The effective dose (E) is a hypothetical equivalent dose uniformly applied to the whole body, which would result in the same health detriment through stochastic effects as the partial-body exposition being considered. It is calculated by summating the equivalent doses (H_T) delivered to the irradiated organs and tissues multiplied by their respective tissue weighting factors (W_T) ¹.

For example, if we assume that the abdomen and pelvis contain about a half of the body's radiosensitive tissues (based on tissue weighting factors), an abdominopelvic CT with a 20 mSv equivalent dose would translate to an effective dose of 10 mSv, i.e. the 20 mSv equivalent dose of this CT has an effect similar to a uniform exposition of the entire body to an equivalent dose of 10 mSv. By definition, the effective dose of a partial-body exposition is always smaller than the equivalent dose of that exposition.

Effective dose is an attempt to summarise the detrimental effect of a radiation exposition in a single number ². It is a very rough estimate based on arbitrary weighting factors and it does not account for patient's age, sex, or size ^1,2. As such, it can only be used for gross orientation and order-of-magnitude comparison between different imaging techniques ². It is neither practical nor meaningful to calculate the effective dose for a particular exposition of a particular patient. The effective dose has been criticised for being of doubtful utility and often misused ².