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Radioactivity Activity Calculator

Reviewed by the CalculatorHub team, edited by James Graham. Activity formula from standard nuclear physics; constants from NIST CODATA 2018. Last verified 15 June 2026.

The radioactive activity A of a sample is the number of nuclear disintegrations per second, measured in becquerels (Bq). It is calculated as A = lambda * N, where the decay constant lambda = ln(2) / T (T is the half-life in seconds) and N is the current number of undecayed atoms. Alternatively, if you know the mass in grams and molar mass, N = (m / M) * NA where NA = 6.02214076 x 10^23 mol^-1. Activity decreases exponentially over time following A(t) = A0 * (1/2)^(t/T). Enter the half-life and number of atoms (or use the mass mode) to compute initial activity in both becquerels and curies.

C-14: 1.807e11 s. I-131: 694,656 s (8.02 days)
Enter atom count. 1 microgram I-131 ~ 4.6e15 atoms
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Activity formula

lambda = ln(2) / T
A = lambda * N

A is in becquerels (Bq = decays/s) when T is in seconds and N is the number of atoms. 1 curie (Ci) = 3.7 x 10^10 Bq (the activity of 1 g of radium-226).

Activity of common isotopes (per gram)

  • Iodine-131 (T = 8.02 days): about 4.6 x 10^15 Bq/g, or 124 GBq/g.
  • Carbon-14 (T = 5,730 years): about 1.66 x 10^11 Bq/g, or 4.5 Ci/g.
  • Uranium-238 (T = 4.47 billion years): about 1.24 x 10^4 Bq/g, very low activity.
  • Polonium-210 (T = 138.4 days): about 1.66 x 10^14 Bq/g, extremely toxic at small masses.

Radioactivity activity: frequently asked questions

What is radioactive activity?

Radioactive activity is the rate at which a radioactive material undergoes decay, measured in becquerels (Bq) where 1 Bq = 1 nuclear disintegration per second. Activity equals the decay constant lambda times the number of undecayed nuclei N: A = lambda * N = (ln 2 / T) * N.

What is a becquerel and how does it relate to the curie?

The becquerel (Bq) is the SI unit of radioactive activity: 1 Bq = 1 decay per second. The older curie (Ci) was defined as the activity of 1 gram of radium-226: 1 Ci = 3.7 x 10^10 Bq. Nuclear medicine often uses gigabecquerels (GBq) or millicuries (mCi).

How do I calculate activity from mass?

Activity A = (ln2 / T) * (m * NA / M), where T is the half-life in seconds, m is the mass in grams, NA is Avogadro's number (6.022 x 10^23 mol^-1), and M is the molar mass in g/mol. This calculator allows entry of atom count directly; use the mass entry option for gram-based calculations.

Why does short half-life mean high activity?

Activity is A = lambda * N = (ln2 / T) * N. A shorter half-life means a larger decay constant lambda, so more atoms decay per second for the same number of atoms present. This is why short-lived isotopes like iodine-131 (8 days) are intensely radioactive per gram, while uranium-238 (4.47 billion years) is barely active per gram.

How does activity decrease over time?

Activity follows the same exponential law as atom count: A(t) = A0 * (1/2)^(t/T) = A0 * exp(-lambda * t). After one half-life, activity drops to half; after two half-lives, to one quarter; and so on. This is why radioactive waste management requires storing short-lived isotopes for many half-lives before disposal.

Official sources

Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.