Radiation Activity Calculator: Convert Bq and Ci

A radiation activity calculator converts between becquerels (Bq) and curies (Ci), the units used to describe how quickly a radioactive material decays. Enter activity in either unit and the calculator returns the equivalent value in the other; it does not calculate dose, exposure, or health risk.

Formula

The conversion is based on the definition of the curie:

• Activity in Ci = activity in Bq ÷ 37,000,000,000
• Activity in Bq = activity in Ci × 37,000,000,000

In the calculator, a is the entered activity in becquerels (Bq). The first result is activity in curies (Ci), rounded to three decimal places; the second repeats the entered Bq value as a whole number. One Bq is one nuclear transformation per second. One Ci is 3.7 × 10¹⁰ transformations per second, or 37 billion Bq.

Worked Example

Using the calculator’s default input, 37,000,000,000 Bq:

1. Divide by the number of becquerels in one curie: 37,000,000,000 ÷ 37,000,000,000.
2. The activity is 1 Ci. Rounded to three decimal places, the calculator displays 1.000 Ci.
3. The equivalent activity in becquerels is 37,000,000,000 Bq.

These are two expressions of the same decay rate, not two different measurements of radiation dose.

What Activity Measures

Radioactive atoms transform spontaneously. Activity describes the rate at which those transformations happen in a sample. The SI unit, the becquerel, is deliberately small: a sample with an activity of 1 Bq undergoes an average of one transformation per second. A curie is a much larger unit, historically tied to the activity of a quantity of radium-226, and is still encountered in some technical and older documents.

Activity is not the same as the amount of radioactive material by mass. Equal masses of different radionuclides can have very different activities because their atoms decay at different rates. Even for one radionuclide, activity changes over time as unstable atoms decay. To infer mass from activity, you need information such as the radionuclide’s identity and its decay characteristics.

Nor does activity alone tell you how much radiation reaches a person or object. The radiation type and energy, distance, shielding, exposure duration, and whether radioactive material enters the body all matter to dose. Dose is commonly expressed in gray (Gy) for absorbed energy per kilogram and sievert (Sv) for a radiation-weighted measure of biological effect. Neither can be found by simply converting Bq to Ci.

How to Use the Calculator

Enter a nonnegative activity in Bq. The calculator divides by 37 billion and reports the corresponding curies, while also showing the Bq input for clarity. For the reverse conversion, multiply a Ci value by 37 billion; for example, 0.25 Ci equals 9,250,000,000 Bq.

• Check the unit printed beside the number in your source. A value in kBq, MBq, or GBq must be converted to Bq before entering it: 1 kBq = 1,000 Bq, 1 MBq = 1,000,000 Bq, and 1 GBq = 1,000,000,000 Bq.
• Keep scientific notation straight. For example, 2.5 × 10⁶ Bq is 2,500,000 Bq, or 0.000067568 Ci.
• Retain sensible significant figures. A calculator can show extra decimal places, but those digits do not make an imprecise source measurement more accurate.

The relationship is the same in US customary and metric contexts because Bq and Ci are activity units, not length, mass, or volume units. No separate customary-unit conversion is needed.

Decay and Time: What the Conversion Does Not Do

Converting units does not account for radioactive decay. For a radionuclide with half-life T, activity after elapsed time t can be estimated by A(t) = A₀ × (1/2)^t/T, where A₀ is the starting activity and A(t) is the activity later. The time units for t and T must match. This calculation requires knowing which radionuclide is involved and its half-life; the Bq-to-Ci conversion alone cannot supply that information.

Real samples may contain several radionuclides, and measurements have uncertainty. In those cases, a single activity number may summarize a more complex mixture. Use the measurement date and the measurement’s stated uncertainty when comparing figures, and do not treat rounded calculator output as a new measurement.

Common Mistakes and Limits

• Confusing activity with dose: Bq and Ci count transformations per second. They do not directly state a person’s dose or risk.
• Missing a prefix: 1 MBq is one million Bq, not one thousand. Convert prefixes carefully before using the calculator.
• Mixing up the direction: Bq to Ci requires division by 37 billion; Ci to Bq requires multiplication.
• Ignoring time: An activity figure refers to a particular time. It may decrease as the radionuclide decays.
• Assuming the result identifies the material: Many radionuclides can have the same activity. The conversion cannot identify an isotope, sample mass, radiation energy, or shielding requirement.

This tool is a unit converter, not an instrument reading or a substitute for interpreting a laboratory report. If the source value is actually in counts per minute, counts per second, or dose units such as Gy or Sv, it cannot be converted to Bq or Ci without additional measurement and calibration information.

Frequently Asked Questions

How many becquerels are in one curie?
One curie is exactly 37,000,000,000 Bq, or 3.7 × 10¹⁰ Bq.

How do I convert Bq to Ci?
Divide the activity in Bq by 37 billion. For example, 74,000,000 Bq ÷ 37,000,000,000 = 0.002 Ci.

How do I convert Ci to Bq?
Multiply the activity in Ci by 37 billion. For example, 0.5 Ci equals 18,500,000,000 Bq.

Does a higher Bq value always mean a higher dose?
No. Activity is the rate of nuclear transformations in a source. Dose depends on factors including radiation type, energy, exposure conditions, distance, shielding, and how the material is encountered.

Can activity be converted directly to grams?
Not without knowing the radionuclide and its decay properties. Different isotopes have different activities per unit mass, so activity alone does not determine sample mass.

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in a | Activity | Bq | 37000000000 | 0
out ci | Activity | Ci | a / 37000000000 | 3
out bq | Activity | Bq | a | 0