One becquerel is 2.7027 × 10⁻¹¹ curie, because a curie is 3.7 × 10¹⁰ becquerel. The becquerel measures how fast a source is decaying — it says nothing at all about dose, which is the third leg of the most consequential unit confusion in physics.
About the units
The becquerel is the SI unit of radioactivity: one nuclear decay per second. It is named after Henri Becquerel, who discovered radioactivity in 1896. It is an extremely small unit — a banana contains about 15 Bq of naturally occurring potassium-40, and a human body around 5,000 Bq. The curie is the older unit, defined as 3.7 × 10¹⁰ decays per second, which was the measured activity of one gram of radium-226. That figure was fixed by convention in 1953 after better measurements gave a slightly different value for radium, so the curie is now a defined number rather than a physical property.
The exact factor
The conversion is 1 Ci = 3.7 × 10¹⁰ Bq exactly, so 1 Bq = 2.7027027 × 10⁻¹¹ Ci. The prefixed forms are what appear in practice: 1 MBq is 27 µCi, and 1 mCi is 37 MBq. What the becquerel does not tell you is any dose: activity says how many decays occur per second, while the resulting dose depends on the energy and type of radiation emitted, whether the source is inside or outside the body, and which tissue it reaches. A becquerel of a short-lived gamma emitter and a becquerel of a long-lived alpha emitter present entirely different hazards.
Where you meet this conversion
Environmental monitoring, food safety and source inventories. Contamination limits for foodstuffs are set in becquerel per kilogram, and there are two separate EU regimes worth keeping apart. Import control of food from the Chernobyl-affected third countries runs at 600 Bq/kg of caesium-137, and 370 Bq/kg for milk and infant food, under Regulation (EU) 2020/1158. The general levels that would apply after a future nuclear accident are set higher in Regulation (Euratom) 2016/52: 400 Bq/kg for infant food, 1,000 for dairy and liquids, and 1,250 for other foodstuffs. The readings from Swedish reindeer and mushrooms after Chernobyl were quoted in the same unit for decades afterwards. Radon in indoor air is measured in becquerel per cubic metre, with 200 Bq/m³ as the Swedish reference level. Sealed industrial and medical sources are labelled in becquerel or curie depending on their age and origin.