Radioactivity units — becquerel and curie
Bq is one decay per second. Ci is 3.7 × 10¹⁰ of them. Twelve orders of magnitude, one unit conversion.
Activity measures the rate of nuclear decays. The SI unit (becquerel) is trivially small — one decay per second — so real sources are always quoted in prefixed forms (kBq, MBq, GBq) or in the legacy curie.
The exact factors
1 Bq = 1 decay per second > 1 Ci = 3.7 × 10¹⁰ Bq = 37 GBq (defined by original radium standard) > 1 mCi = 37 MBq; 1 µCi = 37 kBq
Where each dominates
Bq: environmental measurements, food safety limits, radon in air. kBq/MBq: laboratory tracers. GBq: nuclear medicine doses, industrial radiography sources. Ci: US regulatory language, older US medical literature. TBq/kCi: reactor inventories.
Typical values
Banana ⁴⁰K activity: ~15 Bq. Radon action level (EPA): 148 Bq/m³ = 4 pCi/L. Bone scan dose: 800 MBq ⁹⁹ᵐTc. Cs-137 sealed source (calibration): 10–100 MBq.
Worked example — bone scan preparation
Prescription: 20 mCi ⁹⁹ᵐTc-MDP. Equivalent 740 MBq. The dose calibrator reads in mCi; the modern regulatory paperwork wants MBq. Miss the 37× factor and you either dispense 27 mCi (30× overdose in old units — but actually right in MBq) or underdose by the same factor. Real errors traced to this exact conversion have reached patients.
Recap
1 Ci = 37 GBq. Modern practice is Bq; US clinical still runs on mCi/µCi.
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FAQ
Is activity the same as dose?+
No — activity is decays per second; dose is energy per kg received by tissue. Both matter separately.
Why 3.7 × 10¹⁰?+
Historically defined as the activity of 1 g of radium-226. Later redefined as an exact number but retained the same value.
This article was written by The Turbo Unit Converter engineering desk (Applied metrology & engineering units) and last reviewed on 2026-07-07 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.