Question

What is the difference between the units rem and rad?
Rem is an SI unit, and rad is a conventional unit.
Rem is used to measure biological risk, and rad is used to measure absorbed radiation.

Answer 1

Short answer
- rad measures absorbed dose (energy deposited per mass). It is a non‑SI (conventional) unit. 1 rad = 0.01 Gy.
- rem measures dose equivalent (an absorbed dose adjusted for biological effect of the radiation). It is also a non‑SI unit. 1 rem = 0.01 Sv.

So rem is not an SI unit — both rad and rem are older, conventional units; their SI equivalents are the gray (Gy) for absorbed dose and the sievert (Sv) for dose equivalent / biological effect.

More detail
- Absorbed dose (rad, Gy): how much energy the tissue absorbs. 1 Gy = 1 J/kg = 100 rad, so 1 rad = 0.01 J/kg = 0.01 Gy.
- Dose equivalent / biological effect (rem, Sv): equals absorbed dose × radiation weighting/quality factor (accounts for different biological effectiveness of alphas, neutrons, gammas, etc.). 1 Sv = 100 rem, so 1 rem = 0.01 Sv.

Example
- 1 Gy of gamma radiation (quality factor ≈ 1) → 1 Sv ≈ 100 rem.
- 0.01 Gy = 1 rad; if that radiation has Q = 1, that is 1 rem. If the radiation is alpha with Q ≈ 20, 0.01 Gy (1 rad) → ≈ 20 rem.

Usage note
- Gy and Sv are the preferred SI units; rad and rem are still used in some older literature and by some regulators/practitioners.