DePlaybook › Strategies › Fitts' Law
ORIGIN LARGE · CLOSE LOW ID · FAST SMALL · FAR HIGH ID · SLOW ID = log₂(2D/W) · BIGGER + CLOSER WINS
Strategy ⚡ Low

Fitts' Law

The further away and smaller a target, the longer it takes to hit — and the more likely a user is to miss.

Touch targets CTA placement Mobile design Navigation design Button sizing Motor accessibility

01 — TL;DR

Two sentences.

Fitts' Law predicts the time required to acquire a target as a function of its size and distance — bigger and closer targets are faster and less error-prone to click or tap than smaller and further ones. The formula: ID = log₂(2D/W), where D is distance to the target and W is its width — making target size and position the strongest design variables on motor accuracy.

The formula in plain English

Doubling the distance to a target roughly doubles the time it takes to hit it; doubling its size roughly halves that time. Screen edges and corners are effectively infinite targets in the perpendicular direction — the cursor or finger cannot overshoot them — which is why operating systems anchor their most-used controls to edges and corners.

Trigger phrase

When users are missing buttons, struggling to tap small targets, or taking longer than expected to complete simple pointing tasks — and the fix is a design decision about size and placement, not a copy or content decision.

02 — When to Use

Apply this when…

→ A primary CTA is small relative to its importance — the most important action on a screen should be the easiest to hit
→ You're sizing mobile touch targets and need a principled basis for minimum sizes rather than relying on visual aesthetics
→ Navigation elements are being placed and you need to evaluate whether their position reflects their frequency of use
→ A form or panel with many interactive elements needs review for whether targets users hit repeatedly are optimally sized and positioned
→ You're evaluating accessibility for users with motor impairments — Fitts' predicts which elements will produce the most acquisition difficulty
→ Two layout options are being compared and you need an objective basis for which produces faster, more accurate interactions

When NOT to apply it

Fitts' Law is the wrong lens for interactions that require deliberate precision — surgical interfaces, fine-grain drawing tools, professional measurement instruments — where oversized targets would reduce precision rather than improve usability. It also doesn't govern reading comprehension or decision time: it's a motor acquisition law, not an information-architecture or content-density law.

03 — How It Works

The mechanism

Paul Fitts published his original experiment in 1954, asking participants to tap alternately between two target plates while measuring how movement time varied with target distance and width. Doubling the distance roughly doubled movement time; doubling the target width roughly halved it. The law has since been replicated hundreds of times across mice, touchscreens, eye-tracking interfaces, and VR — making it one of the most robustly validated quantitative laws in human-computer interaction.

01
Size matters more than aesthetics
The primary action on any screen should be large enough to be acquired quickly and accurately, regardless of whether that size "looks proportionate" to surrounding elements. Visual balance is a legitimate design goal, but for primary actions the hierarchy must resolve in favour of interaction efficiency.
02
Proximity matters as much as size
Elements users interact with frequently should be positioned close to where their pointer or finger already is — which in practice means near screen edges, corners, and adjacent to the content users are already engaged with. Movement distance is half of the index of difficulty equation.
03
Edges and corners are premium real estate
Cursors and fingers cannot overshoot screen edges, so any element at an edge is effectively infinite in the perpendicular dimension. This is why macOS pins its menu bar to the top edge, Windows pins its taskbar to the bottom, and the Dock lives at an edge — they exploit the fastest acquisition positions on any screen.
04
Beware effective vs nominal target size
A target's nominal size is what the design system specifies; its effective size is the area within which a tap reliably lands on the intended element rather than an adjacent one. Small targets clustered close together have a much smaller effective size — spacing matters as much as nominal dimensions.
05
Measure ID, completion time, and error rate
Calculate Fitts' index of difficulty for key elements before testing — ID = log₂(2D/W) — to predict which will fail. Then measure task completion time and missed-tap rate in usability testing. High error rates on a specific element indicate a Fitts' failure, regardless of how large it appears visually.

Fitts' vs cognitive load

Fitts' Law governs motor acquisition time — how quickly and accurately a user can point to an element. It does not govern decision time — how long they spend deciding what to do. A large, well-positioned button users cannot understand will still produce slow interactions. Fitts' and Cognitive Load Theory are complementary but distinct: Fitts' governs the act of interacting; CLT governs the work of deciding which target to interact with.

04 — Real Example

Apple's iPhone bottom navigation and the thumb-reach zone

When Apple redesigned the iPhone interface for larger screens beginning with the iPhone 6 in 2014, they faced a fundamental Fitts' Law tension: larger screens provide more real estate but move content further from the natural resting position of the thumb on a one-handed device. A navigation bar at the top of a 6-inch screen places the most frequent navigation items at the furthest possible distance from the thumb — maximising Fitts' index of difficulty for the most frequent interaction in the interface.

