Becquerel Curie Radiation Unit Converter

1 Ci is exactly 3.7×10¹⁰ Bq, and 1 mCi is 37 MBq

The curie (Ci) is defined as 3.7×10¹⁰ Bq, or 37 billion becquerels (37 GBq). This is a defined value, not a rounded one, so 1 Ci and 37 GBq are exactly equal. NIST SP 811 gives the factor as 3.7 E+10 in its table of conversion factors, and the U.S. NRC glossary describes 1 Ci as 37 billion disintegrations per second, as in 1 gram of radium, for example. The becquerel (Bq) is the SI unit of activity of a radionuclide: 1 Bq means that, on average, 1 atomic nucleus decays per second. The 12th CGPM in 1964 accepted the continued use of the curie as a non-SI unit, and the 15th CGPM in 1975 adopted the special names becquerel and gray.

Both unit families change by factors of 1,000 per prefix, and 1 Ci is 37 GBq. So any pair in which the curie prefix is 10⁹ smaller than the becquerel prefix always differs by a factor of 37: 1 mCi = 37 MBq, 1 μCi = 37 kBq, 1 nCi = 37 Bq. The converter above puts each of these pairs on the same row.

Where becquerels and curies meet

The becquerel is the SI unit, but plenty of equipment, international guidelines and older records are still written in curies. The same quantity therefore shows up as different numbers in different documents, and you have to put the units on the same footing before you can compare them. These are the common cases.

• Radon testing: U.S. test kits and reports give radon in pCi/L, while other countries and the WHO use Bq/m³. A result of 4 pCi/L is 148 Bq/m³ (see "Concentration units" below).
• PET scans and radiopharmaceuticals: the ACR–ACNM–SNMMI–SPR practice parameter for FDG-PET/CT states the adult dose as 185–740 MBq (5–20 mCi), giving both units. Scanner screens, labels and papers mix MBq and mCi, so you multiply or divide by 37 to match them.
• Radioactive iodine for thyroid cancer: the 2015 American Thyroid Association guidelines state doses in mCi, about 30 mCi in recommendation 55A and up to 150 mCi in recommendation 56. To compare them with documents written in GBq, read them as 1.11 GBq and 5.55 GBq.
• Sealed sources and equipment labels: labels on industrial and calibration sources and on smoke detectors still carry Ci, mCi and μCi. U.S. federal regulations (10 CFR 30.15) limit the americium-241 in an ionization smoke detector sold to the public to 1 μCi (37 kBq).
• Accident and environmental reports: releases from nuclear accidents are reported in PBq (10¹⁵ Bq), and older documents use MCi (megacurie). The roughly 85 PBq of cesium-137 released at Chernobyl is about 2.3 MCi.
• Laboratory counting: instruments such as liquid scintillation counters report disintegrations per minute (dpm). Since 1 Bq = 60 dpm, 1 pCi = 2.22 dpm and 1 μCi = 2,220,000 dpm. The "times per minute" figure in the converter's result band is exactly dpm.

How to use the converter

1. Type your number into the field for its unit. The left column holds the becquerel family and the right column holds the curie family. The field you typed in gets a darker border, and the other thirteen fields are recalculated from it.
2. The result band at the top shows your value in the unit of the opposite family that usually reads as 1 or more and less than 1,000 (values of 1,000 MCi or more, or below 1 pCi, stay in the end unit). Enter 10 mCi and it shows 370 MBq, and that unit label is highlighted in its field.
3. The sentence under the result band gives the average number of decays per second and per minute, and compares your value with the nearest reference point.
4. The size ladder in the middle draws 10⁻³ Bq to 10¹⁸ Bq on a logarithmic scale. Each step is a factor of 1,000, and a marker sits at the height of your value. The "1 Ci" tick on the right sits about half a step above "10⁹" on the left, and that offset is the factor of 37. Click a reference name to enter its value.
5. Choose 4, 6 or 8 under "Significant digits". Rounded fields carry a "≈". Values below 0.0001 or at 1 billion and above appear in exponent form, such as 3.7×10¹⁰, and you can type them as 3.7e10, 3.7×10^10 or 3.7×10¹⁰.
6. The "Concentration in air or water" row converts Bq/m³, pCi/L and Bq/L separately. Its bar compares an air concentration with the WHO 100, US EPA 148 and EU 300 Bq/m³ lines.
7. "Copy result" copies every unit value on one line. "Copy link" copies an address that carries the current value, unit and digit setting.

The math: all you need is 37 and 1,000

To convert becquerels to curies, divide by 3.7×10¹⁰. To convert curies to becquerels, multiply by 3.7×10¹⁰. By hand, it is quicker to match prefixes in pairs. If the two units share a row (MBq and mCi, say), divide or multiply by 37. If they do not, first move in steps of 1,000 until they share a row, then use 37.

Dividing by 37 often leaves a "≈" because 1 ÷ 37 = 0.027027… repeats the digits 027 forever. The converter computes the exact fraction first, rounds only once at the significant digits you choose, and shows "=" only when the displayed value equals the exact value.

Reference points on the size ladder

Activities span many orders of magnitude, so the numbers alone are hard to size up. These are the reference points on the ladder and where each one comes from. Values given as a range are drawn as a short bar on the ladder.

The ladder compares sizes, not hazards. Two sources with the same activity can give people very different doses, depending on the radionuclide, the type and energy of the radiation, and whether the source is outside the body or has been taken in.

