Ever wondered how to measure if a product is truly user-friendly? The System Usability Scale (SUS) is the gold standard for evaluating usability—and it’s simpler than you think.
What Is the System Usability Scale (SUS)?
The System Usability Scale (SUS) is a widely adopted, reliable tool used to assess the perceived usability of a product, system, or service. Developed in 1986 by John Brooke at Digital Equipment Corporation, SUS has become one of the most frequently used questionnaires in usability testing across industries—from software and websites to medical devices and consumer electronics.
Origins and Development of SUS
Brooke created the SUS as a quick, cost-effective way to measure usability without requiring complex metrics or extensive user testing. It was designed to be technology-agnostic, meaning it can be applied to virtually any interactive system, regardless of platform or function.
The original research was conducted during usability evaluations of voice recognition systems, but the model proved so robust that it was quickly adopted across human-computer interaction (HCI) fields. Over the decades, SUS has been validated through numerous academic studies and real-world applications, cementing its status as a benchmark in usability assessment.
Structure and Format of the SUS Questionnaire
The SUS consists of 10 statements, each rated on a 5-point Likert scale ranging from “Strongly Disagree” to “Strongly Agree.” The statements alternate between positive and negative phrasing to reduce response bias. For example:
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- 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 users complete the survey, a standardized scoring algorithm calculates a final SUS score between 0 and 100. A score above 68 is considered above average, while scores above 80.3 are deemed excellent based on extensive benchmarking data.
Why SUS Stands Out Among Usability Metrics
Unlike other usability tools that require specialized training or lengthy test protocols, SUS is quick to administer, easy to score, and highly reliable. Its brevity—just 10 questions—makes it ideal for integration into usability studies, beta tests, or post-task evaluations without overwhelming participants.
“The beauty of the SUS lies in its simplicity and consistency. It doesn’t tell you *why* a system is hard to use, but it tells you *how hard* it is with remarkable accuracy.” — Jakob Nielsen, Nielsen Norman Group
Moreover, because it’s been used for over three decades, there’s a vast body of comparative data available, allowing researchers to benchmark their results against industry standards.
How to Administer the System Usability Scale
Administering the SUS correctly is crucial to obtaining valid and reliable results. While the questionnaire itself is short, the context in which it’s used can significantly affect the outcome.
Best Practices for Administering SUS
To ensure accurate data collection, follow these best practices:
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- Administer after a task-based session: SUS should be given immediately after users complete a set of representative tasks. This ensures their feedback reflects actual experience rather than general impressions.
- Use a neutral environment: Whether in-person or remote, minimize distractions and ensure participants feel comfortable sharing honest feedback.
- Provide clear instructions: Explain that there are no right or wrong answers and that their honest opinion is valuable.
- Keep it anonymous: Anonymity encourages more truthful responses, especially when evaluating internal tools or sensitive applications.
For more guidance, the official SUS documentation and scoring templates are available through resources like Usability.gov, a trusted source for UX professionals.
When to Use SUS in the Design Process
SUS is most effective when used iteratively throughout the design lifecycle:
- Early Prototypes: Evaluate low-fidelity wireframes or clickable mockups to identify major usability issues before investing in development.
- Mid-Development: Test functional prototypes to compare design alternatives (A/B testing) and track improvements over time.
- Post-Launch: Gather user feedback on live systems to assess real-world usability and inform future updates.
Because SUS provides a quantitative score, it’s particularly useful for tracking progress across versions. For instance, if Version 1 scores 52 and Version 2 scores 76, you have clear evidence of improved usability—even if the qualitative reasons aren’t immediately obvious.
Common Pitfalls to Avoid
Despite its simplicity, several common mistakes can compromise SUS results:
- Using SUS in isolation: SUS gives you a score, not insights. Always pair it with qualitative methods like interviews or think-aloud protocols to understand *why* users rated the system the way they did.
- Administering too early: If users haven’t had enough interaction with the system, their responses may be based on assumptions rather than experience.
- Modifying the wording: Even small changes to the SUS statements can invalidate the scoring model. Stick to the original wording unless you’re conducting academic research with proper validation.
For validated translations and adaptations, refer to the SUS Translations Database, which offers officially reviewed versions in over 40 languages.
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Scoring and Interpreting the System Usability Scale
One of the key strengths of the SUS is its straightforward scoring method. However, understanding how to interpret the results is just as important as calculating them correctly.
Step-by-Step SUS Scoring Guide
Here’s how to calculate a SUS score manually:
- For odd-numbered questions (1, 3, 5, 7, 9), subtract 1 from the user’s response (which ranges from 1 to 5).
- For even-numbered questions (2, 4, 6, 8, 10), subtract the user’s response from 5.
- Sum the converted values across all 10 questions.
- Multiply the total by 2.5 to convert it to a 0–100 scale.
For example, if a user’s adjusted sum is 30, multiplying by 2.5 gives a SUS score of 75.
While manual calculation is educational, most practitioners use automated tools like the SUS Calculator to streamline the process and reduce errors.
