Three units, two different things being measured
Radiation dose gets confusing fast because the units measure two related but distinct things. The gray (Gy) measures the raw physical energy that radiation deposits in tissue — pure physics, no biology involved. The sievert (Sv) takes that energy and weights it by how biologically damaging that particular type of radiation is, since alpha particles do far more harm per unit of energy than X-rays or gamma rays. For the X-rays and gamma rays most people actually encounter, a gray and a sievert are numerically the same, which is part of why the two get conflated.
The rem is simply an older unit for the same thing the sievert measures — biological dose. It's still used in the United States and a few other countries, largely for historical reasons.
Convert and compare a real dose
Enter any dose and see it converted across units, plus how it compares to flights and X-rays.
How the units convert
Because a full sievert is a large dose, almost every everyday figure you'll see is quoted in millisieverts or microsieverts. Medical and regulatory documents in the US often use rem or millirem instead — divide by 100 to convert rem to sieverts.
Reference doses that put a number in context
| Exposure | Approx. dose |
|---|---|
| Eating a banana | ~0.0001 µSv |
| Dental X-ray | ~5 µSv |
| Transatlantic flight | ~40 µSv |
| Chest X-ray | ~100 µSv |
| Natural background (annual) | ~2.4 mSv |
| CT scan (abdomen) | ~8 mSv |
| Annual limit, radiation worker | ~20 mSv |
| Onset of radiation sickness | ~1 Sv |
| Often fatal without treatment | ~5 Sv |
The scale spans roughly ten orders of magnitude between eating a banana and a dose that causes acute radiation sickness — which is exactly why context matters more than the raw number. A dose reported in microsieverts is essentially always trivial; one reported in whole sieverts is a medical emergency.
Where these doses come from
The isotopes behind both medical doses and background radiation decay according to their half-life — see the half-life guide for how that works — and the materials themselves trace back to the nuclear fuel cycle covered in the uranium enrichment guide.
Frequently asked questions
Is a sievert the same as a rem?
They measure the same thing on different scales — 1 sievert equals 100 rem. The sievert is the modern SI unit; rem is the older unit still used in some countries.
What's the difference between a sievert and a gray?
A gray measures raw absorbed energy; a sievert weights that by biological harm. For X-rays and gamma rays they're numerically equal; for alpha particles, sieverts are much higher than grays.
How big is a typical radiation dose?
Background radiation is a few millisieverts a year; a chest X-ray is a fraction of a millisievert; a long flight is a few microsieverts — all far below the roughly 1 sievert dose that causes radiation sickness.