Apple's response was Reachability — a gesture that temporarily shifts the top of the screen down — and, more significantly, a progressive shift of primary navigation to the bottom of the screen. Bottom tab bars, bottom sheet interactions, and the migration of primary controls into thumb-reach zones across Apple's own apps reflect a systematic application of Fitts' Law to one-handed mobile interaction.

Mobile · Navigation · iOS
Bottom navigation drops Fitts' ID by ~60%
Bottom-bar placement keeps primary navigation within the ~75mm thumb-reach zone of a standard adult hand. That single positioning decision moves the index of difficulty for the most frequent interaction in iOS from approximately 4 bits (top of screen, far from thumb) to approximately 1.5 bits (bottom bar, within thumb reach) — a difference that compounds across every navigation tap a user performs across the lifetime of the device.
~60% lower ID

05 — In the Wild

Test yourself & see real examples

No examples yet — be the first.

Spotted a product where the most important action is buried in a small, distant corner — or one where the primary CTA is so large and well-placed it practically invites the click? Submit a screenshot and annotate what you see. Every approved example gets attributed to you.

✓ Reviewed before publishing ✓ Your name on every example you submit ✓ Violation or fix — both welcome

Seen Fitts' Law applied well or violated in a real product? Help grow the evidence base.

06 — Common Mistakes

Where teams go wrong

✗ Sizing buttons for visual balance rather than interaction priority. A primary CTA at the same visual weight as surrounding secondaries has been designed for aesthetic balance, not interaction efficiency. Visual balance is legitimate, but when the most important thing to click is not the easiest thing to click, the design is optimising for how it looks rather than how it works.
✗ Placing the primary CTA where it looks centred rather than where fingers already are. On mobile, the visual centre of a tall screen is exactly where the thumb cannot comfortably reach. "Below the fold" and "bottom of screen" are not penalties — they are the thumb-reach zone Fitts' identifies as prime interaction real estate.
✗ Clustering small interactive elements without considering compound difficulty. When multiple small targets sit a few pixels apart — toggle switches, checkboxes, dense icon rows — the effective target size is reduced by the proximity of adjacent targets. Evaluate dense controls on their effective ID, not their nominal size.
✗ Ignoring Fitts' in right-click and contextual menus. Items in a contextual menu are typically narrow, uniformly sized, and require precise positioning — and the most-used items are rarely closest to the trigger. Making frequent actions larger and closer to the trigger reduces error rates and completion times for the interactions users perform most often.
✗ Treating the WCAG 44px minimum as a ceiling instead of a floor. 44 × 44 CSS pixels is the minimum for accessible motor acquisition — not the recommended size for primary interactions. Critical actions on mobile should comfortably exceed it; meeting the minimum is not the same as designing for speed and accuracy.

07 — Variations & Related Principles

Connected ideas

Fitts' Law is the primary quantitative model for motor acquisition time in interaction design. Its closest relationships are with the design principles and cognitive models that together govern the full cost of a user interaction — from the decision to act to the moment the target is hit.

Decision before movement
Hick's Law
Decision time vs movement time
Cognitive Load Theory
Where users look vs reach
Selective Attention
Accessible target sizing
Universal Design
Convention vs acquisition
Jakob's Law
Spacing affects effective size
Law of Proximity

The most important pairing is Fitts' Law with Hick's Law. Hick's governs the time before movement begins — the cognitive cost of selecting among options. Fitts' governs the time after the decision is made — the motor cost of executing the selected action. Every user interaction has both a Hick's component (how long to decide) and a Fitts' component (how long to reach). Designing for one while ignoring the other produces an interaction that is fast in one dimension and slow in the other.

08 — 10-Min Exercise

Run it right now

⏱ 10 minutes · Solo · No prep

The Fitts' Audit

Open your product's most important screen — the primary dashboard, the key conversion page, or the main action screen. Identify the three most important actions a user needs to take.

1. For each action, evaluate two Fitts' properties: how large is the target (measure or estimate in pixels), and how far is it from where the user's pointer or thumb naturally rests when arriving at this screen — the centre for mouse, the bottom third for mobile thumb.

2. Rank your three actions by their Fitts' optimisation: which has the largest target and the shortest distance to acquisition? Is the ranking of Fitts' optimisation the same as the ranking of interaction importance?

3. If the most important action is not also the most Fitts-optimal — not the largest and most accessible target on the screen — you've found a misalignment. Note where the gap is: too small, too far, or both.

4. Sketch the fix. Could the target be larger? Could it be moved closer to where the user's pointer or thumb naturally rests? Even a 20% increase in size or repositioning to a scrollable edge can produce a meaningful reduction in acquisition time and error rate for the most important interaction in your product.

10 minutes