Ever wondered how to measure if your app, website, or software is actually user-friendly? Enter the System Usability Scale (SUS) — a simple, powerful tool trusted by UX experts worldwide to quantify usability. In this deep dive, we’ll unpack everything you need to know about SUS, from its origins to real-world applications.
What Is the System Usability Scale (SUS)?
The System Usability Scale (SUS) is a widely used, reliable questionnaire designed to assess the perceived usability of a product, system, or interface. Developed in the late 1980s by John Brooke at Digital Equipment Corporation, SUS has since become a gold standard in usability evaluation across industries — from software and mobile apps to medical devices and automotive interfaces.
Origins and Development of SUS
The SUS was first introduced in 1986 as a quick, easy-to-administer tool to evaluate the usability of computer systems. At a time when usability testing was often qualitative and subjective, Brooke’s 10-item questionnaire offered a quantitative alternative. Despite its brevity, the SUS proved remarkably effective at capturing users’ overall satisfaction with a system’s ease of use.
Brooke’s original research was published in a technical report titled SUS: A Quick and Dirty Usability Scale, which laid the foundation for decades of academic and industry adoption. You can read more about the original study on ResearchGate.
Structure of the SUS Questionnaire
The SUS consists of 10 statements, each rated on a 5-point Likert scale ranging from “Strongly Disagree” (1) to “Strongly Agree” (5). The statements alternate between positive and negative phrasing to reduce response bias. For example:
- 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.
After collecting responses, a specific scoring formula is applied to generate a final SUS score between 0 and 100.
Why SUS Stands Out Among Usability Metrics
Unlike more complex usability frameworks, the System Usability Scale is lightweight, language-neutral, and platform-agnostic. It doesn’t require specialized training to administer or interpret, making it accessible to designers, developers, and product managers alike. Its reliability and validity have been confirmed through numerous peer-reviewed studies, cementing its status as a cornerstone of UX research.
“The SUS has stood the test of time because it’s simple, quick, and provides a surprisingly accurate snapshot of usability.” — Dr. James Lewis, Human Factors Researcher at IBM
How to Administer the System Usability Scale
One of the greatest strengths of the System Usability Scale is its ease of deployment. Whether you’re running a lab study, remote usability test, or post-launch survey, SUS can be integrated seamlessly into your research workflow.
When to Use SUS in Your Research
The ideal time to administer the SUS is immediately after a user completes a task or series of tasks with your product. This ensures their experience is fresh in their mind, leading to more accurate feedback. Common use cases include:
- After a usability testing session
- Following a product demo or onboarding flow
- Post-release user feedback collection
- Comparative testing between two design versions (A/B testing)
Because SUS is not tied to specific functionality, it can be used across different stages of development — from prototypes to fully launched products.
Step-by-Step Guide to Administering SUS
Here’s how to effectively deploy the System Usability Scale:
- Prepare the environment: Ensure participants have completed a realistic set of tasks using your system.
- Distribute the questionnaire: Present the 10 SUS items clearly, either on paper or via digital survey tools like Google Forms, Qualtrics, or SurveyMonkey.
- Collect responses: Allow users to answer honestly without influence.
- Calculate the score: Apply the standard SUS scoring formula (explained in the next section).
- Interpret results: Compare scores against benchmarks or previous iterations.
For best results, aim for a minimum of 8–12 participants to achieve statistically meaningful data, though even smaller samples can provide directional insights.
Common Pitfalls to Avoid
While SUS is straightforward, mistakes in administration can skew results. Common errors include:
- Administering SUS before users have meaningful interaction with the system
- Using modified versions of the questionnaire without validation
- Translating SUS without proper linguistic validation
- Overinterpreting small differences in scores (e.g., 68 vs. 70)
To maintain reliability, always use the original SUS wording and follow standardized procedures.
How to Calculate and Interpret SUS Scores
The power of the System Usability Scale lies not just in data collection, but in how that data is transformed into actionable insights. Understanding the scoring mechanism is crucial for accurate interpretation.
The SUS Scoring Formula Explained
The calculation for the System Usability Scale follows a specific algorithm:
- For odd-numbered questions (1, 3, 5, 7, 9), subtract 1 from the user response.
- For even-numbered questions (2, 4, 6, 8, 10), subtract the user response from 5.
- Sum the converted values across all 10 items.
- Multiply the total by 2.5 to normalize the score to a 0–100 scale.
For example, if a user gives all “3” responses (neutral), the raw sum would be 20, and the final SUS score would be 50 — right at the average benchmark.
