SI coherent derived unit with special name and symbol
Name Symbol Derived quantity Expressed in terms of SI base units
gray Gy absorbed dose,
specific energy (imparted),
m2 s−2


The gray is the SI coherent derived unit of absorbed dose of ionising radiation.

One gray is equal to the absorption of one joule of ionising radiation energy per kilogram of matter.

The gray is named after British physicist Louis Harold Gray (1905 – 1965).

Absorbed dose

The gray is a measure of the amount of energy absorbed by a unit mass of matter when exposed to a source of ionising radiation.

1 \ \text{Gy} \ = \ 1 \ \dfrac{\text{J}}{\text{kg}} = 1 \ \dfrac{\text{m}^2}{\text{s}^2}

The gray is used for measuring the delivered dose of ionising radiation in applications such as radiotherapy, food irradiation and radiation sterilisation and predicting likely acute effects, such as acute radiation syndrome in radiological protection.

As a measure of low levels of absorbed dose, it also forms the basis for the calculation of the radiation protection unit the sievert, which is a measure of the health effect of low levels of ionising radiation on the human body.

The gray is also used in radiation metrology as a unit of the radiation quantity kerma; defined as the sum of the initial kinetic energies of all the charged particles liberated by uncharged ionising radiation in a sample of matter per unit mass.

Diagnostic imaging

X-rays can be used for diagnostic imaging. Medical imaging using x-rays can reveal anatomical details that would be invisible by other means, such as bone fractures and tumours. The amount of x-rays used depends on the type of imaging carried out. X-ray exposure is carefully controlled to minimise the carcinogenic side effects of the ionising radiation.

X-ray usage Typical ionising radiation dose
abdominal x-ray 0.7 mGy
abdominal CT scan 8 mGy
CT scan of abdomen and pelvis 14 mGy

Radiation therapy

Radiation therapy is a medical treatment for cancer. It involves exposing tumours to carefully measured amounts of ionising radiation. The ionising radiation used can be x-rays or γ-rays (gamma rays). The amount of radiation applied varies depending on the type and stage of cancer being treated.

Therapy Typical ionising radiation dose
lymphoma 20 to 40 Gy
preventive (adjuvant) doses for breast, head, and neck cancers 45 to 60 Gy
solid epithelial tumor 60 to 80 Gy

Radiation protection

The absorbed dose also plays an important role in radiation protection, as it is the starting point for calculating the stochastic health risk of low levels of radiation, which is defined as the probability of cancer induction and genetic damage. The gray measures the total absorbed energy of radiation, but the probability of stochastic damage also depends on the type and energy of the radiation and the types of tissues involved. This probability is related to the equivalent dose in sieverts (Sv), which has the same dimensions as the gray. It is related to the gray by weighting factors described in the articles on equivalent dose and effective dose.

For x-rays and gamma rays the gray is numerically the same value when expressed in sieverts, but for alpha particles 1 Gy is equivalent to 20 Sv, and a radiation weighting factor is applied accordingly.

Radiation poisoning

The gray is conventionally used to express the severity of what are known as “tissue effects” from doses received in acute exposure to high levels of ionising radiation. These are effects that are certain to happen, as opposed to the uncertain effects of low levels of radiation that have a probability of causing damage. A whole-body acute exposure to 5 Gy or more of high-energy radiation usually leads to death within 14 days. This dose represents 350 J for a 70 kg adult.

Absorbed dose in matter

The gray is used to measure absorbed dose rates in non-tissue materials for processes such as radiation hardening, food irradiation and electron irradiation. Measuring and controlling the value of absorbed dose is vital to ensuring correct operation of these processes.


Kerma (kinetic energy released per unit mass) is used in radiation metrology as a measure of the liberated energy of ionisation due to irradiation, and is expressed in grays. Importantly, kerma dose is different from absorbed dose, depending on the radiation energies involved, partially because ionisation energy is not accounted for. Whilst roughly equal at low energies, kerma is much higher than absorbed dose at higher energies, because some energy escapes from the absorbing volume in the form of bremsstrahlung (x-rays) or fast-moving electrons.

Absorbed dose vs dose equivalent

The quantity “dose equivalent” H is the product of the absorbed dose D of ionising radiation and the dimensionless factor Q (quality factor) defined as a function of linear energy transfer:

H \ = \ Q \ D

Thus, for a given radiation, the numerical value of H in J/kg may differ from that of D in J/kg depending on the value of Q.

In order to avoid any risk of confusion between the absorbed dose D and the dose equivalent H, special names for the respective SI units are used:

  • The special unit name gray, symbol Gy, should be used instead of joules per kilogram for the unit of absorbed dose D.
  • The special unit name sievert, symbol Sv, should be used instead of joules per kilogram for the unit of dose equivalent H.

Radiation-related units

Name Symbol Quantity Expressed in terms of SI base units Expressed in terms of other SI units
becquerel Bq activity referred
to a radionuclide
s-1 1/s
coulomb per kilogram C/kg exposure
(x- and γ-rays)
kg−1 s A C/kg
gray Gy absorbed dose m2 s-2 J/kg
sievert Sv dose equivalent m2 s-2 J/kg
gray per second Gy/s absorbed dose rate m2 s−3 W/kg