"Put the main menu and the buy buttons at the top left, it's the web standard!"
This is a claim you hear over and over again in company design meetings. But reality hides a bitter spoiler: on modern smartphones, the top-left corner is by far the worst place you could put an action that matters to your business.
Anatomy and usability: the origins of the law
Behind a mobile app or website that feels pleasant and natural to use there isn't just good aesthetic taste. There is a scientific, mathematical rule formulated back in 1954 by the psychologist Paul Fitts: Fitts's Law.
Put simply, this principle states that the time needed to reach a target depends on the distance to travel and on the size of the target itself. The further away and smaller a button is, the harder it is for the user to tap it. Translated into the world of today's smartphones, this law becomes the cornerstone of mobile design.
Physical reality and design friction
To apply this principle correctly to web design, we need to look at the interaction between people and devices at three critical points:
1. The hardware reality
Phone screens get bigger every year, but our hands stay the same size. Behavioural data show that the vast majority of people use their smartphone holding it in one hand and navigating the interface with their thumb alone.
2. The invisible friction
If you put your Call to Action (the button to buy, sign up or get in touch) or the main menu icon at the top of the screen, you are forcing your user into real thumb contortions, or into using their other hand to complete the action. That's a micro-frustration. And micro-frustrations, piled up, make sales plummet.
3. The solution: designing for the thumb
The solution is to move all the key actions into the so-called Thumb Zone (the thumb's comfort zone), that is, the lower half of the screen. Following Fitts's Law, the golden rule is simple: the more important a button is to your revenue, the bigger it should be and the closer to the natural resting position of the user's finger.
The tech giants already know it
There is a key detail that confirms this theory: it is no coincidence at all that Apple recently overhauled Safari's design, moving the whole address and navigation bar to the bottom.
If you pay attention, all the apps you use most often during the day (such as Instagram, Spotify or TikTok) have their entire main navigation comfortably within thumb's reach, in the lower part of the screen. They have understood that physical comfort and ergonomics translate directly into time spent in the app and into conversions. When you design for mobile, stop looking at your computer screen and start looking at your hands.
