Ever wonder how companies measure whether their apps or websites are actually easy to use? Enter the System Usability Scale—a simple yet powerful tool that’s been the gold standard for usability testing since the 1980s.
What Is the System Usability Scale?
The System Usability Scale (SUS) is a widely used, reliable, and quick method for measuring the perceived usability of a product, system, or interface. Developed in 1986 by John Brooke at Digital Equipment Corporation, the SUS has become one of the most popular usability assessment tools across industries—from software and mobile apps to medical devices and automotive interfaces.
Origins and Development of SUS
The SUS was born out of a need for a lightweight, yet effective way to evaluate usability without requiring extensive user testing or complex metrics. At the time, usability professionals were using a variety of inconsistent and often subjective methods. Brooke aimed to create a standardized questionnaire that could be applied across different systems and yield comparable results.
- Originally developed for internal use in usability labs.
- First published in 1986 in a technical report.
- Designed to be technology-agnostic—usable for software, hardware, or hybrid systems.
Despite its age, the SUS remains relevant because of its simplicity and robustness. It doesn’t require users to have technical knowledge, and it can be administered quickly after a user completes a task.
How the SUS Works: A Simple 10-Item Questionnaire
The SUS consists of 10 statements, each rated on a five-point Likert scale ranging from “Strongly Disagree” to “Strongly Agree.” The statements alternate between positive and negative phrasing to reduce response bias. After users complete a task or interact with a system, they answer questions like:
- I think that I would like to use this system frequently.
- I found the system unnecessarily complex.
- I thought the system was easy to use.
The scoring is standardized: odd-numbered items are scored by subtracting 1 from the user response (1–5), while even-numbered items are reverse-scored (5–1). The total score is then multiplied by 2.5 to convert it to a 0–100 scale.
“The beauty of the SUS lies in its simplicity. You don’t need a PhD in statistics to use it, but it still gives you meaningful, actionable data.” — Jakob Nielsen, Nielsen Norman Group
Why the System Usability Scale Is Still Relevant Today
Despite the emergence of newer usability metrics and advanced analytics tools, the System Usability Scale remains a cornerstone in UX research. Its enduring popularity stems from its balance of reliability, ease of use, and broad applicability.
Universality Across Platforms and Industries
One of the biggest strengths of the SUS is its versatility. Whether you’re testing a mobile banking app, a hospital patient portal, or a smart home device, the SUS can be applied with minimal adaptation.
- Used in tech, healthcare, automotive, finance, and education sectors.
- Validated across cultures and languages—translated into over 30 languages.
- Effective for both digital and physical interactive systems.
For example, a study published in International Journal of Human–Computer Interaction confirmed that SUS scores correlate strongly with user satisfaction across diverse user groups and technological contexts.
Speed and Efficiency in Usability Testing
In fast-paced development environments, time is critical. The SUS can be administered in under 5 minutes, making it ideal for agile teams and rapid prototyping cycles.
- Can be used after a single task or a full user session.
- Requires only 5–10 users to get reliable insights (Nielsen, 2000).
- Provides a quantitative score that’s easy to track over time.
This efficiency makes the SUS a favorite among UX researchers who need to deliver quick feedback without sacrificing data quality.
How to Administer the System Usability Scale Correctly
While the SUS is simple to use, administering it correctly is crucial for obtaining valid and reliable results. Missteps in timing, context, or interpretation can lead to misleading data.
Best Practices for Administering SUS
To get the most accurate results, follow these best practices:
- Administer after a task or session: Users should have hands-on experience with the system before answering.
- Keep it anonymous: Encourage honest feedback by not requiring names.
- Use neutral instructions: Avoid leading questions or biased language when introducing the survey.
- Collect demographic data separately: This helps in segmenting results without influencing SUS responses.
For example, if you’re testing a new e-commerce checkout flow, have users complete a purchase (or simulated purchase) before presenting the SUS questionnaire.
Common Mistakes to Avoid
Even experienced researchers can make errors when using the SUS. Here are some common pitfalls:
- Administering too early: Giving the SUS before users have meaningful interaction skews results.
- Modifying the wording: Changing the original statements invalidates the scoring model.
- Ignoring context: A low SUS score without qualitative feedback offers limited insight.
- Using it in isolation: SUS should complement, not replace, other usability methods like think-aloud protocols or task success rates.
As noted by Sauro and Lewis in their book Quantifying the User Experience, altering the SUS items—even slightly—can compromise its psychometric properties.
