Fitts’s Law
For aimed movement, acquisition time tends to increase with target distance and decrease with target size along the movement axis.
Direct explanation
What it means
A large nearby control is generally faster to acquire than a small distant one. The model captures a speed-accuracy tradeoff, not simply a rule to make everything large.
Target geometry affects speed, error, fatigue, and access. It provides a testable reason to prioritize important controls and protect them from cramped layouts.
Research anchor: Paul M. Fitts, 1954
Research and interpretation
What the research found
Movement time in reciprocal aimed movements varied predictably with movement amplitude and target width.
Fitts-style models are extensively replicated and adapted across devices, with formulation and parameters fitted to the technique and task.
Popular advice versus careful use
Common shorthand: Make every button large and put important actions in a corner.
Optimize effective target geometry for the actual input method, frequency, and error cost, then validate with users.
MT = a + b · log₂(1 + D/W)Target acquisition lab
Compare size and distance
Start each trial from the fixed origin, then select the target. The sequence rotates through large, small, near, and far targets.
Current setup: Large and near
Ready at origin
This demonstration reveals the mechanism. A few personal trials are not a controlled experiment and should not be treated as evidence about an individual.
How designers apply it
- 01
Give primary mobile actions adequate target size and separation.
- 02
Place frequent desktop actions near the current focus or stable edges.
- 03
Increase the effective hit area of icons without inflating their visual weight.
Original UI examples
mobile map
Map recenter control
A 24 px icon sits inside a 48 px touch target near the thumb zone.
The control remains visually quiet while its effective width and spacing improve acquisition.
desktop editor
Timeline editor
Trim handles widen on hover and keyboard focus without moving the media boundary.
A difficult precise target becomes easier while expert keyboard adjustment remains available.
Confirming a mobile delivery
Less effective
Use a small text link beside another destructive link.
Better fit
Provide separated, comfortably sized controls with clear labels and a reversible confirmation path.
Larger is not sufficient if labels or consequences are unclear.
Limits and failure modes
The law models aimed movement after a target is identified; it does not include visual search, comprehension, or consequence evaluation.
Common mistakes
- Applying the formula to visual search or decision time.
- Assuming infinite screen-edge benefits apply to touchscreens.
When it should not dominate
- To predict how long users take to understand a label.
- As the sole basis for sizing rare destructive actions.
Responsible use
Accessibility
Meet WCAG target-size requirements and also test spacing, zoom, tremor, switch access, keyboard paths, and alternative input; minimum conformance is not optimal usability.
On mobile: Account for finger occlusion, reach, posture, and adjacent-target errors.
Ethics and context
Use the principle to reduce unnecessary effort and improve comprehension. Validate the result with representative users rather than treating a general tendency as a universal instruction.
On desktop: Edges and corners can increase pointer-effective width only when the control truly reaches the boundary.
Designer checklist
- Inspect the actual interactive hit box.
- Measure errors across input methods.
- Protect frequent and consequential targets from crowding.
Check your understanding
Which statement applies Fitts’s Law most carefully?
Practice and continue learning
See ideas in real interfaces
These external interfaces are editorial references, not examples created to demonstrate this principle.
Sources and further reading
- 01Open source
The Information Capacity of the Human Motor System in Controlling the Amplitude of Movement
Paul M. Fitts · 1954 · Journal of Experimental Psychology
Primary research · primary or original source
- 02Open source
Understanding Success Criterion 2.5.8: Target Size (Minimum)
W3C Accessibility Guidelines Working Group · 2023 · W3C
Standard
Continue the graph
Related principles
law
Steering Law
Movement time through a constrained path tends to increase with path length and decrease with path width along the trajectory.
principle
Stimulus-Response Compatibility
Responses are generally faster and less error-prone when the relationship between a signal and its control matches learned or spatial expectations.
heuristic
Doherty Threshold
Rapid, meaningful system feedback helps sustain an interactive rhythm; the often-cited 400 ms value is historical guidance, not a universal boundary.