Scientists measure radiation in different ways. Sometimes, they measure the dose that a person receives from a radioactive source, and sometimes they measure the amount of radioactivity in water, or in soil, or in the air.  Each measure describes a different aspect of radiation, and each has its own unit.

There are different but interrelated units for measuring radioactivity and its effects:

• Radioactivity refers to the amount of ionizing radiation released by a material. Whether it emits alpha or beta particles, gamma rays, x-rays, or neutrons, a quantity of radioactive material is expressed in terms of its radioactivity (or simply its activity). This represents how many atoms in the material decay in a given time period. The units of measurement for radioactivity are the becquerel (Bq, international unit) and the curie (Ci, U.S. unit).
• Exposure describes the amount of radiation traveling through the air. Many types of radiation monitors measure exposure. The units for exposure are the coulomb/kilogram (C/kg, international unit) and the roentgen (R, U.S. unit).
• Absorbed dose describes the amount of radiation absorbed by an object or person. The unit for absorbed dose is the graygrayA gray is the international unit used to measure absorbed dose (the amount of radiation absorbed by an object or person). The U.S. unit for absorbed dose is the rad. One gray is equal to 100 rads. (Gy, international unit) or the  radradThe U.S. unit used to measure absorbed radiation dose (the amount of radiation absorbed by an object or person). The international equivalent is the Gray (Gy). One hundred rads are equal to 1 Gray. (U.S. unit).  One gray is equal to 100 rads.
• ​Effective dose describes the amount of radiation absorbed by person, adjusted to account for the type of radiation received and the effect on particular organs. The unit used for effective dose is sievertsievertAn international unit used to measure effective dose. The U.S. unit is rem. (Sv, international unit) or remremThe U.S. unit to measure effective dose. The international unit is sieverts (Sv). (U.S. unit). 
  
  One sievert is equal to 100 rems. More commonly, dose is measured in much smaller units: millisieverts or millirems.  A millisievert is one thousandth of a sievert. A milliremmilliremThe millirem is the U.S. unit used to measure effective dose. One millirem equals 0.001 rem. The international unit is milliSievert (mSv). (mrem) is one thousandth of a rem. 

Use the Radiation Dose Calculator to estimate your yearly dose from sources of ionizing radiation.