page_title: Fitts's law
section_title: Implications for UI design
hierarchical_section_title: Fitts's law / Implications for UI design
caption_attribution_description: English: Example of a radial menu. The sectors have English labeling.
context_page_description: Fitts's law is a predictive model of human movement primarily used in human–computer interaction and ergonomics. This scientific law predicts that the time required to rapidly move to a target area is a function of the ratio between the distance to the target and the width of the target. Fitts's law is used to model the act of pointing, either by physically touching an object with a hand or finger, or virtually, by pointing to an object on a computer monitor using a pointing device.
  Fitts's law has been shown to apply under a variety of conditions; with many different limbs, manipulanda, physical environments, and user populations.
context_section_description: Multiple design guidelines for GUIs can be derived from the implications of Fitts's law.  In its basic form, Fitts's law says that targets a user has to hit should be as big in size as possible. This derived from the W parameter. More specifically, the effective size of the button should be as big as possible, meaning that their form has to be optimized for the movement direction of the user onto the target.
  Layouts should also group functions that are used commonly with each other close. Optimizing for the D parameter in this way allows for smaller travel times.
  Placing layout elements on the four edges of the screen allows for infinitely large targets in one dimension and therefore present ideal scenarios. As the user's pointer will always stop at the edge, they can move the mouse with the greatest possible speed and still hit the target. The target area is effectively infinitely long along the movement axis. Therefore, this guideline is called “Rule of the infinite edges”. The use of this rule can be seen for example in MacOS, which places the menu bar always on the top left edge of the screen instead of the current programs windowframe.
  This effect can be exaggerated at the four corners of a screen. At these points two edges collide and form a theoretically infinitely big button. Microsoft Windows places its "Start" button in the lower left corner and Microsoft Office 2007 uses the upper left corner for its "Office" menu. These four spots get called the “magic corners”.
  MacOS places the close button on the upper left side of the program window and the menu bar fills out the magic corner with another button.
  A UI that allows for pop-up menus rather than fixed drop-down menus reduces travel times for the D parameter. The user can continue interaction right from their mouse position and don't have to move to a different preset area. Many operating systems use this when displaying right-click context menus. As the menu starts right on the pixel which the user clicked on, this pixel is referred to as the "magic" or "prime pixel".
  James Boritz et al. (1991) compared radial menu designs. In a radial menu all items have the same distance from the prime pixel. The research suggests that in practical implementations the direction in which a user has to move their mouse has also to be accounted for. For right-handed users selecting the left most menu item was significantly more difficult than the right-sided one. No differences were found for transitions from upper to lower functions and vice versa.
caption: Radial menu
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