Fitts Law is a predictive model of human movement that describes the time required to rapidly move to a target area as a function of the ratio between the distance to the target and the width of the target. In simpler terms, the larger and closer an interactive element is, the faster and easier it is for users to interact with it.

Fitts’s Law can be expressed mathematically as:

T = a + b log2 (D/W + 1)

where T is the time to complete the movement, a and b are constants, D is the distance to the target, and W is the width of the target.

This principle, first described by psychologist Paul Fitts in 1954, has become a cornerstone of interaction design and user interface development. It helps designers understand and predict how users will interact with their designs, making it easier to create interfaces that are both efficient and enjoyable to use.

Why is Fitts’s Law important?

Fitts’s Law is important because it provides designers with scientific guidance for creating more usable and accessible interfaces. By understanding and applying this principle, designers can:

Increase usability: users can more easily interact with buttons, links, and other interactive elements without frustration or errors
Improve accessibility: larger touch targets benefit all users, especially those with motor impairments or using mobile devices
Enhance efficiency: reducing the time and effort required to complete actions leads to faster task completion
Reduce errors: appropriately sized and spaced targets minimize accidental clicks and misclicks

How to apply Fitts’s Law?

Fitts’s Law can be applied by following these steps:

Make important actions prominent: place frequently used buttons and controls where they’re easy to reach, such as at the edges or corners of screens
Size targets appropriately: ensure interactive elements are large enough to be easily clicked or tapped, with minimum sizes of 44x44 pixels for touch targets (following WCAG guidelines)
Provide adequate spacing: add sufficient space between interactive elements to prevent accidental clicks and make selection easier
Position elements strategically: place related actions close together to reduce mouse or finger travel distance
Test with real users: validate your design decisions through usability testing, paying special attention to how users interact with buttons, links, and other interactive elements

These steps are not fixed or rigid; you can iterate and refine them as needed. You can also use different tools and techniques to support each step, such as heat maps, click tracking, and usability testing sessions.

Examples of Fitts’s Law

Fitts’s Law has been applied by many successful products and interfaces across different domains and industries. Here are some examples:

Mobile operating systems: iOS and Android both require minimum touch target sizes (44x44 pixels and 48x48 pixels respectively) to ensure users can reliably tap buttons and links
Video streaming platforms: YouTube places large, easy-to-click play/pause buttons in the center of videos, making video control effortless
E-commerce websites: Amazon uses large “Add to Cart” and “Buy Now” buttons that are easy to click, reducing friction in the purchase process
Operating system interfaces: macOS places the menu bar at the very top edge of the screen, making it infinitely selectable (users can throw their cursor to the top without overshooting)

Conclusion

Fitts’s Law is a fundamental principle in interaction design that states the time required to move to a target area is a function of the distance to and size of the target. It helps designers create more usable, accessible, and efficient interfaces by providing scientific guidance for sizing and positioning interactive elements. By applying Fitts’s Law, designers can significantly improve the user experience and reduce interaction friction.