Radiation Dose Unit Converter

Radiation Dose Units — What They Measure

Radiation dose is measured in different ways depending on whether you want to know the raw energy deposited or the biological harm caused.

Absorbed Dose (energy deposited in tissue)

• Gray (Gy): the SI unit. 1 Gy = 1 joule of energy absorbed per kilogram of tissue.
• Rad: the older CGS unit. 1 Gy = 100 rad (1 rad = 0.01 Gy).

Equivalent Dose (biological effect) Different radiation types cause different amounts of biological damage even when the same energy is deposited. The equivalent dose accounts for this by multiplying absorbed dose by a radiation weighting factor (wR):

• Equivalent dose (Sv) = Absorbed dose (Gy) × wR
• Sievert (Sv): the SI unit for equivalent dose.
• Rem: the older unit. 1 Sv = 100 rem.
• Millisievert (mSv): 1 Sv = 1,000 mSv. Most practical doses are expressed in mSv.

Radiation Weighting Factors (wR)

• X-rays and gamma rays: wR = 1 (reference radiation)
• Beta particles (electrons): wR = 1
• Protons: wR = 2
• Neutrons: wR = 5 to 20 (depends on energy — higher energy neutrons are more damaging)
• Alpha particles: wR = 20 (most biologically damaging per unit energy)

Activity (how many decays per second)

• Becquerel (Bq): 1 Bq = 1 nuclear decay per second. The SI unit for radioactivity.
• Curie (Ci): 1 Ci = 3.7 × 10¹⁰ Bq (based on the activity of 1 gram of radium-226).
• 1 Ci = 37 GBq (gigabecquerels).

Annual Background Radiation The average person receives about 2.4 mSv per year from natural background radiation (cosmic rays, radon gas, naturally occurring isotopes in food and the environment). This varies significantly by location — high-altitude cities like Denver, Colorado receive roughly twice the global average.