Concentration units: per volume for air and water, per mass for food and soil

When you describe the activity in a given amount of air or water, as with radon or radioactive substances in water, you use a concentration unit such as Bq/m³, Bq/L or pCi/L. Since 1 pCi is 0.037 Bq and 1 L is 0.001 m³, 1 pCi/L = 0.037 ÷ 0.001 = 37 Bq/m³. The 37 appears again because the pico (10⁻¹²) and the liter (10⁻³ m³) line up with the 10⁹ in 1 Ci = 37×10⁹ Bq.

The EPA action level of 4 pCi/L is exactly 148 Bq/m³ once converted. If a detector shows pCi/L, multiply by 37 to read it in Bq/m³, and divide by 37 to go the other way. Water is often given in Bq/L, and 1 Bq/L = 1,000 Bq/m³ ≈ 27.027 pCi/L. The units convert, but radon in water and radon in air reach the body by different routes, so water is not judged on the same scale as the air levels. The converter likewise leaves the reference bar empty when you enter a Bq/L value.

Food, soil and building materials are usually given in Bq/kg, and U.S. soil data often use pCi/g. The fields above do not convert per-mass concentrations, so use the 37 and 1,000 in this table. A per-volume value and a per-mass value cannot be converted into each other unless you know the density.

Common mistakes

• MBq versus mBq: a capital M is mega (10⁶) and a lowercase m is milli (10⁻³), a factor of a billion apart. In the converter, MBq is the fourth row and mBq is the bottom row.
• mCi versus μCi: milli and micro differ by a factor of 1,000. Depending on the font or handwriting, μ can look like u or m, so check the original.
• Activity versus dose: Bq and Ci count decays per second. The energy absorbed by the body is measured in Gy (gray), and the dose weighted for biological effect in Sv (sievert). For the same Bq, Sv depends on the radionuclide, the type and energy of the radiation, and the route and time of exposure, so unit conversion alone cannot turn one into the other. The same NIST table also lists the rad (1 rad = 0.01 Gy) and the rem (1 rem = 0.01 Sv), both dose units.
• Label values and time: activity falls over time according to the half-life. The label value of a radiopharmaceutical or a source applies at the reference time printed on the label, so you must correct for decay over the elapsed time to get today's value.
• Dropping the denominator: if "radon 148" omits the "/m³", you cannot tell whether it means Bq/m³ or Bq/L, and those differ by a factor of 1,000.
• Hz versus Bq: both have dimension 1/s, but note (d) to Table 4 of the SI Brochure says the hertz is only for periodic phenomena and the becquerel only for stochastic processes in activity referred to a radionuclide.

Limits and cautions

• This converter changes units only. It does not judge radiation dose (Sv), health risk or safety.
• It does not calculate decay over time from the half-life, nor the specific activity of a radionuclide (Bq per gram).
• The ladder reference points are representative values to help you gauge size. Actual doses and releases are ranges that depend on the guideline or assessment.
• The radon bar only compares a number against annual-average levels. Whether a measurement exceeds a level is decided by an official procedure that fixes the measurement period and method.
• Values up to 10²⁴ Bq (10²⁴ Bq/m³ for concentrations) are accepted, with exponents from −30 to 30. The ladder draws 10⁻³ to 10¹⁸ Bq, and values outside that range are marked with an arrow at the end.

Basis: 1 Ci = 3.7×10¹⁰ Bq is a defined value and does not change. The sources below were checked on October 20, 2026.

Sources

• 01NIST SP 811 — Appendix B.9 Factors for units listed by kind of quantityCurie (Ci) → becquerel (Bq) 3.7 E+10; rad → Gy 1.0 E-02, rem → Sv 1.0 E-02 in the same table
• 02BIPM — The International System of Units (SI Brochure, 9th edition, 2019)Table 4 note (d): hertz for periodic phenomena, becquerel for stochastic processes in activity referred to a radionuclide. Appendix 1: 12th CGPM (1964) accepted the curie as a non-SI unit, 15th CGPM (1975) adopted the names becquerel and gray
• 03U.S. NRC — Glossary: Curie (Ci)1 Ci = 37 billion (3.7×10¹⁰) disintegrations per second, as in 1 gram of radium
• 04Health Physics Society — Radioactivity in the bodyAdult of 70 kg: potassium-40 about 4.2 kBq, carbon-14 about 3.08 kBq
• 05eCFR — 10 CFR 30.15 Certain items containing byproduct material(a)(7) ionization smoke detectors: americium-241 up to 1 μCi
• 06ACR–ACNM–SNMMI–SPR Practice Parameter — FDG-PET/CTAdult FDG dose 185–740 MBq (5–20 mCi)
• 072015 American Thyroid Association Management Guidelines (Thyroid 2016)Recommendation 55A about 30 mCi, recommendation 56 up to 150 mCi
• 08IAEA Safety Guide RS-G-1.9 — Categorization of Radioactive SourcesTable 2: teletherapy cobalt-60, typical 4.0×10³ Ci (1.5×10² TBq)
• 09UNSCEAR 2020/2021 Report, Vol. II (Fukushima)Cesium-137 released to the atmosphere: 6–20 PBq
• 10UNSCEAR 2008 Report, Annex D (Chernobyl)Cesium-137 released: about 85 PBq
• 11UNSCEAR 2000 Report, Annex B (Natural radiation sources)Indoor radon concentration, arithmetic mean 40 Bq/m³
• 12WHO — Radon and health (fact sheet)Annual-average reference level for homes 100 Bq/m³, or 300 Bq/m³ or less if that is not feasible
• 13U.S. EPA — Health Risk of RadonFix the home at 4 pCi/L (150 Bq/m³) or more
• 14EUR-Lex — Council Directive 2013/59/EuratomArticle 74: national reference level for indoor radon, annual average, 300 Bq/m³ or less

Frequently asked questions

Related tools