Understanding SUS Score Benchmarks
Interpreting SUS scores requires context. According to research by Sauro and Lewis (2006), here’s a general guideline:
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- 0–25: Poor usability — significant redesign needed
- 25–50: Below average — noticeable usability problems
- 50–75: Acceptable — meets basic usability expectations
- 75–100: Excellent — top-tier usability performance
It’s also helpful to compare your score to industry averages. For example, the average SUS score across all products is approximately 68. Web applications typically score around 70–75, while mobile apps often range between 65–72.
Statistical Significance and Sample Size
Because SUS produces a single numerical score per user, you can apply statistical analysis to compare groups or track changes over time. However, small sample sizes can lead to misleading conclusions.
As a rule of thumb:
- 5–10 users: Suitable for formative testing (identifying major issues)
- 15–20 users: Recommended for summative testing (benchmarking)
- 30+ users: Ideal for high-confidence comparisons and statistical significance testing
Tools like confidence intervals and t-tests can help determine whether differences between versions or products are meaningful. For deeper analysis, consider using software like R or SPSS, or online calculators such as those provided by MeasuringU, a leading resource in UX measurement.
Advantages of Using the System Usability Scale
The enduring popularity of the SUS is no accident. Its widespread adoption is rooted in a combination of practical benefits that make it indispensable in usability research.
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Reliability and Validity
Decades of research have confirmed that SUS is both reliable (consistent results across repeated tests) and valid (accurately measures what it claims to measure). Studies have shown high internal consistency, with Cronbach’s alpha typically above 0.9, indicating strong reliability.
Its validity has been demonstrated across diverse domains, including healthcare, finance, education, and e-commerce. Whether testing a mobile banking app or a surgical robot interface, SUS consistently delivers meaningful insights.
Speed and Simplicity
One of the biggest advantages of the SUS is its brevity. Most users complete the questionnaire in under 5 minutes, minimizing participant fatigue and making it easy to integrate into tight research schedules.
Additionally, the scoring algorithm is simple enough to be implemented in spreadsheets or automated dashboards, enabling real-time reporting during usability studies.
Universality and Cross-Industry Applicability
Unlike proprietary tools, SUS is technology-neutral and language-flexible. It can be used to evaluate desktop software, mobile apps, websites, kiosks, voice assistants, and even physical products with digital interfaces.
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This universality allows organizations to maintain a consistent usability metric across departments and product lines. For example, a large corporation might use SUS to compare the usability of its HR portal, customer service dashboard, and employee training platform—all using the same scoring framework.
Limitations and Criticisms of the System Usability Scale
Despite its strengths, the SUS is not without limitations. Understanding these weaknesses helps researchers use the tool more effectively and know when to supplement it with other methods.
Lack of Diagnostic Detail
One of the most common criticisms of SUS is that it doesn’t explain *why* a system received a particular score. A low score indicates poor usability, but it doesn’t pinpoint whether the issue lies in navigation, terminology, layout, or functionality.
To address this, usability experts recommend combining SUS with qualitative techniques such as:
- Think-aloud protocols
- User interviews
- Task success rate tracking
- Heatmaps and session recordings
This mixed-methods approach provides both the “what” (from SUS) and the “why” (from qualitative data).
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Sensitivity to Context and Expectations
Users’ SUS ratings can be influenced by factors unrelated to actual usability, such as brand perception, prior experience with similar systems, or even their mood during testing.
For example, a user familiar with advanced software may rate a beginner-friendly app as “too simplistic,” lowering the SUS score despite good usability. Conversely, a loyal customer may give higher ratings due to brand loyalty rather than objective ease of use.
Researchers must account for these biases by carefully screening participants and contextualizing results within broader user research findings.
Scoring Ambiguities and Misinterpretations
Although the scoring formula is well-documented, errors still occur—especially when researchers modify the questionnaire or miscalculate the final score.
Common mistakes include:
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- Forgetting to reverse-score even-numbered items
- Using a 7-point scale instead of 5-point
- Changing the wording of statements, which invalidates the standard scoring model
To avoid these pitfalls, always use the original SUS wording and verify calculations using trusted tools like the SUS-Score.com platform, which offers free calculators and templates.
Comparing SUS to Other Usability Evaluation Methods
While SUS is one of the most popular usability scales, it’s not the only one. Understanding how it compares to alternatives helps researchers choose the right tool for their needs.
SUS vs. SUPR-Q: Measuring Usability and Satisfaction
The SUPR-Q (Standardized User Experience Percentile Rank Questionnaire) builds on SUS by adding dimensions like trust, loyalty, and appearance. While SUS focuses purely on usability, SUPR-Q provides a broader picture of the overall user experience.
SUPR-Q is particularly useful for websites where aesthetics and credibility matter as much as functionality. However, it requires a license and is less flexible than the public-domain SUS.
SUS vs. UMUX and UMUX-Lite
The UMUX (Usability Metric for User Experience) is a shorter alternative based on two core usability dimensions: efficiency and ease of use. The UMUX-Lite version reduces this to just two questions, making it even faster to administer.
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While UMUX-Lite correlates highly with SUS (r = 0.87), it lacks the depth and nuance of the full 10-item SUS. It’s best used when time is extremely limited, such as in large-scale surveys or mobile pop-up feedback forms.
