The disclosure provides a system and method for adjusting a backlight level for an image being displayed on an electronic device. The system comprises: a display for displaying an image to be backlit; memory storing a numeric representation of the image on a pixel-by-pixel basis; and a backlight adjustment module to calculate a pixel-by-pixel, running average of an intensity of the image using the numeric representation and to determine a backlight level for the image using the running average, the running average being calculated according to an equation that calculates a new average using a previous average and a current pixel intensity value to avoid an overflow condition when calculating the running average.