Weber’s law was originally postulated during research that Weber carried out in 1834 to try and calculate the threshold for perceiving a change in weight and was later applied to the general measurement of sensation and perceptions by Weber’s student Fechner. It states that the perception of change in any given stimulus is always dependent on what that stimulus is. In other words, whether a change will be noticed is affected by how big, heavy or significant etc. that something was beforehand and how significant the change. Our sensorial capacity has limits and there is an “absolute threshold” that describes the minimum amount by which stimulus intensity must be changed in order to produce a noticeable variation in sensory experience. Weber's law shows that a difference of approximately 10 percent is the average point where people are stirred to respond.

Weber originally tested this using the sensation of weight but it can be applied to a variety of sensory modalities (brightness, loudness, mass, length, etc.). It can also be applied to numerical values such as prices, the deletion of sections of text, or any other perceptions we might have. We can even take the well-known example of when a friend goes to the hairdresser without telling you: the likelihood of you noticing the new hair cut will depend entirely on how different it is from beforehand. If it’s just a trim without any changes to style or colour then there’s a strong likelihood that you won’t even notice. However, if she cuts it from waist to chin length then the change from the original is significant enough that it will be impossible to miss.

Weber’s law is often used in marketing, particularly with regards to price increases for products and services. It implies for example that it is possible to increase prices by small enough amounts – that fall under the “absolute threshold” – without your customers even noticing.