The dynamic range of the gray level of a digital image is limited by the noise it contains. Two statistical methods called “runs test” and “join‐count statistic” are used to measure the noise level in a digital image. A residual image is formed by subtracting an original image from its smoothed version. Theoretically, the noise level in the residual image should be identical to that in the original image. The noise level is determined by examining each bit plane of the residual image individually starting from the least significant bit up to the bit plane whose statistic does not show a random pattern. Images from three digital modalities: computerized tomography, magnetic resonance, and computed radiography are used to evaluate the gray‐level dynamic range. Both methods are easy to implement and fast to perform.