Fitts's Law¶

Status: established
Last updated: 2026-06-03
Sources: Fitts Jep54.Pdf, Lawsofux.Pdf
Tags: [ux-design, design-principles, heuristics, fitts-law, motor-control, target-acquisition, index-of-difficulty, information-theory, touch-targets]

Summary¶

Fitts's law states that the time to move rapidly to a target is a function of the distance to the target and its size: targets that are larger and closer are faster to acquire. Fitts (1954) derived the law by treating the human motor system as an information channel of limited capacity and defined an index of difficulty, measured in bits, that combines movement amplitude and target tolerance. Across three movement tasks he found that the rate of information output stays approximately constant — about 10 to 12 bits per second — over a wide range of amplitudes and accuracies. In UX practice the law becomes a design rule: make interactive elements large enough and place frequent actions close to where the pointer or attention already sits (Yablonski, 2024). The motor-control science also grounds the response-execution material in Selection And Control Of Action.

Body¶

Context¶

This article rests on two sources at different altitudes. Fitts (1954), in a Journal of Experimental Psychology paper, applies information theory to rapid aimed movement and reports three experiments measuring how movement time depends on target distance and size; the RAW file is the public-domain 1992 APA Centennial reprint of that paper. Yablonski (2024), in the Fitts's Law chapter of Laws of UX, takes the resulting law and works it into interface guidance, from form layout to touchscreens and in-car displays. Within this knowledge base the article is the design-heuristic counterpart to the response-execution theory in Selection And Control Of Action, which carries Fitts's law as motor-control science; it connects to Information Processing (the action-selection stage) and the wartime origins recounted in Human Factors Ergonomics Discipline, and it is the applied sibling of Hicks Law, with which it is often paired.

Key Points¶

Fitts (1954) frames the motor system as a communication channel. Drawing on Shannon's theory and Miller's reading of it, he treats the amplitude, duration, and variability of movements as interrelated rather than independent, and argues that earlier work conflicted because it overlooked that interrelation; the information capacity of the motor system is inferred from the variability of movements a participant tries to keep uniform (PDF pp. 1–2, orig. pp. 262–263).

The central construct is the index of difficulty. Fitts defines it as Id = −log₂(Ws / 2A) = log₂(2A / Ws) bits per response, where Ws is the target tolerance (width) and A is the average movement amplitude (distance). The choice of 2A for the denominator is deliberate: it keeps the index positive in all practical cases and adds one bit per response, so the index corresponds to the number of successive fractionations needed to specify the target out of a range twice the amplitude (PDF pp. 5–6, orig. pp. 266–267). A companion index of performance expresses the result as a rate — the average information per movement divided by the time per movement (PDF pp. 6–7, orig. pp. 267–268).

Three experiments test the thesis, each varying amplitude and tolerance through geometric steps. In reciprocal tapping, participants moved a stylus back and forth between two target plates under 16 conditions and with two stylus weights; in disc transfer they moved plastic discs between pins; in pin transfer they moved pins between holes (PDF pp. 3–5, orig. pp. 264–266). Across all three tasks, movement time increased as amplitude grew and as the tolerance was tightened, while the rate of information output stayed approximately constant over a wide working range — about 10 to 12 bits per second — falling off only outside an optimum range. The conditions studied spanned 1 to 10 bits per response (PDF pp. 7–8, orig. pp. 268–269).

Fitts (1954) reads this constant rate as a fixed information-handling capacity of the motor system together with its visual and proprioceptive feedback loops, which reconciles otherwise conflicting reports on movement duration (PDF pp. 7–8, orig. pp. 268–269).

Yablonski (2024) carries the law into interface design. He states it as: the time to engage with an object is relative to its size and the distance to it, traces it to Fitts's 1954 work and the index of difficulty, and draws the design implications directly (PDF pp. 35–46, orig. pp. 15–26). Touch targets should be large enough to select accurately, and the distance between a likely starting point and frequently used controls should be minimised; he illustrates this with form labels and inputs, with Apple CarPlay's generous spacing, and with "infinite" targets such as screen edges that cannot be overshot. He notes the law is not limited to mouse or touch input and extends to spatial-computing interfaces, and that mobile interfaces are especially subject to it because of their limited input precision (PDF pp. 35–46, orig. pp. 15–26).

Conclusion¶

Fitts (1954) and Yablonski (2024) describe the same relationship at two altitudes and agree on its shape: movement time rises with distance and falls with target size, captured quantitatively by the index of difficulty. Fitts supplies the empirical and information-theoretic foundation — a roughly constant motor-channel capacity of 10 to 12 bits per second — while Yablonski turns it into a concrete design lever: size and proximity govern how quickly users can act, so interactive elements should be appropriately large and positioned close to where the user's attention and pointer already are. The formal motor-control account is developed further in Selection And Control Of Action.

References¶

Fitts, P.M. (1954) 'The information capacity of the human motor system in controlling the amplitude of movement', Journal of Experimental Psychology, 47(6), pp. 381–391. doi: 10.1037/h0055392. fitts1954information

Yablonski, J. (2024) Laws of UX: Using Psychology to Design Better Products & Services. 2nd edn. Sebastopol, CA: O'Reilly Media. yablonski2024lawsux

Open Questions¶

How does Fitts's law extend to spatial-computing, gesture, and gaze interfaces, where "distance" and "size" are less well defined (Yablonski, 2024)?
How should Fitts's-law targeting gains be balanced against the choice-reduction pressure of Hicks Law when laying out dense control panels?
Later refinements (the Shannon formulation, Id = log₂(2A/W + 1)) post-date Fitts (1954); is a held source for that revision worth acquiring to extend this article?