A method of calculating a gamma-value for a display panel includes displaying a first color test image having a first target gray-level and then measuring a first luminance-value implemented by at least one first color-pixel included in a jth display-area, displaying a second color test image having the first target gray-level and then measuring a second luminance-value implemented by at least one second color-pixel included in the jth display-area, deriving an estimated driving current flowing in the second color-pixel at the first target gray-level based on RGB gray level-driving current curves, deriving a second target gray-level when the estimated driving current flows in the first color-pixel based on the RGB gray level-driving current curves, and calculating a target gamma-value to be applied to the first color-pixel included in the jth display-area based on the first target gray-level, the second target gray-level, the first luminance-value, and the second luminance-value.