What is Fitts's law?
Fitts's law began as a measurement of arm movements to targets in 1954. It reached screens through a 1978 device comparison and a 1992 revision of its index of difficulty.
Answer to the question
What is Fitts's law?
Fitts's law says that the time to reach a target depends on its distance and its width. Paul Fitts reported it in 1954 from tapping and transfer tasks. A 1978 study placed the mouse on the same scale, and a 1992 paper revised its index of difficulty for screens.
Key takeaways
- Fitts (1954) tested tapping, disc transfer and pin transfer tasks and reported a best rate of about 10 to 12 bits per second.
- Card and colleagues (1978) found the mouse fastest of four text-selection devices, with mouse and joystick positioning times fitting Fitts's law.
- MacKenzie (1992) proposed an index of difficulty that adds one inside the logarithm, so overlapping targets cannot produce a negative value.
- MacKenzie's review of six pointing studies found that performance measures varied enormously, which made cross-study comparison difficult.
- Laws of UX applies the law to touch targets, form buttons and screen edges, which are design extensions of the original experiments.
Fitts's law says that the time needed to reach a target depends on how far away the target is and how wide it is. Paul Fitts measured that relationship in a laboratory, decades before anyone pointed at a screen. Distance and width trade against each other, so a far target can be as quick to hit as a near one if it is proportionally wider. Jon Yablonski's Laws of UX lists Fitts's law among ten design heuristics, as guidance for sizing and spacing touch targets.
A Law First Measured With Stylus, Discs and Pins
Fitts was an American psychologist whose wartime work on aircraft controls is told in When the Design Is to Blame, Not You. His paper in the Journal of Experimental Psychology asked a different question: whether the human motor system has a roughly fixed capacity for handling information when it controls the amplitude of a movement. He took the measure from communication engineering and treated each movement as a message worth a number of bits.
Three tasks supplied the data. In the tapping task Fitts ran, sixteen right-handed college men struck two target plates in turn with a stylus, at amplitudes from two to sixteen inches and target widths from a quarter of an inch to two inches. In a disc transfer task, a separate group of sixteen men moved washers from one pin to another. In a pin transfer task, twenty students, ten men and ten women, moved pins between sets of holes. Errors were not permitted in the two transfer tasks, so the tolerance was fixed and the question was how long a movement of a given amplitude took.
Fitts defined difficulty in bits per response from the ratio of amplitude to tolerance, then divided it by average movement time to obtain a rate in bits per second. The rate stayed roughly constant across a central range of amplitudes and tolerances and fell off outside it. Across the three tasks the best performance was about ten to twelve bits per second. A constant rate means movement time rises in step with difficulty, which is the straight-line relationship later researchers kept fitting.
In its modern form Fitts's law predicts movement time as one constant plus a second constant multiplied by the logarithm of a ratio of distance to width. The two constants are fitted separately for each device and task. The logarithm gives the law its shape. Doubling a target's width removes the same amount of difficulty at any distance, and doubling the distance adds the same amount at any width.
The ratio carries the prediction, not either quantity alone. A small button beside the pointer and a large button across the screen can sit at similar difficulty, and a designer who enlarges a target or shortens the path is acting on the same ratio. Fitts's own index used the logarithm of twice the amplitude over the tolerance. The version common in computing differs slightly, and its history comes later in this article.
The rate in bits per second, which the field came to call the index of performance, gave researchers a single number for comparing tasks and devices. It is also the reciprocal of the slope of the fitted line, which links the two ways of describing the same data.
Computing Takes Up the Law and Ranks the Mouse
Computing took up Fitts's law in 1978. Stuart Card and his co-authors compared four ways of selecting text on a display in the journal Ergonomics: a mouse, a rate-controlled isometric joystick, step keys and text keys. The authors reported that the mouse was fastest on all measures and made the fewest errors. Positioning times for the mouse and the joystick fitted Fitts's law, and the mouse's slope was close to values found in other eye-hand tasks, which the authors read as a sign that its positioning time was near the minimum achievable. For the key-based devices, positioning time was proportional to the number of keystrokes needed.
The study turned a laboratory result about arm movements into a way to rank interfaces. Devices as different as a mouse and a joystick could be set on one scale because Fitts's law described both, and the fitted constants showed how they differed. Keys fell outside the scale because their cost was counted in keystrokes, not in distance and width.