Understanding SUS Score Benchmarks
Once calculated, SUS scores can be interpreted using established benchmarks. According to research by Sauro and Lewis (2006), the average SUS score across thousands of studies is approximately 68.
- Below 50: Poor usability — significant redesign needed
- 50–60: Marginal — below average, but may be acceptable depending on context
- 60–70: Acceptable — meets basic usability expectations
- 70–80: Good — solid user experience
- 80–90: Excellent — high level of usability
- 90–100: Outstanding — rare and exceptional
It’s important to note that these are general guidelines. Context matters — a medical device may require a higher threshold than an internal tool.
Comparative Analysis Using SUS
One of the most powerful uses of the System Usability Scale is comparative testing. By measuring SUS scores before and after a design change, teams can quantify the impact of UX improvements.
For instance, if Version A of an app scores 58 and Version B scores 76 after a redesign, that 18-point jump indicates a substantial usability improvement. This kind of data is invaluable when justifying design decisions to stakeholders.
“SUS allows us to speak the language of business — numbers. Instead of saying ‘users seemed confused,’ we can say ‘usability improved by 22 points.'” — UX Lead, Tech Startup
Advantages of the System Usability Scale
The enduring popularity of the System Usability Scale isn’t accidental. Its widespread adoption is rooted in a set of compelling advantages that make it ideal for both academic and commercial settings.
Simplicity and Speed
With only 10 questions and a typical completion time of 5–10 minutes, SUS is one of the fastest usability assessment tools available. This efficiency makes it perfect for tight project timelines and large-scale studies where time is a constraint.
Its brevity also increases response rates and reduces participant fatigue, leading to higher-quality data. Unlike longer surveys, SUS doesn’t overwhelm users, which improves the reliability of their feedback.
Reliability and Validity
Despite its simplicity, the System Usability Scale has been rigorously tested for both reliability (consistency of results) and validity (accuracy in measuring what it claims to measure).
Studies have shown that SUS has high internal consistency, with Cronbach’s alpha typically above 0.9, indicating strong reliability. It also correlates well with other usability metrics, such as task success rates and user satisfaction scores.
For more on the psychometric properties of SUS, see the comprehensive review by Bangor, Kortum, and Miller (2008) available on ACM Digital Library.
Flexibility Across Domains
The System Usability Scale is not limited to digital interfaces. It has been successfully applied to:
- Mobile applications
- Web platforms
- Consumer electronics
- Medical devices
- Automotive infotainment systems
- Industrial control panels
This cross-platform versatility makes SUS a universal tool for any team focused on user-centered design.
Limitations and Criticisms of the System Usability Scale
While the System Usability Scale is a powerful tool, it’s not without its limitations. Understanding these weaknesses helps ensure it’s used appropriately and supplemented when necessary.
Lack of Diagnostic Detail
One of the most common criticisms of the System Usability Scale is that it provides a global score without pinpointing specific usability issues. A low SUS score tells you that something is wrong, but not what exactly needs fixing.
For example, a score of 52 could result from navigation problems, poor error messages, or slow performance — but SUS alone won’t reveal the root cause. That’s why it’s best used alongside qualitative methods like think-aloud protocols or interviews.
No Built-in Emotional or Behavioral Metrics
The System Usability Scale focuses on perceived ease of use and learnability, but it doesn’t capture emotional responses (e.g., frustration, delight) or behavioral intentions (e.g., likelihood to recommend, reuse). This is where complementary tools like the Net Promoter Score (NPS) or the User Experience Questionnaire (UEQ) can add depth.
For instance, a product might score well on SUS but still fail to delight users — a critical insight for long-term engagement.
Language and Cultural Sensitivity
Although SUS has been translated into over 30 languages, direct translations can sometimes miss cultural nuances. A phrase like “I found the system unnecessarily complex” may carry different connotations in collectivist vs. individualist cultures.
To maintain validity, translated versions should undergo back-translation and cognitive debriefing with native speakers. Resources for validated translations are available through the MeasuringU website.
Best Practices for Using the System Usability Scale
To get the most value from the System Usability Scale, it’s essential to follow best practices in design, administration, and interpretation.
Integrate SUS into Your UX Research Workflow
SUS should not be a one-off measurement. Instead, embed it into your product development lifecycle:
- Use it in early prototypes to catch major usability flaws
- Run it after each design iteration to track progress
- Deploy it post-launch to monitor long-term user satisfaction
This longitudinal approach turns SUS into a diagnostic dashboard for your product’s usability health.
