What is Fitts's Law?
In 1954 the psychologist Paul Fitts published a series of experiments on human motor control. Participants tapped back and forth between two targets with a stylus while he varied the distance between them and their width. The result was remarkably consistent: movement time grew with distance and shrank with target width, following a simple logarithmic relationship.
Decades later, human–computer interaction researchers showed that the same law predicts pointing with a mouse, a trackpad and a finger. That is why it is one of the most cited principles in interface design.
The Fitts's Law formula
The version most used in HCI today is the Shannon formulation, popularised by I. Scott MacKenzie:
MT = a + b · log2(D / W + 1)
- MT — movement time, how long it takes to reach the target.
- D — distance from the starting point to the centre of the target.
- W — width of the target measured along the direction of movement.
- a, b — constants measured for a given device and person.
The logarithm term is called the index of difficulty, measured in bits. Doubling the distance, or halving the size, adds roughly one bit — and a fixed amount of time — to every click.
What Fitts's Law means for web design
Make primary targets large
Give buttons generous padding and make whole cards clickable instead of only the small “Read more” link inside them.
Reduce the distance
Put the submit button right below the last form field and keep actions next to the content they affect.
Use edges and corners on desktop
The pointer stops at the screen edge, so targets placed there behave as if they were infinitely deep — the reason operating systems put menus and docks at the edges.
Space small targets apart
Icons packed together cause mis-taps. Spacing turns the empty area into a safety margin.
Design for the thumb
On phones many people browse with one hand. Primary actions near the bottom and centre of the screen are easier to reach than the top corners.
Recommended tap target sizes
| Guideline | Minimum size | Notes |
|---|---|---|
| WCAG 2.2 — 2.5.8 Target Size (Minimum), level AA | 24 × 24 CSS px | Smaller targets pass only if they have enough spacing; inline links in text are exempt. |
| WCAG 2.2 — 2.5.5 Target Size (Enhanced), level AAA | 44 × 44 CSS px | The safer target for primary controls. |
| Apple Human Interface Guidelines | 44 × 44 pt | Recommended hit area for controls on iOS. |
| Android / Material Design | 48 × 48 dp | Recommended touch target size. |
A practical rule for websites: 44 × 44 px for every primary button and icon button, at least 24 × 24 px with spacing for secondary controls, and normal text size for links inside sentences.
Fitts's Law examples: good and bad
| Pattern | Why it works or fails |
|---|---|
| ✓ Full-width checkout button on mobile | Large target in the natural thumb zone: fast and hard to miss. |
| ✓ Entire product card is clickable | The target becomes the whole card, not a small text link. |
| ✗ Tiny “×” to close a pop-up | Small, far in the corner — people miss it and get frustrated. |
| ✗ Icon buttons of 24 px with 4 px gaps | Neighbouring targets are hit by mistake, especially on touch screens. |
| ✗ Primary call to action as a text link | Small target and weak visual signal — fewer clicks. |
How to check Fitts's Law on your website
Open your page in the browser's developer tools, switch to a phone viewport and inspect the size of each button and link. Anything below 44 px in either direction on a primary control is worth fixing; anything below 24 px without spacing fails WCAG 2.2 AA.
Lensux opens your page in a real browser on desktop and mobile, measures every interactive element and lists the ones under 44 px with their size and position. Links inside sentences are excluded, as WCAG allows.
Frequently asked questions
What is a good button size according to Fitts's Law?
Fitts's Law doesn't set a number, it says bigger is faster. In practice, 44 × 44 px is the recommended minimum for primary controls on touch screens, and WCAG 2.2 requires at least 24 × 24 px (or enough spacing) at level AA.
Does Fitts's Law apply to touch screens?
Yes. Research has extended it to finger input. Because a fingertip is much less precise than a mouse pointer and covers what it touches, touch targets need to be larger than mouse targets.
Is 44 px or 48 px better?
Both are good targets: 44 pt comes from Apple's guidelines and 48 dp from Android's. Choose one for your design system and apply it consistently; either is far better than the 24 px minimum.
Sources
- 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
- MacKenzie, I. S. (1992). Fitts' law as a research and design tool in human–computer interaction. Human–Computer Interaction, 7(1), 91–139
- W3C — Understanding WCAG 2.2: Target Size (Minimum)
- Apple — Human Interface Guidelines: Accessibility
- Laws of UX — Fitts's Law