System Usability Scale (SUS): The Complete Guide to Measuring User Experience

What Is the System Usability Scale?

The System Usability Scale (SUS) is a standardized questionnaire designed to measure how easy a product or system is to use. Developed by John Brooke in 1986, SUS has become one of the most widely used and trusted tools in the field of user experience research.

The brilliance of SUS lies in its simplicity. Despite being developed over 35 years ago, the questionnaire remains largely unchanged and continues to provide valuable insights into perceived usability. Unlike usability testing methods that focus on specific tasks or issues, SUS provides a high-level overview of user satisfaction with the overall system.

SUS is valued for several key reasons:

• Standardization: The consistent questionnaire allows for benchmarking across products and over time
• Brevity: Just 10 questions mean minimal burden on participants
• Reliability: Decades of research have validated SUS as a dependable measure
• Versatility: Applicable to websites, software, hardware, and even physical products

When users find a product easy to use, they are more likely to complete tasks, return to the product, and recommend it to others. Conversely, poor usability creates friction that drives users away. SUS provides a quantifiable way to understand how users perceive your interface.

For organizations focused on conversion rate optimization, measuring usability is essential--users who struggle with your interface are unlikely to convert, regardless of how compelling your offers may be.

The 10 Standard SUS Questions

The SUS questionnaire consists of 10 statements that participants rate on a 5-point Likert scale, from "Strongly Disagree" to "Strongly Agree." Five questions are positively worded (odd-numbered), and five are negatively worded (even-numbered), which helps reduce response bias.

The 10 standard SUS questions are:

Odd questions contribute positively when rated highly, while even questions contribute positively when rated low. This counterintuitive aspect is often misunderstood.

For effective landing page optimization, understanding how users perceive your site through structured feedback like SUS is invaluable for identifying friction points that impact conversion rates.

How to Calculate SUS Scores

Calculating the SUS score follows a specific formula that accounts for the mixed wording of the questions.

Step-by-Step Scoring Process

Step 1: Transform individual responses

For each participant's responses:

• For odd-numbered questions (1, 3, 5, 7, 9): Subtract 1 from the response score
• For even-numbered questions (2, 4, 6, 8, 10): Subtract the response score from 5

Step 2: Sum all transformed scores

Add up all 10 transformed values for each participant.

Step 3: Calculate the final SUS score

Multiply the sum by 2.5 to get the final SUS score out of 100.

Formula: SUS Score = (Sum of transformed scores) × 2.5

The Average SUS Score

The average SUS score across a wide range of industries and products is approximately 68. Scores above this threshold are generally considered above average, while scores below 68 suggest below-average usability.

Best Practices for Administering SUS

Timing of Administration

The SUS should be administered at a specific point in the research session:

1. After participants complete realistic tasks with the product
2. Before any extensive debriefing or discussion
3. While their experience is still fresh but they've had meaningful interaction

Administering SUS immediately after task completion captures genuine experience without contamination from post-test discussions.

Sample Size Recommendations

• Exploratory/formative testing: 8-12 participants can reveal trends
• Benchmarking studies: 20+ participants provide greater statistical reliability
• Comparative studies: Aim for similar sample sizes across conditions

Question Customization

While the standard SUS questions are effective, you can adapt them:

• Maintain the core structure and intent of each question
• Avoid changing wording that alters the meaning
• Consider adding supplementary questions if you need specific feedback

Any modifications should be validated to ensure they don't compromise reliability.

For teams implementing AI-powered interfaces, SUS provides a standardized way to measure whether intelligent features are actually usable by end users.

Common Mistakes to Avoid

1. Administering at the Wrong Time

Giving SUS before users have meaningful interaction, or after extensive debriefing, compromises validity. Administer it at the recommended point in your research session.

2. Ignoring the Scoring Formula

Simply averaging responses without applying the transformation formula produces incorrect results. Always follow the proper scoring methodology.

3. Over-Interpreting Small Differences

Small differences in SUS scores (within 5 points) may not be meaningful. Focus on substantial differences and trends rather than minor fluctuations.

4. Using SUS in Isolation

SUS should complement, not replace, other usability research methods. Combine it with qualitative research to get the complete picture.

When to Use SUS

SUS is particularly valuable for:

• Establishing a baseline: Measure current usability before making improvements
• Comparing versions: Evaluate whether new designs perform better than old ones
• Benchmarking: Compare your product against competitors or industry standards
• Tracking progress: Monitor usability trends over multiple releases
• Communicating value: Share usability outcomes with stakeholders in clear terms

For user flow optimization, SUS provides quantitative data to validate whether your redesigned flows actually improve the perceived usability of your digital products.

Sources

1. MeasuringU - Measuring Usability with the System Usability Scale (SUS)
2. Dovetail - Implementing and Using the System Usability Scale (SUS): A Guide
3. UXArmy - System Usability Scale (SUS) In 2025