SUS vs. NASA-TLX: Cognitive Load vs. Usability
The NASA-TLX (Task Load Index) measures perceived mental workload rather than usability. It’s commonly used in high-stakes environments like aviation, healthcare, and military systems.
While both tools use Likert scales and produce numerical scores, NASA-TLX focuses on effort, frustration, and time pressure—factors that may not directly correlate with ease of use. In some cases, a system can be easy to use (high SUS) but still impose high cognitive load (high NASA-TLX), especially under stress.
For comprehensive evaluation, some researchers use both SUS and NASA-TLX in tandem.
Real-World Applications of the System Usability Scale
The true value of SUS lies in its practical application across industries. From tech startups to government agencies, organizations use SUS to drive design decisions and improve user satisfaction.
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Case Study: Improving a Healthcare App
A U.S.-based telehealth company used SUS to evaluate a new patient portal. Initial testing with 15 users yielded an average SUS score of 54—well below the benchmark.
Follow-up interviews revealed confusion around appointment scheduling and medication tracking. The design team simplified the navigation, added tooltips, and improved error messages.
After revisions, a second round of testing showed an average SUS score of 82—a dramatic improvement indicating excellent usability. This data was instrumental in securing stakeholder buy-in for the redesigned interface.
Case Study: Evaluating E-Learning Platforms
An educational institution compared three e-learning platforms using SUS. Faculty and students tested each system after completing a sample course module.
Results showed:
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- Platform A: SUS 62
- Platform B: SUS 78
- Platform C: SUS 69
Despite Platform A’s advanced features, its complexity hurt usability. Platform B, though less feature-rich, scored highest due to intuitive design. The university selected Platform B, prioritizing usability over functionality.
Case Study: Benchmarking Mobile Banking Apps
A financial consultancy conducted a comparative study of 10 mobile banking apps using SUS. Each app was tested by 20 users performing common tasks like transferring money and checking statements.
The average SUS score across all apps was 71, with top performers exceeding 80. The lowest-scoring app (SUS 48) had inconsistent navigation and poor error recovery.
The findings were published in a public report, helping banks understand where they stood relative to competitors and highlighting areas for improvement.
Future of the System Usability Scale
As technology evolves, so too does the role of usability measurement. While the core SUS remains unchanged, its application continues to adapt to new contexts and research needs.
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Integration with Automated UX Testing Tools
Modern UX platforms are beginning to integrate SUS into automated testing workflows. Tools like UserTesting, Lookback, and Maze allow researchers to embed SUS at the end of unmoderated usability tests, automatically collecting and analyzing scores alongside behavioral data.
This integration enables faster iteration and larger sample sizes, making SUS more scalable than ever.
Adaptation for Emerging Technologies
Researchers are exploring how SUS can be applied to voice interfaces, augmented reality (AR), virtual reality (VR), and AI-driven systems. While the original 10-item format still works in many cases, some adaptations are being tested to better capture the nuances of these modalities.
For example, a modified SUS for voice assistants might include questions about speech recognition accuracy and naturalness of interaction, while maintaining the core structure.
Ongoing Research and Validation
Academic interest in SUS remains strong. Recent studies have examined its psychometric properties, cross-cultural validity, and sensitivity to subtle design changes.
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Organizations like the User Experience Professionals Association (UXPA) continue to support research that validates and extends the use of SUS, ensuring it remains relevant in a rapidly changing digital landscape.
What is the System Usability Scale used for?
The System Usability Scale (SUS) is used to measure the perceived usability of a product, system, or service. It helps designers and researchers evaluate how easy and satisfying a system is to use, identify usability issues, compare design alternatives, and track improvements over time.
Is the System Usability Scale reliable?
Yes, the SUS is widely regarded as a reliable and valid tool for measuring usability. It has high internal consistency (Cronbach’s alpha > 0.9) and has been validated across numerous studies and industries. Its simplicity and consistency make it a trusted metric in both academic and commercial settings.
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How many users do I need for a SUS test?
For formative testing (identifying major issues), 5–10 users are sufficient. For summative testing (benchmarking or comparing versions), 15–20 users are recommended. For statistically significant results, 30 or more users provide greater confidence in the findings.
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 model. If modifications are necessary (e.g., for translation or accessibility), use officially validated versions or conduct psychometric validation to ensure reliability.
Where can I find the official SUS questionnaire?
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The original SUS questionnaire is in the public domain and can be freely used. Official resources, including scoring templates and translations, are available at SUS-Score.com and Usability.gov.
In conclusion, the System Usability Scale remains a cornerstone of usability evaluation. Its blend of simplicity, reliability, and universality makes it an essential tool for anyone involved in user experience design. While it has limitations—particularly in diagnostic depth—its ability to deliver a quick, standardized measure of usability is unmatched. When used correctly and in conjunction with qualitative methods, SUS empowers teams to build better, more user-friendly products. As technology advances, the SUS continues to evolve, proving that even a 35-year-old tool can remain relevant in the fast-paced world of digital design.
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