If you’ve ever wondered how to measure the ease of use of a product or system, the System Usability Scale (SUS) is your golden ticket. Simple, reliable, and widely trusted, this tool has become the go-to for UX professionals worldwide.
What Is the System Usability Scale (SUS)?
The System Usability Scale (SUS) is a 10-item questionnaire designed to assess the perceived usability of a system, product, or service. Developed by John Brooke in 1986, it remains one of the most widely used tools in user experience (UX) research due to its simplicity and effectiveness.
Origins and Development of SUS
Brooke created the SUS while working at Digital Equipment Corporation (DEC) to find a quick and reliable way to evaluate usability across different systems. Unlike more complex usability metrics, SUS was designed to be technology- and domain-independent, making it applicable to software, websites, mobile apps, and even hardware interfaces.
- Originally developed in the late 1980s.
- First published in a technical report by Brooke in 1986.
- Designed to be agnostic to specific technologies or industries.
Its longevity is a testament to its robustness. Over the decades, SUS has been validated across countless studies and remains a benchmark in usability measurement. You can read the original research via ResearchGate.
How SUS Works: The 10-Item Questionnaire
The SUS consists of 10 statements rated on a 5-point Likert scale, ranging from “Strongly Disagree” to “Strongly Agree.” Each item alternates between positive and negative phrasing to reduce response bias.
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.I think that I would need the support of a technical person to be able to use this system.I found the various functions in this system were well integrated.I thought there was too much inconsistency in this system..
I would imagine that most people would learn to use this system very quickly.I found the system very cumbersome to use.I felt very confident using the system.I needed to learn a lot of things before I could get going with this system.Notice how odd-numbered questions are positively worded, while even-numbered ones are negatively worded.This design helps balance user perception and minimizes the impact of acquiescence bias..
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Scoring the System Usability Scale
Scoring SUS is straightforward but requires attention to detail. Each response is scored from 0 to 4, depending on whether it’s a positive or negative statement.
- For odd-numbered items: Subtract 1 from the user’s scale position (1–5 becomes 0–4).
- For even-numbered items: Subtract the user’s scale position from 5 (5–1 becomes 0–4).
- Sum all 10 items and multiply the total by 2.5 to get a score from 0 to 100.
For example, if a user’s raw sum is 40, multiplying by 2.5 gives a SUS score of 100. If the sum is 20, the SUS score is 50.
“The beauty of SUS lies in its simplicity—only 10 questions, yet it delivers a reliable usability score.” — UX Researcher, Nielsen Norman Group
Why the System Usability Scale Is So Widely Used
The System Usability Scale has earned its place as a staple in UX evaluation because it strikes a perfect balance between simplicity and accuracy. It doesn’t require extensive training to administer or interpret, making it accessible even to non-specialists.
Speed and Efficiency in Usability Testing
One of the biggest advantages of the SUS is how quickly it can be administered. Most users complete the questionnaire in under 5 minutes, making it ideal for time-constrained usability studies.
- Can be used at the end of a usability test session.
- Does not interfere with task-based testing.
- Provides immediate quantitative data.
This efficiency allows researchers to collect usability data from large sample sizes without significantly increasing study duration. It’s especially useful in agile environments where rapid feedback is essential.
Reliability and Validity Across Domains
Despite its simplicity, the SUS has been shown to be both reliable and valid across a wide range of applications. Studies have confirmed its internal consistency (Cronbach’s alpha typically above 0.9) and its ability to discriminate between usable and unusable systems.
- Validated in software, healthcare, automotive, and consumer electronics.
- Used in academic research and industry alike.
- Proven effective across different languages and cultures.
According to a meta-analysis by Bangor, Kortum, and Miller (2008), SUS scores are consistent across diverse user groups and platforms. Their findings, published in Journal of Usability Studies, confirm that SUS is a psychometrically sound instrument.
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Cost-Effective and Easy to Implement
Unlike proprietary usability tools, the SUS is free to use and does not require licensing. This makes it an attractive option for startups, academic researchers, and small teams with limited budgets.
- No cost associated with using the questionnaire.
- Easy to integrate into surveys or testing platforms like Qualtrics or Google Forms.
- Can be translated and localized with proper validation.
Because it’s in the public domain, the SUS has been adapted into over 30 languages, further expanding its global reach.
How to Administer the System Usability Scale Effectively
While the SUS is simple to use, administering it correctly is crucial to obtaining meaningful results. Poor timing, incorrect instructions, or misinterpretation can skew data and lead to flawed conclusions.
Best Practices for Timing and Context
The timing of the SUS administration significantly affects the quality of responses. It should be given immediately after the user completes a set of tasks with the system.
- Administer right after a usability test session.
- Avoid delays—memory of the experience fades quickly.
- Ensure users have had enough interaction to form an opinion.
For example, if testing a mobile banking app, ask users to complete key tasks like checking a balance or transferring money before giving them the SUS. This ensures their feedback is based on actual usage, not first impressions.
Clear Instructions and Neutral Wording
To maintain consistency, always provide standardized instructions. Avoid leading questions or emotional language that might influence responses.
- Use the exact wording from the original SUS.
- Explain that there are no right or wrong answers.
- Ensure anonymity to encourage honest feedback.
Example instruction: “Please rate your agreement with the following statements based on your experience with the system today. There are no right or wrong answers—just your honest opinion.”
