The Weber–Fechner laws are two related scientific laws in the field of psychophysics, known as Weber's law and Fechner's law. Both relate to human perception, more specifically the relation between the actual change in a physical stimulus and the perceived change. This includes stimuli to all senses: vision, hearing, taste, touch, and smell.

Ernst Heinrich Weber states that "the minimum increase of stimulus which will produce a perceptible increase of sensation is proportional to the pre-existent stimulus," while Gustav Fechner's law is an inference from Weber's law (with additional assumptions) which states that the intensity of our sensation increases as the logarithm of an increase in energy rather than as rapidly as the increase.

Both Weber's law and Fechner's law were formulated by Gustav Theodor Fechner (1801–1887). They were first published in 1860 in the work Elemente der Psychophysik (Elements of Psychophysics). This publication was the first work ever in this field, and where Fechner coined the term psychophysics to describe the interdisciplinary study of how humans perceive physical magnitudes. He made the claim that "...psycho-physics is an exact doctrine of the relation of function or dependence between body and soul."

Ernst Heinrich Weber (1795–1878) was one of the first persons to approach the study of the human response to a physical stimulus in a quantitative fashion. Fechner was a student of Weber and named his first law in honor of his mentor, since it was Weber who had conducted the experiments needed to formulate the law.

In sensation and contrast, change is defined as "contrast over time". The sense detect the change, specifically in Weber's law, it detects the relative change - not absolute change. In Weber's law, to notice a change in stimulus (e.g. brightness or weight), the change must be constant proportion of the original stimulus. Weber's law states that just noticeable difference is proportional to the magnitude of the initial stimulus. The brain is a percentage of change detector. The differential threshold is the smallest difference needed to differentiate two stimuli for each sense has been studied by using similarly methods to signal detection. For instance, holding an object that weighs 1,2,10, and 11lbs. Let another person hold the lightest object (1lb). Now, swap out that object for something double the weight. The person will always respond saying that the second object is heavier than the first. However, it makes it difficult when the difference is a small percentage of the overall weight. For example, holding an object that is 10lb. Next, swap it out and hold the object that is 11lb. It would be difficult for one to tell the difference between the two for which is heavier.This shows the idea that bigger stimuli require a larger difference in order to be noticed. This is Weber's Law.

Fechner formulated several versions of the law, all communicating the same idea. One formulation states:

Simple differential sensitivity is inversely proportional to the size of the components of the difference; relative differential sensitivity remains the same regardless of size.

What this means is that the perceived change in stimuli is inversely proportional to the initial stimuli.