Laws of UX
Fitts's Law
The time to hit a target depends on its size and distance — small, far targets are hard to reach.
Where it comes from
It's named for the American psychologist Paul Fitts, who in 1954 measured how quickly people could move to and tap a target of a given size and distance. His equation has held up for seventy years and underpins much of how we think about pointing, tapping, and clicking.
Why it matters for your website
Your most important actions should be the easiest to hit. Fitts's Law shows the time to reach a target is a function of its size and distance. Small or awkwardly placed buttons directly cost you clicks — especially on touchscreens.
Two things make a target easy to hit: how big it is, and how far the cursor or thumb has to travel to reach it. The corners and edges of a screen are special — they're effectively infinite in size, because the pointer stops dead against them however hard you overshoot. That's why desktop menus, docks, and close buttons have lived in corners for decades.
On touch, the calculus shifts but the law doesn't. A finger is a blunt instrument — roughly a centimetre of contact — so targets that look tappable on a designer's large monitor can be near-impossible on a phone held one-handed on a train. Undersized links sitting close together turn every tap into a gamble, and a missed tap on a 'Buy' button is a sale you nearly made.
Wrong vs right
A 'Continue' button the same small size as everything around it, set in the middle of a busy form where the thumb has to travel to find it.
A large, full-width primary button anchored where the thumb naturally rests, so it's both big and close.
Tiny close (×) icons and inline links sized at 16px, packed tightly together, so people routinely tap the wrong one.
Touch targets at least 44–48px with clear spacing between them, so each is easy to hit without catching its neighbour.
A critical action tucked in the far corner of a wide desktop layout, miles from where the user's attention and cursor already are.
The primary action placed near the content it follows, where the cursor is already heading.
Understanding Fitts's Law
Fitts's Law puts a number on something every interface designer feels intuitively: the time to acquire a target grows as the target gets smaller and as the distance to it gets larger. Formally the relationship is logarithmic — doubling the distance doesn't double the difficulty — but the practical lesson is blunt. Important controls should be large, and they should be close to where the user's attention and pointer already are.
The law has a few famous corollaries. Screen edges and corners behave as though they're infinitely large, because you can't overshoot them — which is why so many durable UI patterns live there. Targets that sit along the path the user is already travelling are cheaper to reach than ones requiring a detour. And grouping related controls reduces the total travel between them.
On touchscreens the principle becomes unavoidable. There's no precise cursor, just a soft fingertip, so size and spacing aren't polish — they're the difference between a control that works and one that frustrates. This is where Fitts's Law meets accessibility: generous touch targets help people with motor difficulties, larger fingers, or an unsteady grip, and they help everyone on a small screen. It pairs directly with touch-target sizing and the thumb zone.
How Kweri checks it
Kweri can measure a good deal of this directly — it inspects the rendered size of interactive elements and flags buttons, links, and tap targets that fall below recommended minimums or sit too close together to hit reliably. What it can't fully judge by measurement is travel distance in context: whether a button is *positioned* where a user's attention and pointer naturally are for a given task. So Kweri reliably catches undersized and crowded targets, and raises placement as a prompt where the layout suggests an important action is harder to reach than it should be.
FAQ
What is Fitts's Law?
Fitts's Law states that the time to move to and select a target depends on the target's size and its distance from the starting point: bigger, closer targets are faster to hit than smaller, more distant ones. In design it's used to size and place important controls.
What is the formula for Fitts's Law?
The common form is MT = a + b·log₂(2D/W), where MT is movement time, D is the distance to the target, W is the target's width, and a and b are constants. The takeaway is that difficulty rises with distance and falls with size — logarithmically, not linearly.
How does Fitts's Law apply to touchscreens?
Strongly. A fingertip is far less precise than a cursor, so touch targets need to be large enough (commonly 44–48px) and spaced apart enough to hit reliably. Undersized, crowded targets cause mis-taps and lost actions.
Why are screen corners and edges important in Fitts's Law?
Because you can't overshoot them — the pointer stops at the boundary no matter how far you push. That effectively makes edge and corner targets infinitely large and very fast to acquire, which is why menus and docks often live there.
How is Fitts's Law different from Hick's Law?
Fitts's Law is about the time to physically reach a target once you've decided. Hick's Law is about the time to decide between options in the first place. They often apply to the same screen and complement each other.
What's a good minimum size for a button or tap target?
On touch, a widely used floor is roughly 44–48px with adequate spacing, drawn from platform guidelines and WCAG's target-size criterion. Bigger is generally better for primary actions; small, tightly packed targets are the common failure.
Related principles
Tap targets need to be big enough, and spaced enough, to hit reliably with a finger.
Primary actions should sit where the thumb naturally reaches on a phone held one-handed.
The more choices you show, the longer people take to decide — and the likelier they pick nothing.
Attribution & sources
Identified by Paul Fitts (1954). Catalogued from Laws of UX (Jon Yablonski).
Established by psychologist Paul Fitts in 1954 and applied to interface design ever since; popularised for designers by Jon Yablonski's Laws of UX.
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