitted.
What is the decay constant λ?
The decay constant λ = ln(2) / t½ = 0.693 / t½. It represents the probability per unit time that a nucleus will decay. A larger λ means faster decay and shorter half-life. If you know λ, you can find activity A = λN and half-life t½ = 0.693/λ.