Rewriting the Index of Difficulty for the Screen
I. Scott MacKenzie's 1992 paper in Human-Computer Interaction is the standard account of how the law was adapted. It sets the model in its historical and theoretical context and examines what happens when observations keep departing from its predictions. It proposes a revised index of task difficulty that it argues is more soundly based in information theory than Fitts's original. The revised form takes the logarithm of the distance-to-width ratio plus one, which cannot return a negative difficulty when targets overlap, a case the original form can produce.
The same paper reviews six studies that used Fitts's law to predict cursor-positioning and selection times with mice, trackballs, joysticks, touchpads, helmet-mounted sights and eye trackers. Their performance measures varied enormously, which made comparison across studies difficult, and the paper lists the sources of experimental variation that might explain the spread. Anyone quoting a throughput figure for a device is quoting a number that depends on how the test was run.
A 2004 review in the International Journal of Human-Computer Studies carries the subtitle perspectives on 27 years of Fitts's law research in human-computer interaction and the title Towards a standard for pointing device evaluation. The title names the aim: device tests that can be compared between laboratories.
What Designers Take From the Ratio of Distance and Width
Laws of UX turns the result into advice about layout. Its examples include placing a submit button close to the last field of a form and treating the edge of a screen as a target the pointer cannot overshoot. Those are design extensions of Fitts's law. The experiments above measured arm and hand movements to targets of fixed distance and width, and the screen-edge case rests on the pointer stopping at the display boundary, which makes the target effectively deep. The book sets the same logic beside touch targets, where size and spacing decide how reachable a control is.
Within the terms of Fitts's law, two levers exist. A designer can make the target wider, or shorten the path to it, and each acts on the same ratio. Spacing is the third consideration the book adds, on the reasoning that controls placed apart are less likely to be hit by mistake.
| Study | Task or devices | What it reported |
|---|---|---|
| Fitts, 1954 | Tapping, disc transfer, pin transfer | A best rate of about ten to twelve bits per second |
| Card and colleagues, 1978 | Mouse, joystick, step keys, text keys | Mouse fastest, positioning times fitted the law |
| MacKenzie, 1992 | Review of six pointing studies | Revised index of difficulty, wide variation in measures |
The memory costs of crowded screens are covered in What is cognitive load theory? and What Cognitive Load Costs at Checkout. The cockpit studies behind it appear in When the Design Is to Blame, Not You.
Common questions
What is Fitts's law?
Fitts's law is the finding that the time needed to reach a target rises with the ratio of its distance to its width. It is used to compare pointing devices and to reason about the size and placement of buttons.
Who discovered Fitts's law?
Paul Fitts, an American psychologist, published it in the Journal of Experimental Psychology in 1954. His experiments used tapping, disc transfer and pin transfer tasks to test whether the motor system handles information at a roughly constant rate.
What is the formula for Fitts's law?
Movement time equals one constant plus a second constant times the logarithm of a distance-to-width ratio. The constants are fitted for each device. The form common in computing adds one to the ratio inside the logarithm, following MacKenzie's 1992 revision.
Does Fitts's law apply to touchscreens?
The studies discussed here covered tapping and transfer tasks, a mouse, a joystick, trackballs, touchpads and other pointing devices. Guidance on touch targets comes from design practice, such as Laws of UX, which applies the same distance and size logic to fingers.
Sources
Yablonski, J. (2024). Laws of UX: Using Psychology to Design Better Products & Services (2nd ed.). Sebastopol, CA: O'Reilly Media. Seed book; vault hub Books/Laws of UX.
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. doi:10.1037/h0055392 https://doi.org/10.1037/h0055392
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. doi:10.1080/00140137808931762 https://doi.org/10.1080/00140137808931762
MacKenzie, I. S. (1992). “Fitts' law as a research and design tool in human-computer interaction.” Human-Computer Interaction, 7(1), 91-139. doi:10.1207/s15327051hci0701_3 https://doi.org/10.1207/s15327051hci0701_3
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. doi:10.1016/j.ijhcs.2004.09.001 https://doi.org/10.1016/j.ijhcs.2004.09.001