Unit converter
Activity, dose, dose equivalent, exposure and surface contamination — converted within each quantity, never across.
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Why conversions go wrong
Most conversion mistakes in radiation protection are not arithmetic. They happen when two numbers that look interchangeable describe different physical quantities. A becquerel counts nuclear transformations per second. A gray counts joules deposited per kilogram. A sievert is a gray multiplied by a weighting factor chosen for biological effect. A roentgen counts charge liberated per kilogram of dry air. Converting inside any one of those is a fixed factor. Converting between them is physics, and it needs information the number itself does not carry.
This converter therefore refuses to cross between quantities. Pick what you have, and every unit of that quantity appears at once. The one bridge it does offer — exposure to air kerma — is labelled as a physical step rather than a conversion, because it multiplies by W/e, the mean energy expended per ion pair formed in dry air, 33.97 joules per coulomb.
The factors, and which are exact
The curie is defined as exactly 3.7 × 10¹⁰ becquerel. The rad is exactly 0.01 gray and the rem exactly 0.01 sievert. The roentgen is exactly 2.58 × 10⁻⁴ coulomb per kilogram. None of these is rounded, so a conversion between them loses nothing. The disintegration per minute is exactly one sixtieth of a becquerel, which is why 6000 dpm per 100 cm² is exactly 1 Bq/cm² — a coincidence worth memorising, because those two units appear on the same survey report more often than any other pair.
Surface contamination
Surface units carry a hidden area. dpm/100 cm² is quoted per probe window, Bq/cm² per single square centimetre, and Bq/m² per square metre. A factor of 100 between the first two and 10⁴ between the last two is the single most common error on a decommissioning record. If a number looks two orders of magnitude out of place, this is usually why.
Gray and sievert
For photons and electrons the radiation weighting factor is 1, so a dose in gray and a dose equivalent in sievert are numerically equal. That equality tempts people to treat the units as synonyms. They are not. For alpha particles the factor is 20, for neutrons it varies with energy from about 2 to 20, and applying the wrong one changes a compliance decision by more than an order of magnitude. A converter cannot know which particle you mean, so it does not guess.
What this tool does not do
It does not convert absorbed dose to effective dose, which additionally needs tissue weighting factors and an exposure geometry. It does not convert air kerma to a personal dose equivalent such as H*(10), which needs a conversion coefficient that depends on photon energy. Both of those are answered by published coefficient sets, not by a fixed factor.