Scoring and Interpreting the System Usability Scale
One of the most appealing aspects of the System Usability Scale is its straightforward scoring method. However, understanding what the numbers mean is just as important as calculating them correctly.
Step-by-Step Scoring Guide
Here’s how to calculate a SUS score manually:
- For each participant, list their responses to the 10 items (1–5).
- For odd-numbered items: subtract 1 from the response (e.g., 5 becomes 4).
- For even-numbered items: subtract the response from 5 (e.g., 1 becomes 4).
- Sum all 10 adjusted scores.
- Multiply the total by 2.5 to get the final SUS score (0–100).
For example, if a user’s adjusted sum is 68, their SUS score is 68 × 2.5 = 85.
What Do the Scores Mean?
SUS scores are interpreted on a 0–100 scale, but there are established benchmarks:
- Below 50: Poor usability — significant redesign needed.
- 50–69: Marginal — acceptable but with notable issues.
- 70–79: Good — above average, minor improvements possible.
- 80–89: Excellent — highly usable, competitive advantage.
- 90–100: Outstanding — rare, near-perfect usability.
According to research by Sauro and Lewis, the average SUS score across thousands of studies is around 68, placing most systems in the “marginal to good” range.
“A SUS score of 70 is often seen as the threshold for ‘acceptable usability.’ Anything above that indicates a user-friendly system.” — Jeff Sauro, MeasuringU
Advantages of Using the System Usability Scale
The System Usability Scale has stood the test of time for good reason. Its advantages make it a go-to tool for usability professionals worldwide.
Reliability and Validity
Despite its brevity, the SUS is both reliable and valid. Studies have shown high internal consistency (Cronbach’s alpha typically > 0.9), meaning the items measure the same underlying construct—usability.
- High test-retest reliability: users give similar scores when retested.
- Strong correlation with other usability metrics like task success and time-on-task.
- Validated across numerous domains and user populations.
A meta-analysis published in Human Factors Journal confirmed that SUS scores predict user satisfaction and perceived ease of use with high accuracy.
Cost-Effective and Scalable
Unlike eye-tracking or biometric testing, the SUS requires no special equipment. It can be deployed via online survey tools like Google Forms, SurveyMonkey, or integrated into usability testing platforms.
- Low cost per participant.
- Easy to automate and analyze with spreadsheets or statistical software.
- Scalable for large user studies or A/B testing scenarios.
This makes it accessible even for startups or teams with limited UX budgets.
Limitations and Criticisms of the System Usability Scale
No tool is perfect, and the System Usability Scale is no exception. While widely praised, it has several limitations that users should be aware of.
Lack of Diagnostic Detail
The SUS provides a global usability score but doesn’t tell you *why* a system scored low. It’s a “what” metric, not a “why” metric.
- Doesn’t identify specific usability problems (e.g., confusing navigation, slow load times).
- Requires follow-up qualitative research to uncover root causes.
- Can mask critical issues if averaged across diverse user groups.
For instance, a SUS score of 65 might hide the fact that older users struggled significantly while younger users found it easy.
Subjectivity and Cultural Bias
Because the SUS relies on self-reported perceptions, it’s influenced by user expectations, mood, and cultural norms.
- Users from high-context cultures may give more moderate ratings.
- Novice users might rate a system lower due to lack of familiarity.
- Response bias (e.g., acquiescence bias) can affect results.
Researchers must interpret scores in context and consider demographic factors when analyzing data.
Alternatives and Complements to the System Usability Scale
While the System Usability Scale is powerful, it’s often used alongside other tools to get a complete picture of usability.
Nielsen’s Usability Attributes
Jakob Nielsen’s framework breaks usability into five key components:
- Learnability: How easy is it for users to accomplish basic tasks the first time?
- Efficiency: Once learned, how quickly can users perform tasks?
- Memorability: After not using it, can users re-establish proficiency?
- Errors: How many errors do users make, and how severe are they?
- Satisfaction: How pleasant is it to use the system?
The SUS primarily measures satisfaction and perceived ease, so combining it with direct observation of errors and efficiency provides a fuller picture.
UMUX and UMUX-Lite
The Usability Metric for User Experience (UMUX) is a newer, four-item scale based on ISO standards. UMUX-Lite is a two-item version that correlates highly with SUS but is even shorter.
- UMUX-Lite: “This system is easy to use” and “This system is efficient.”
- Scored on a 1–7 scale, then converted to 0–100.
