Introduction
Radiation dose measures the energy absorbed by tissue, in grays (Gy) or rads, and the biological effect, in sieverts (Sv) or rem. Background radiation is ~3 mSv/year; a chest X-ray is 0.02 mSv; a head CT is 2 mSv; a lethal dose (50% 30-day mortality) is ~4,000 mSv. The factor-of-1000 swings between casual exposure and medical intervention are where unit errors become dangerous.
Why radiation dose units exist and how they diverged
Gray (Gy) replaced the older rad (1 Gy = 100 rad) as the SI absorbed-dose unit. Sievert (Sv) replaced the rem (1 Sv = 100 rem) for equivalent dose, which weights absorbed dose by radiation type (alpha > neutron > gamma ≈ beta = 1). Alpha particles deposit energy densely along a short track — high damage per gray — so the weighting factor is 20. Gamma and X-rays deposit energy sparsely; weighting factor 1, so 1 Gy gamma = 1 Sv.
For medical imaging, the effective dose (Sv) accounts for organ sensitivity. A CT of the abdomen is more carcinogenic per Gy than a head CT because the abdomen contains more radiation-sensitive organs.
How to convert radiation dose
Gray to rad: multiply by 100. Sievert to rem: multiply by 100. mSv to Sv: divide by 1,000. For gamma and X-ray radiation (most medical contexts), 1 Gy absorbed = 1 Sv effective. For alpha particles (inhaled radon, alpha emitters in the body), weighting factor 20: 1 Gy = 20 Sv.
Units supported by this radiation dose calculator
- Sievert (Sv)
- Millisievert (mSv)
- Microsievert (µSv)
- Rem
- Millirem (mrem)
- Gray (Gy)
- Milligray (mGy)
- Rad
- Millirad
Common radiation dose conversion mistakes
- Gy vs Sv. Gy is energy deposited per kg of tissue. Sv adjusts for biological effectiveness. For gamma they're numerically equal; for alpha, Sv is 20× higher.
- mSv vs µSv. Millisievert (mSv) is 1/1000 Sv. Microsievert (µSv) is 1/1,000,000 Sv. A mix-up between these is a 1000× error. Dental X-ray: 5 µSv = 0.005 mSv. Full-body CT: 10 mSv = 10,000 µSv. Know which prefix you're reading.
- Rad vs rem. Like gray vs sievert — rad is absorbed, rem is weighted. Older US units, largely replaced by SI but still in some reference material.
- Acute vs chronic exposure. 1 Sv in seconds is acute radiation sickness. 1 Sv spread over a lifetime is slightly raised cancer risk. Same total energy, very different effect.
- Activity (Becquerel, Curie) vs dose. Activity is decays per second; dose is energy absorbed. A source's Becquerel reading doesn't directly tell you the dose — that depends on distance, shielding, and exposure time.
Real-world radiation dose examples
- Background radiation (US avg): 3.1 mSv/year.
- Dental X-ray: 0.005 mSv (5 µSv).
- Chest X-ray: 0.02 mSv.
- Mammogram: 0.4 mSv.
- Head CT: 2 mSv.
- Abdominal CT: 10 mSv.
- Coast-to-coast US flight: 0.035 mSv (cosmic radiation).
- Airline crew, annual occupational: ~3 mSv.
- Annual occupational limit (US radiation workers): 50 mSv.
- CT of the heart (coronary angiography): 16 mSv.
- Acute radiation sickness threshold: ~1,000 mSv = 1 Sv.
- 50% 30-day mortality: ~4,000 mSv = 4 Sv (without medical care).
- Certain death within weeks: ~10,000 mSv = 10 Sv.
Tips for accurate radiation dose conversion
- For medical imaging, the risk-benefit calculation favors imaging when clinically indicated; avoid elective repeat scans.
- Background radiation varies by location — Denver is ~4.5 mSv/year (altitude); sea level is ~3 mSv/year.
- Radon is the biggest source of indoor radiation (1-2 mSv/year from homes with average levels; much more with poor ventilation).
Related: Energy Converter · Frequency Converter · SI Prefix Converter.