Weber’s Law
Explains that users only notice changes when they exceed a set percentage threshold of the original stimulus, from color tweaks to timing delays.
Definition
Weber’s Law, or the Weber-Fechner Law, describes how the smallest detectable difference between two stimuli is a constant proportion of the original stimulus’s intensity.
In UX, that means tweaks to color, size, volume, or timing need to cross a percentage threshold before users actually notice them.
This isn’t just academic, our senses adapt logarithmically, so a 10% brightness bump on a dim icon registers, but a 10% bump on a full-screen hero image can fly under the radar.
Understanding Weber’s Law is fundamental for designing perceptible changes in interfaces, from visual hierarchy shifts to haptic feedback, and ensuring your refinements don’t vanish into user blindness.
Real world example
Think about adjusting the brightness slider on your iPhone: small drags at low brightness make a visible difference, but the same pixel shift at near-max brightness barely registers. That proportional threshold is Weber’s Law in action.
Real world example
In user onboarding flows where progress indicators must visibly change state.
in crowded dashboards when key metrics need perceptible highlighting.
on error and success states, color, icon size, or vibration feedback must cross a threshold to be noticed.
in interactive micro-animations that signal state changes (e.g., toggles, button presses).
in timing delays for hover effects or tooltips to feel responsive rather than sluggish.
What are the key benefits?
Everything you need to make smarter growth decisions, without the guesswork or wasted time.
Increase color contrast by at least 20% for critical UI states.
Scale up icon sizes proportionally in dense interfaces to ensure legibility.
Set animation duration changes above the perceptual JND (around 10–20%).
What are the key benefits?
Everything you need to make smarter growth decisions, without the guesswork or wasted time.
Don’t rely on 1–2% brightness tweaks for important alerts.
Avoid tiny font-weight shifts that users won’t perceive in body text.
Don’t make millisecond-level speed tweaks and call it ‘faster’ UX.
Frequently asked questions
Growth co-pilot turns your toughest product questions into clear, data-backed recommendations you can act on immediately.
What exactly is the Just Noticeable Difference (JND)?
JND is the minimal change in a stimulus (like brightness or volume) that users can reliably detect, it scales proportionally with the original intensity.
How do I measure JND in my product?
Run A/B tests with incremental changes (10–20% steps) and track detection rates via user feedback or analytics heatmaps.
Does Weber’s Law apply to timing as well as visuals?
Yes, delay changes need proportional increases (e.g., hover tooltips might need a 100–120ms jump) to feel faster or slower.
Can I combine multiple small cues to surpass the JND?
Absolutely, pairing color, size, and haptic feedback can collectively exceed the JND when individual changes fall short.
Is Weber’s Law relevant for typography?
Definitely, font-size and weight shifts need to cross perceptual thresholds (around 10% size change) to register with users.
Stop Invisible Tweaks
If your UI improvements are too subtle, they’re wasted. Run your interface through CrackGrowth’s diagnostic to pinpoint where your changes need to cross the JND threshold.