Combine SUS with Qualitative Feedback
To overcome SUS’s lack of diagnostic detail, always pair it with open-ended questions. After the 10-item scale, ask:
- What did you find most confusing?
- What one change would improve your experience?
- Was there anything that frustrated you?
This mixed-methods approach gives you both the “what” (from SUS) and the “why” (from qualitative feedback).
Use SUS for Benchmarking and Goal Setting
Establish internal benchmarks based on past projects or industry standards. For example, if your company’s average SUS score is 72, aim to exceed that with new releases.
You can also benchmark against competitors. Conduct blind usability tests with rival products and compare SUS scores to identify strengths and weaknesses.
Real-World Applications of the System Usability Scale
The System Usability Scale isn’t just a theoretical tool — it’s actively used by leading organizations to improve real products.
SUS in Healthcare Technology
In medical device design, usability can be a matter of life and death. The FDA recommends using validated tools like SUS to assess the usability of devices such as insulin pumps, infusion systems, and diagnostic software.
For example, a study published in the Journal of Biomedical Informatics used SUS to evaluate an electronic health record (EHR) system, identifying key pain points that led to a redesigned interface with a 24-point SUS improvement.
SUS in Mobile App Development
Mobile app teams use SUS to compare design alternatives quickly. A fintech startup, for instance, tested two onboarding flows and found that Flow A scored 61 while Flow B scored 83. This data directly influenced their final design decision.
Because mobile users have low tolerance for friction, even small SUS improvements can translate into higher retention and conversion rates.
SUS in Enterprise Software
Internal tools often suffer from poor usability due to lack of user testing. Companies like SAP and Oracle use SUS to evaluate enterprise resource planning (ERP) systems, ensuring employees can perform tasks efficiently.
One global manufacturer reported reducing training time by 30% after redesigning their inventory management system based on low SUS scores and follow-up interviews.
Future of the System Usability Scale
As technology evolves, so too does the role of usability measurement. While newer tools and AI-driven analytics emerge, the System Usability Scale remains relevant — but not unchanged.
Integration with Automated UX Testing
Modern UX platforms are beginning to integrate SUS with behavioral analytics. Tools like Hotjar, Lookback, and UserTesting now allow researchers to trigger SUS surveys automatically after session recordings, creating a seamless feedback loop.
Some platforms even use machine learning to correlate SUS scores with user behavior patterns, such as hesitation, scrolling, or error rates.
Adaptations and Derivatives of SUS
While the original SUS remains unchanged, several validated derivatives have emerged:
- mSUS (Mobile SUS): Slight wording adjustments for mobile contexts
- ASQ (After-Scenario Questionnaire): Shorter, scenario-specific version
- UMUX (Usability Metric for User Experience): A 4-item alternative with similar psychometric properties
These adaptations maintain the spirit of SUS while addressing specific use cases.
Will SUS Remain Relevant in the Age of AI?
Even with advances in AI and predictive analytics, human perception remains central to usability. Algorithms can track clicks and time-on-task, but they can’t measure perceived ease of use — which is exactly what SUS captures.
As long as users have opinions about how easy or hard a system is to use, the System Usability Scale will remain a vital tool in the UX toolkit.
What is the System Usability Scale?
The System Usability Scale (SUS) is a 10-item questionnaire used to measure the perceived usability of a system. It produces a single score from 0 to 100, with higher scores indicating better usability. It’s known for being quick, reliable, and easy to administer.
How do you calculate a SUS score?
To calculate a SUS score: subtract 1 from odd-numbered responses, subtract even-numbered responses from 5, sum the converted values, then multiply by 2.5. The result is a score between 0 and 100.
What is a good SUS score?
A score of 68 is average. Scores above 70 are considered good, 80+ is excellent, and 90+ is outstanding. Below 68 suggests room for improvement.
Can I modify the SUS questionnaire?
While you can adapt the wording slightly (e.g., for mobile apps), major changes can invalidate the score. It’s best to use the original version to ensure comparability with benchmarks.
Is the System Usability Scale free to use?
Yes, the System Usability Scale is in the public domain and free for both commercial and academic use. No permission is required to administer or publish results.
The System Usability Scale continues to be one of the most trusted tools in user experience research. Its simplicity, reliability, and flexibility make it indispensable for anyone serious about building usable products. While it has limitations — particularly in diagnostic depth — its value as a quick, standardized metric is unmatched. When combined with qualitative insights and modern research tools, SUS provides a powerful lens into how users truly experience your system. Whether you’re a startup founder, UX designer, or product manager, mastering the System Usability Scale is a step toward creating products that don’t just work — but delight.
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