Detectors will be saturated when exposed to intense light. There is certain dependence among saturated area, light intensity and saturation threshold. The energy distribution of far-field spot is described with Airy function, due to the constraint of pupil. Saturated area formula has been obtained by the study of the envelope of Airy function. Exposed to a beam of intense light, the saturated area is proportional to the 2/3-th power of light intensity and inversely proportional to the 2/3-th power of saturation threshold. The formula is verified by irradiation experiment. The result has certain significance in saturation estimation and laser blinding.