Formula and meaning
In reciprocal tapping experiments, Fitts (1954) found that movement time depends on the ratio of distance (D) to target width (W). The common Shannon form is MT = a + b·log₂(D/W + 1); the log term is the index of difficulty in bits. Doubling a target's size or halving its distance removes roughly one bit. Like Hick's law, the idea treats people as channels in the sense of information theory.
From the lab to HCI
Card, English and Burr (1978) showed the mouse fitted Fitts's law almost perfectly, which helped justify its commercial adoption. MacKenzie (1992) popularised the Shannon form and effective-width correction, and Soukoreff and MacKenzie (2004) proposed a throughput standard for comparing devices. Screen edges act as infinitely wide targets for a mouse because the cursor stops there, which is why menu bars sit at the edge.
Mobile touch and limits
Fingers are less precise than cursors. Bi, Li and Zhai (2013) separated the spread of finger contact in their FFitts model, improving predictions for small targets. On mobile, avoiding mis-taps often matters more than raw speed, which links to touch target size and the thumb zone. Fitts's law covers single aimed movements; following narrow cascading menus is better described by the steering law of Accot and Zhai (1997). Large targets especially help users with tremor, connecting the law to web accessibility.
FAQ
- What is Fitts's law in simple terms?
- The farther and smaller a target is, the longer it takes to hit and the more errors people make, so frequent actions should be big and close.
- Does it apply to touchscreens?
- The trend holds, but fingers are imprecise, so designers combine corrected models such as FFitts with generous touch areas.
Sources reviewed: 2026-10-27
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), 381–391. The original paper that predicted movement time from distance and width; start here for the formula.
- Card, S. K., English, W. K., & Burr, B. J. (1978). Evaluation of mouse, rate-controlled isometric joystick, step keys, and text keys for text selection on a CRT. Ergonomics, 21(8), 601–613. The experiment that used Fitts's law to explain why the mouse won, the first big HCI application.
- MacKenzie, I. S. (1992). Fitts' law as a research and design tool in human-computer interaction. Human–Computer Interaction, 7(1), 91–139. The review that popularised the Shannon formulation and effective-width correction used in practice.
- Soukoreff, R. W., & MacKenzie, I. S. (2004). Towards a standard for pointing device evaluation, perspectives on 27 years of Fitts' law research in HCI. International Journal of Human-Computer Studies, 61(6), 751–789. Proposes a throughput standard for fairly comparing pointing devices.
- Accot, J., & Zhai, S. (1997). Beyond Fitts' law: Models for trajectory-based HCI tasks. In Proceedings of the ACM SIGCHI Conference on Human Factors in Computing Systems (pp. 295–302). ACM. Introduces the steering law for tunnel-like paths such as cascading menus.
- Bi, X., Li, Y., & Zhai, S. (2013). FFitts law: Modeling finger touch with Fitts' law. In Proceedings of the SIGCHI Conference on Human Factors in Computing Systems (pp. 1363–1372). ACM. The FFitts model adds finger imprecision, useful when sizing mobile buttons.
- Welford, A. T. (1968). Fundamentals of skill. Methuen. A classic on motor skill that discusses variants of Fitts's formula.