Understanding radiation dose
Radiation dose is measured in sieverts (Sv), a unit of biological harm. Because a full sievert is a large dose, everyday exposures are usually quoted in millisieverts (mSv, a thousandth) or microsieverts (µSv, a millionth). The older unit, the rem, still appears in some countries — one sievert equals 100 rem.
It helps to separate two ideas. The gray (Gy) measures the raw energy absorbed by tissue. The sievert takes that and weights it by how damaging the radiation type is — so a gray of alpha radiation counts for far more sieverts than a gray of gamma. For the X-rays and gamma rays most people encounter, the two are numerically the same.
Putting a dose in perspective
Context is everything with radiation. Natural background radiation gives most people a few millisieverts a year just from the ground, food and cosmic rays. A chest X-ray is a fraction of a millisievert; a long-haul flight, a few microsieverts. These are thousands of times below the levels that cause harm. The calculator above flags where your figure sits on that scale.
This is the same decay physics behind the radioactive decay calculator — the isotopes that emit these doses follow the half-life curves shown there — and the materials originate in the fuel cycle explored by the fuel equivalence calculator.
Frequently asked questions
What's the difference between a sievert and a gray?
A gray measures absorbed energy; a sievert measures biological harm, weighting the gray by radiation type. For X-rays and gamma they're equal; for alpha, the sievert value is much higher.
How much radiation is dangerous?
Background is ~2–3 mSv/year. Around 1 Sv at once can cause radiation sickness and ~5 Sv is often fatal untreated. Everyday exposures are thousands of times smaller.
How many millisieverts in a sievert?
1 Sv = 1,000 mSv = 1,000,000 µSv = 100 rem.