Weber’s law states that the just noticeable difference between two stimuli is proportional to the magnitude of the stimuli. The larger the original stimulus, the larger the change must be before it is detected.

Ernst Weber observed that the ability to discriminate weights, brightness, and other sensory qualities follows a constant ratio rather than a constant absolute difference. To notice an increase in the weight of a backpack, the added load must be a roughly constant fraction of the existing weight. The same principle applies, within limits, across sensory modalities. The fraction itself—the Weber fraction—varies from one modality to another and is smallest for the most sensitive discriminations.

Weber’s law holds best for mid-range stimulus intensities and breaks down near threshold and at very high intensities. Fechner later used it as a foundation for the first quantitative scale of sensation. The law remains a basic principle of psychophysics and a practical guide for designing displays, product differences, and sensory experiments.

A change must be proportionally larger when the background stimulus is larger. That is Weber’s law.