Radiation Dose Conversion Calculator
Convert radiation dose measurements between rem, sievert, rad, and gray, with adjustment for radiation type weighting factor. Used by health physicists, radiologists, and radiation safety officers to compare dose values across unit systems.
About this calculator
Radiation dosimetry uses two distinct physical quantities. Absorbed dose (gray, Gy; or rad) measures energy deposited per unit mass of tissue: 1 Gy = 1 J/kg = 100 rad. Equivalent dose (sievert, Sv; or rem) accounts for biological effectiveness by multiplying absorbed dose by a radiation weighting factor w_R: H (Sv) = D (Gy) × w_R. For X-rays and gamma rays w_R = 1, so 1 Gy = 1 Sv. For protons w_R = 2; for fast neutrons w_R = 2–20 depending on energy; for alpha particles w_R = 20. The legacy units rem and rad follow the same relationship: H (rem) = D (rad) × w_R, with 1 Sv = 100 rem and 1 Gy = 100 rad. This calculator handles all six pairwise conversions between these four units.
How to use
A patient receives 2 gray of fast-neutron therapy (w_R = 10). To convert to sievert: H = D × w_R = 2 Gy × 10 = 20 Sv. To then convert to rem: 20 Sv × 100 = 2,000 rem. Alternatively, starting from 50 rem of gamma radiation (w_R = 1): dose in sievert = 50 × 0.01 = 0.5 Sv; dose in gray = 0.5 Sv / 1 = 0.5 Gy; dose in rad = 0.5 Gy × 100 = 50 rad. Enter your dose value, select the source unit and target unit, and choose the radiation type to apply the correct weighting factor automatically.
Frequently asked questions
What is the difference between gray and sievert in radiation dosimetry?
Gray (Gy) is the SI unit of absorbed dose and simply measures how much energy (in joules) is deposited per kilogram of material, regardless of radiation type. Sievert (Sv) is the unit of equivalent dose and weights the absorbed dose by the biological effectiveness of the radiation — so 1 Gy of alpha particles causes far more cellular damage than 1 Gy of gamma rays. For gamma rays and X-rays the weighting factor is 1 and the numerical values are identical, but for neutrons or alpha particles the sievert value can be 2–20 times higher. Regulatory dose limits are always expressed in sieverts because they represent biological risk.
How do rem and sievert relate to each other historically?
The rem (Roentgen Equivalent Man) is the traditional unit of equivalent dose used in the US customary system, defined before the SI system was adopted. It carries the same biological weighting concept as the sievert: 1 rem = 0.01 Sv, or equivalently 100 rem = 1 Sv. Most US nuclear industry regulations (NRC 10 CFR Part 20) still express limits in rem and mrem (millirem), while international standards and most scientific literature use sievert and millisievert. Annual occupational dose limits are 5 rem (50 mSv) per year, and the public limit is 100 mrem (1 mSv) per year above background.
When should I use equivalent dose versus effective dose in radiation protection?
Equivalent dose (sievert) applies the radiation weighting factor w_R to account for different radiation types, but treats all tissues equally. Effective dose goes one step further, multiplying equivalent dose in each organ by a tissue weighting factor w_T that reflects that organ's sensitivity to radiation-induced cancer — thyroid, lung, bone marrow, and gonads each have different w_T values. Equivalent dose is used when the entire body or a uniform field is irradiated. Effective dose is the correct quantity for comparing partial-body exposures, assessing cancer risk from medical imaging, or setting general public dose limits. This calculator computes equivalent dose conversions; effective dose requires organ-specific dosimetry software.