laws & principles
Weber's law
What is Weber's law?
Weber's law states that the smallest change we can notice in something is proportional to its starting size, not a fixed amount. A £1 rise on a £3 coffee is glaring; the same £1 on a £900 laptop is invisible. The "just-noticeable difference" scales with the baseline, so big things have to change by a lot before anyone perceives it.
Also known as: webers law, weber's law, just noticeable difference
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The demo
Add the same £1 to two prices - a cheap one and a dear one - and ask yourself which rise you'd actually notice. Then slide the base price and watch the same £1 fade from obvious to invisible.
☕ Coffee
💻 Laptop
What this demo shows (text version)
The same fixed change - one pound - is added to two prices: a cheap coffee and an expensive item whose base price you can change with a slider. On the £3 coffee, a £1 rise is a third more and jumps out; on the £900 item, the same £1 is a tiny fraction and goes unnoticed. As you slide the second item's base price down toward the coffee's, the identical £1 change crosses from invisible to obvious.
That's Weber's law: the smallest change we can notice is proportional to the starting size, not a fixed amount. It shapes how we perceive prices, sizes, spacing and durations - emphasis and meaningful differences must scale with their context to be felt - and it has an ethical edge, since small relative changes (creeping price rises, shrinkflation) slip past perception.
What counts as a noticeable change depends on the baseline. The just-noticeable difference is roughly a constant percentage of the starting value, so a small absolute change is obvious against a small base and imperceptible against a large one. This shapes how people perceive prices (£1 added to a cheap item stings; added to an expensive one, nobody notices), sizes, durations, and visual differences (a small element nudged a few pixels reads as misaligned; a large one needs a bigger nudge to look "off"). For design: scale your changes to the context - make differences proportional. To be perceived (emphasis, a meaningful jump), a change on a large base must itself be large; to stay unnoticed, keep it a small fraction. It also has an ethical edge: the same law is how creeping price rises and shrinkflation slip past us.
The same £1 jumped out on the cheap item and vanished on the dear one - because what we notice is a proportion of the starting size, not an absolute amount. That's Weber's law. It's why a tiny tweak reads as "broken alignment" on a small element but not a large one, why emphasis has to scale with context to be felt - and why small, repeated price creeps slip by unnoticed.
Weber's law (from psychophysics) holds that the just-noticeable difference between two stimuli is a roughly constant ratio of the original stimulus, across many senses - weight, brightness, loudness, length, and perceived quantities like price and time. So detecting a change depends on relative, not absolute, magnitude: adding one coin to a few is obvious; adding it to a heavy bag is not. The perception scales with the baseline.
In interfaces it explains several things. Price perception: a fixed surcharge feels large on cheap items and trivial on expensive ones, which retailers exploit (and which makes incremental price rises and shrinkflation hard to notice). Visual perception: misalignments, size differences and spacing changes must be proportionally larger on bigger elements to be seen, and proportionally smaller to go unnoticed. Durations and progress feel different against different baselines too.
Design with proportion in mind: when you want a change to register - emphasis, a size step in a type scale, a meaningful difference between options - make it large enough relative to its context, not just a fixed amount. When you want changes to feel smooth or subtle, keep them a small fraction of the baseline. And recognise the ethics: because small relative changes slip past perception, the same law underlies manipulative incremental pricing - so use it to communicate honestly, not to obscure.