Fitts’s Law
This is a glossary entry for Fitts’s Law, including the formula, practical guidelines, and exam questions.
In a Nutshell
Fitts’s Law states: the time required to move to and select a target depends on target distance and target size. Larger targets that are closer are faster to acquire.
Formal Definition
Core formula:
MT = a + b * log2(D/W + 1)MT: movement timeD: distance to targetW: target width (along the direction of movement)
log2(D/W + 1) is the Index of Difficulty (ID).
Practical implications:
- Place primary actions large and close
- Screen edges and corners are ergonomically favorable (“cursor stops”)
- Touch: larger targets and adequate spacing (typically 44–48px)
Why Fitts’s Law Matters
Fitts’s Law is fundamental to software ergonomics and Human-Computer Interaction (HCI). It guides you in designing interfaces that are both efficient and usable. By optimizing target size and distance, you reduce user movement time and increase satisfaction with your application.
The law appears regularly in software ergonomics and UI design exams. It’s core knowledge under ISO 9241 and an essential tool for evaluating and refining user interfaces.
Exam-Focused Concepts Explained
Formula and the meaning of D/W
The formula MT = a + b * log2(D/W + 1) models movement time. The ratio D/W (distance divided by target width) is critical. A large ratio means either long distance or a small target, which increases movement time. A small ratio means short distance or a large target, which decreases movement time.
Large targets and short distances reduce MT
When you make targets large and place them near the starting position, the D/W ratio shrinks. This produces a smaller Index of Difficulty and therefore shorter movement time. This is why frequently used buttons should be large and prominently positioned.
Edges and corners as “infinite targets”
Screen edges and corners act like infinitely large targets. The cursor cannot overshoot them—the screen boundary stops it. This makes these regions particularly ergonomic for critical actions like menus or primary buttons.
Touch: don’t cluster critical actions
On touch interfaces, users lack precise cursor positioning. Critical actions like “Delete” or “Cancel” should not sit too close to frequently used controls. Typical touch targets measure 44 to 48 pixels to prevent accidental activation.
Metrics: MT, error rate, (optional) throughput
Movement time is your primary metric. Additionally, measure error rate—how often the target is missed. Throughput combines speed and accuracy into a single score and is common in research.
Documentation: mockups with target dimensions and test protocol
In practice, document your UI decisions with mockups that show target dimensions. A test protocol records how you measured movement time and error rate. This documentation ensures your design choices are defensible and reproducible.
Exam-Relevant Topics
- Formula and the meaning of D/W
- Large targets and short distances reduce MT
- Edges and corners as “infinite targets”
- Touch: don’t cluster critical actions
- Metrics: MT, error rate, (optional) throughput
- Documentation: mockups with target dimensions and test protocol
Practical Example
Goal: make "Save" faster to reach than "Cancel"
- Save: large and close to the natural workflow
- Cancel: smaller and farther away
- Separate destructive actions spatially
Common Exam Questions (with Brief Answers)
- Fitts’s formula? MT = a + b * log2(D/W + 1).
- What is ID? log2(D/W + 1) as a difficulty measure.
- Why are edges and corners favorable? The cursor cannot overshoot.