Fitts’s Law

This article is a term explanation of Fitts’s Law – including formula, practical rules, and exam questions.

In a Nutshell

Fitts’s Law describes: The movement time to hit a target depends on target distance and target size. Large, nearby targets are faster to hit.

Compact Technical Description

Base formula:

MT = a + b * log2(D/W + 1)

• MT: Movement time
• D: Target distance
• W: Target size (width along movement axis)

log2(D/W + 1) is the Index of Difficulty (ID).

Practical implications:

• Place primary actions large and near
• Edges/corners are ergonomically favorable (“cursor stops”)
• Touch: larger target sizes and sufficient distances (typically 44–48px)

Exam-Relevant Key Points

• Formula + meaning of D/W
• Large targets / small distance reduce MT
• Edges/corners as “large targets”
• Touch: critical actions not too close together
• Metrics: MT, error rate, (optional) throughput
• Documentation: mockups with target dimensions + test protocol

Practical Example

Goal: "Save" faster to reach than "Cancel"
- Save large + near typical path
- Cancel smaller/further away
- Destructive actions spatially separated

Typical Exam Questions (with Short Answer)

1. Fitts’s formula? MT = a + b * log2(D/W + 1).
2. What is ID? log2(D/W + 1) as difficulty measure.
3. Why are edges/corners good? Cursor cannot “overshoot”.

Most Important Sources

1. https://de.wikipedia.org/wiki/Fitts%E2%80%99sches_Gesetz
2. https://www.nngroup.com/articles/fitts-law/