The Radioactivity Activity Calculator computes decay activity in Becquerels (Bq) and Curies (Ci) from atom count, isotope mass, or half-life. Used in nuclear medicine, radiation protection, radiochemistry, and health physics for dose planning and source characterization.
0.0000320901
s^-1
1.9325e+16
Bq
195,198,855,281,159.75
Bq
5,275.64473733
Ci
1.9520e+17
Bq/g
0.0000320901
s^-1
1.9325e+16
Bq
195,198,855,281,159.75
Bq
5,275.64473733
Ci
1.9520e+17
Bq/g
The calculator for radioactive activity determines the decay rate of a radioactive sample — the number of nuclear disintegrations per second — from the number of radioactive atoms, the isotope's half-life, or the sample mass. Activity is the fundamental quantity in radiation protection, nuclear medicine dose planning, and radiochemical source characterization.
Radioactive activity is directly proportional to the number of unstable nuclei present:
A = λ × N = (ln 2 / t₁/₂) × N
where A is activity (Bq = disintegrations per second), λ is the decay constant (s⁻¹), N is the number of radioactive atoms, and t₁/₂ is the half-life in seconds. When mass is known rather than atom count, N is derived from the molar mass M and Avogadro's number N_A: N = (m × N_A) / M. Combining these: A = (m × N_A × ln 2) / (M × t₁/₂) — the specific activity formula that gives activity per unit mass for any isotope. The half-life calculator computes the time-dependent decay of activity.
Two units are used to express radioactive activity, and conversion between them is essential for interpreting sources from different eras and countries:
1 mCi = 37 MBq and 1 μCi = 37 kBq are the most frequently needed conversions in clinical nuclear medicine. A typical diagnostic Tc-99m scan uses 370–740 MBq (10–20 mCi). The radioactive decay calculator models how activity decreases over time following exponential decay.
Specific activity (SA) describes how radioactive a substance is per unit mass — a property that depends only on the isotope, not the sample size:
SA = (N_A × ln 2) / (M × t₁/₂) (Bq/g)
Short-lived isotopes have enormously high specific activities. Po-210 (t₁/₂ = 138 days) has SA = 1.66 × 10¹⁴ Bq/g; C-14 (t₁/₂ = 5,730 years) has SA = 1.65 × 10¹¹ Bq/g; U-238 (t₁/₂ = 4.47 × 10⁹ years) has SA of only 1.24 × 10⁴ Bq/g. High specific activity isotopes (I-131, F-18, Lu-177) are essential in nuclear medicine because therapeutic or imaging doses can be achieved with microgram quantities of material. The nuclear binding energy calculator and nuclear physics calculators category provide complementary nuclear science tools.
Activity quantification underpins radiation safety and nuclear medicine practice. In diagnostic imaging, administered activities are calibrated to maximize image quality while minimizing patient dose — typically 74–370 MBq for most SPECT procedures and 185–555 MBq for PET F-18 scans. Radiation therapy with Lu-177 DOTATATE uses 7.4 GBq per cycle. Regulatory exemption thresholds specify maximum activity (in Bq or Ci) below which isotopes can be handled without radioactive material licensing. Use this online calculator to convert between mass, atom count, and activity for any isotope and half-life combination.
Decay constant: lambda = ln(2)/t_1/2. Activity from N atoms: A = lambda * N = N*ln(2)/t_1/2 (Bq). Activity from mass: N = (mass_g/A_mass)*NA, then A = lambda*N. Specific activity = lambda*NA/A_mass Bq/g. To convert to Ci: divide by 3.7e10.
1 Bq = 1 decay per second (very low). 1 kBq = 1000/s (lab trace amounts). 1 MBq = 10^6/s (small diagnostic dose). 1 GBq = 10^9/s (standard nuclear medicine dose). 1 TBq = 10^12/s (industrial gamma source). 37 GBq = 1 Ci (1 g Ra-226). U-238 has such low activity that 1 ton contains only 12.4 MBq.
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Results
Tc-99m with 6-hour half-life has a specific activity of 1.93e17 Bq/g. A standard 740 MBq (20 mCi) dose for a bone scan contains only ~38 picograms of Tc-99m — utterly undetectable chemically but very active radioactively.
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Results
In modern carbon, about 1 in 10^12 atoms is C-14 (the rest is C-12). In 1 gram of carbon, ~6.5e10 C-14 atoms give ~249 Bq. C-14 specific activity is 1.65e11 Bq/g for pure C-14.
The spontaneous emission of radiation from unstable nuclei. Activity A = dN/dt = lambda*N is the number of disintegrations per second. It is not a property of the radiation emitted but of the amount and type of radioactive material.
1 Bq = 1 radioactive disintegration per second. Named after Henri Becquerel who discovered radioactivity in 1896. It is the SI unit of radioactivity. Note: 1 Bq represents an extremely small amount of radioactivity — the human body contains about 8,000 Bq of K-40.
1 Ci = 3.7 x 10^10 Bq = 37 GBq. Originally defined as the activity of 1 g of Ra-226 (determined by Marie and Pierre Curie). Still widely used in medical, industrial, and regulatory contexts in the United States.
SA = lambda * NA / M (Bq/g), where M is molar mass. Short half-life means high specific activity — the same mass decays much faster. SA is used to specify source strength per unit mass of radioactive material.
A(t) = A0 * e^(-lambda*t) = A0 * (1/2)^(t/t_1/2). After one half-life, activity halves. After 10 half-lives, activity is less than 0.1% of initial. This is why short-lived medical radioisotopes become safe quickly.
About 8,000 Bq from K-40 (radioactive isotope, 0.012% of all potassium), 3,700 Bq from C-14, and smaller amounts from Rb-87, Ra-226, and thorium-series products. Total ~14,000 Bq per 70 kg person.
Tc-99m (most common): 185-740 MBq (5-20 mCi) for imaging studies. I-131 therapy: 1-10 GBq (30-300 mCi) for thyroid cancer. F-18 for PET scans: 185-370 MBq (5-10 mCi). I-123 for thyroid: 40-200 MBq.
Geiger-Muller (GM) counters for high-dose rate measurement. Scintillation counters (NaI, plastic) for spectroscopy. Ionization chambers for absolute dose measurement. Liquid scintillation counters for low-energy beta emitters (H-3, C-14). HPGE detectors for precise gamma spectroscopy.
In a decay chain at secular equilibrium, all daughter activities equal the parent activity: A_parent = A_daughter. This means decay chain products can contribute significantly to total activity even if present in small amounts.
Regulatory exemption limits vary by country and isotope. In the US (NRC), exempt quantities are typically ~37 kBq (1 microcurie) for common isotopes. Below this activity, materials can be handled without special radiation license. IAEA provides international guidance.
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