The time it takes to tap or click something depends on two things: how big it is, and how far away it is. Bigger and closer is always faster.
Reach for something small on the other side of a table. Now reach for something large right in front of you. Your hand knows the difference before you even move — the first requires careful aim and a slow approach, the second is effortless. Fitts's Law formalises this into something measurable, and it applies to every button, link, and icon in every interface ever built.
What makes this law particularly important in mobile design is that it operates constantly and invisibly. Users don't think “that button is too small” — they think “this app feels clunky.” The experience degrades without a clear explanation. But the explanation is almost always the same: targets that are too small, or too far from where hands already rest.
Paul Fitts was an American psychologist working with the US Air Force in 1954, studying why pilots made errors moving between cockpit controls. He established a precise, logarithmic relationship between movement time, target size, and target distance. Wider and closer targets were reliably faster and more accurate to reach.
The practical translation: make important things bigger and put them where hands already are. Every pixel of size reduction and every millimetre of added distance is measurable cost, paid by every user on every interaction.
“Distance is the enemy of action. Size is its antidote.”
Both LinkedIn and Facebook show you incoming connection requests in a list. The action is identical: accept or decline. The difference in how each app presents that action is a direct illustration of Fitts's Law.
LinkedIn's invitation screen places two small circular icon buttons — a ✕ and a ✓ — to the right of each person row. They're compact, icon-only, and require precise targeting to hit the correct one, especially since they sit side by side. If your thumb lands 4mm off, you might decline someone you meant to accept.
Facebook's friend requests use full-width “Confirm” and “Delete” buttons stacked below each person. These are generous, labelled, and far apart — vertically separated so there's no risk of hitting the wrong one. Tap both screens and feel the difference.
Accept and Decline are 8px apart. One imprecise tap accepts someone you meant to ignore — or vice versa.
Confirm and Delete span the full row width. No precision required, no accidental wrong taps.
The information on each row is identical. The purpose is identical. The difference is entirely in the tap target design — and that difference changes whether users can confidently act without fear of triggering the wrong outcome. LinkedIn's approach prioritises density, fitting more invitations on screen. Facebook's approach prioritises accuracy, making each action easy and unambiguous.
In 2014, Apple launched the iPhone 6 — their first phone with a screen large enough that the top half became genuinely difficult to reach with one hand. Rather than ignoring the problem, they shipped a feature called Reachability built directly into iOS: swipe down on the home indicator and the entire screen slides down, pulling the top half into the natural thumb zone.
Reachability is Apple's explicit acknowledgment, encoded in the operating system, that the top of a large screen violates Fitts's Law severely enough to require a workaround. It is the most direct statement any major platform has ever made about the cost of distance in mobile interaction.
The screen below has critical actions at the top. Swipe down on the home bar at the bottom to activate Reachability — then notice how those top buttons drop into easy thumb reach.
Tap the home bar to activate
Swipe down on the home bar to activate Reachability and pull top actions into thumb reach.
The lesson from Reachability isn't that every app should build a similar feature. The lesson is that screen real estate at the top is not free — it costs movement. When Apple decided the cost was high enough to engineer a system-wide workaround, they were making a Fitts's Law judgment at the platform level. App designers should make the same judgment at the layout level: put primary actions at the bottom, reserve the top for display and navigation.
Target size gets most of the attention in Fitts's Law discussions, but distance is equally important. A generous button at the wrong place can require as much movement time as a small button in the right place.
E-commerce product pages demonstrate this cleanly. A user scrolls through photos, reads the description, checks the reviews — and at some point decides to buy. If the Buy button is embedded in the scrollable content, its distance from the thumb grows with every scroll. By the time they've finished reading reviews, it could be hundreds of pixels away — requiring scrolling back up before buying. A button pinned to the bottom of the screen has zero distance regardless of scroll position. Scroll both products all the way to the reviews, then try to buy.
Scroll up to find the button. It's buried above the fold somewhere.
Scroll to the last review. The button is still right there — same distance from the thumb wherever you are on the page.
Amazon, Apple, Shopify and virtually every major e-commerce app now use sticky bottom CTAs for exactly this reason. The distance from the user's thumb to the Buy button is effectively zero regardless of scroll position — which is Fitts's Law applied to layout.
The question to ask for every interactive element is not “can users reach this?” — they almost always can. The question is “what is the cost of reaching this, and does that cost match the frequency and importance of this action?” Primary actions should have the lowest cost: large targets, bottom-of-screen placement, clearly separated from neighbouring actions so there's no risk of hitting the wrong one.
Fitts, P. M. (1954). The information capacity of the human motor system in controlling the amplitude of movement. Journal of Experimental Psychology, 47(6), 381–391. · Apple Reachability introduced with iOS 8, iPhone 6/6 Plus (2014). · Apple Human Interface Guidelines: minimum touch target 44×44pt. · Google Material Design: minimum touch target 48×48dp.