- Often used in mobile app feedback loops.
While UMUX is promising, the SUS still has broader validation and historical data for comparison.
Real-World Applications of the System Usability Scale
The System Usability Scale isn’t just a theoretical tool—it’s used daily by companies and researchers to improve real products.
Case Study: Improving a Healthcare App
A telehealth startup used the SUS to evaluate its patient onboarding process. Initial testing with 15 users yielded an average SUS score of 52—well below the acceptable threshold.
- Qualitative feedback revealed confusion around document upload steps.
- After redesigning the interface, the SUS score improved to 78.
- Task completion rate increased from 60% to 92%.
This case shows how SUS can track improvement and justify design changes to stakeholders.
Use in Academic and Government Research
The SUS is frequently used in academic studies and government evaluations. For example, the U.S. Department of Veterans Affairs uses SUS to assess the usability of its online health portals.
- Helps ensure digital services meet accessibility and usability standards.
- Supports compliance with Section 508 and ADA requirements.
- Used in FDA submissions for medical device software.
Its adoption in regulated environments speaks to its credibility and rigor.
How to Improve Your System Usability Scale Score
Getting a low SUS score isn’t the end—it’s the beginning of improvement. Here’s how to turn insights into action.
Combine SUS with Qualitative Feedback
Always pair the SUS with open-ended questions like:
- What did you find most difficult?
- What would you change about the system?
- What did you like most?
This helps identify specific pain points behind the score. For example, a user might rate “I found the system unnecessarily complex” as “Strongly Agree,” and in their comment explain that the menu structure was confusing.
Iterate and Retest
Usability is not a one-time fix. After making changes:
- Retest with the same or new users.
- Compare SUS scores before and after.
- Track trends over multiple versions.
Continuous improvement leads to higher SUS scores and better user experiences.
“The best products aren’t born perfect—they evolve through repeated testing and refinement. The SUS is your compass in that journey.” — Don Norman, Design Thinker
Future of the System Usability Scale
As technology evolves, so does usability assessment. But the core principles of the System Usability Scale remain relevant.
Integration with AI and Analytics
Modern UX platforms are integrating SUS with behavioral analytics. For example:
- Automatically trigger SUS after a user completes a key task in an app.
- Correlate SUS scores with clickstream data or session duration.
- Use machine learning to predict SUS scores based on user behavior.
Tools like Hotjar and UserZoom now support SUS integration, making it easier than ever to collect and analyze data.
Potential for Adaptation
While the original SUS remains unchanged, researchers are exploring domain-specific variants:
- mSUS: Mobile System Usability Scale.
- Web SUS: Tailored for web applications.
- Game SUS: For evaluating video game interfaces.
However, purists argue that modifying SUS undermines its comparability. The consensus remains: use the original unless you have strong validation for a variant.
What is the System Usability Scale?
The System Usability Scale (SUS) is a 10-item questionnaire used to assess the perceived usability of a system. It produces a single score from 0 to 100, with higher scores indicating better usability. It’s widely used due to its simplicity, reliability, and broad applicability.
How is the SUS score calculated?
Each of the 10 items is rated from 1 (Strongly Disagree) to 5 (Strongly Agree). Odd-numbered items are scored by subtracting 1 from the response; even-numbered items are reverse-scored (5 minus the response). The total of the 10 adjusted scores is multiplied by 2.5 to get the final SUS score (0–100).
What is a good SUS score?
A SUS score above 70 is considered good. Scores between 70–79 are “good,” 80–89 are “excellent,” and 90+ are “outstanding.” The average SUS score across studies is about 68.
Can I modify the SUS questionnaire?
It’s strongly advised not to modify the wording or structure of the SUS, as this can invalidate the scoring and make comparisons unreliable. If you need a customized version, consider using validated alternatives like UMUX-Lite.
How many users do I need for a reliable SUS score?
As few as 5 users can provide a reliable estimate of usability, especially for formative testing. For higher precision or summative evaluations, 15–20 users are recommended.
The System Usability Scale remains one of the most trusted tools in the UX professional’s toolkit. Its ability to deliver a quick, reliable, and comparable measure of usability has made it indispensable across industries. While it has limitations—such as not diagnosing specific issues—it excels as a high-level indicator of user experience. When combined with qualitative insights and other metrics, the SUS provides a powerful foundation for improving products. Whether you’re a designer, developer, or researcher, understanding and applying the System Usability Scale can help you build systems that are not just functional, but truly user-friendly.
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