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Avoiding Common Pitfalls
Even experienced researchers can make mistakes when using the SUS. Common errors include mis-scoring responses, using modified versions without validation, or applying it to inappropriate contexts.
- Never change the wording of questions without re-validating.
- Don’t use SUS as the sole measure of usability—combine with qualitative data.
- Be cautious when comparing scores across different types of systems.
“A SUS score of 68 is average—but context matters. A 68 for a medical device may be unacceptable, while for legacy enterprise software, it might be excellent.” — Dr. Aaron Bangor, Human Factors Researcher
Interpreting System Usability Scale Scores
Understanding what a SUS score means is just as important as collecting it. A raw number doesn’t tell the full story—context, benchmarks, and qualitative insights are essential for accurate interpretation.
The SUS Score Range and What It Means
SUS scores range from 0 to 100, with higher scores indicating better perceived usability. While there’s no universal pass/fail threshold, general guidelines help interpret results.
- Below 50: Poor usability.
- 50–69: Marginal or acceptable.
- 70–79: Good usability.
- 80–100: Excellent usability.
According to Bangor et al. (2009), the average SUS score across thousands of studies is approximately 68. This serves as a useful benchmark when evaluating new systems.
Using SUS Benchmarks for Comparison
Benchmarks allow you to compare your product’s usability against industry standards or previous versions. For instance, if your app scores 75, it’s above average and likely competitive.
- Compare against historical data from earlier versions.
- Use published benchmarks (e.g., mobile apps average ~70).
- Track improvements over time after UX redesigns.
For example, a redesign that increases SUS from 60 to 80 indicates a significant usability improvement. This kind of data is powerful for convincing stakeholders of UX ROI.
Combining SUS with Qualitative Feedback
While SUS provides a quantitative score, it doesn’t explain why users feel a certain way. That’s where qualitative feedback comes in.
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- Pair SUS with open-ended questions: “What did you find most frustrating?”
- Conduct follow-up interviews to explore low-scoring areas.
- Use thematic analysis to identify recurring usability issues.
For instance, a SUS score of 55 might be accompanied by comments like “I couldn’t find the settings menu” or “The buttons were too small on mobile.” These insights guide targeted improvements.
Limitations and Criticisms of the System Usability Scale
Despite its popularity, the SUS is not without limitations. Understanding its weaknesses helps researchers use it more effectively and know when to supplement it with other tools.
Lack of Diagnostic Detail
The SUS gives a global usability score but doesn’t pinpoint specific problems. A low score tells you something is wrong, but not what.
- Does not identify which features are problematic.
- Cannot replace task success rates or error analysis.
- Should be used alongside observational data.
For example, a user might rate a system poorly due to slow load times, confusing navigation, or poor visual design—but SUS alone won’t reveal which factor dominates.
Sensitivity to User Expectations and Context
SUS scores can be influenced by factors outside the system’s control, such as user expertise, prior experience, or testing environment.
- Novice users may give lower scores than experts.
- High-stress environments (e.g., emergency systems) affect perception.
- Cultural differences in rating tendencies (e.g., leniency bias).
Researchers must account for these variables when interpreting results. A score must always be viewed in context.
Not Designed for All Types of Systems
While SUS is versatile, it may not be ideal for highly specialized or safety-critical systems where usability dimensions like learnability or error tolerance are paramount.
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- Medical devices may require more rigorous assessment tools.
- Complex enterprise software might need extended questionnaires.
- Children or cognitively impaired users may not respond reliably.
In such cases, tools like the Post-Study System Usability Questionnaire (PSSUQ) or the Usability Metric for User Experience (UMUX) may be more appropriate.
Alternatives and Complements to the System Usability Scale
While the SUS is a powerful tool, it’s not the only one available. Depending on your goals, you might consider complementary or alternative usability metrics.
UMUX: The Usability Metric for User Experience
The UMUX is a 4-item questionnaire derived from SUS, designed to be shorter while maintaining reliability. It focuses on two core dimensions: usability and efficiency.
- Only 4 questions, making it even faster to administer.
- Highly correlated with SUS (r > 0.9).
- Available in a Lite version (UMUX-Lite) with just 2 items.
UMUX is ideal when survey space is limited, such as in mobile app feedback prompts. Learn more at MeasuringU.
PSSUQ: Post-Study System Usability Questionnaire
Developed by NASA, the PSSUQ offers a more detailed assessment with 19 items across three subscales: system usefulness, information quality, and interface quality.
- More diagnostic than SUS.
- Ideal for in-depth usability studies.
- Requires more time to complete.
It’s often used in government and aerospace applications where detailed feedback is critical.
Net Promoter Score (NPS) and SUS: A Powerful Combination
While NPS measures loyalty and willingness to recommend, combining it with SUS provides a fuller picture of user satisfaction.
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- NPS identifies promoters, passives, and detractors.
- SUS explains why users feel that way.
- Together, they link usability to business outcomes.
For example, a high NPS with a low SUS might indicate brand loyalty despite poor usability—valuable insight for product strategy.
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. From tech giants to healthcare providers, SUS helps drive user-centered design.
Case Study: Improving a Mobile Banking App
A major European bank used SUS to evaluate its mobile app before and after a redesign. Initial testing revealed a SUS score of 58, indicating poor usability.
- Users struggled with navigation and transaction confirmation.
- After redesign, SUS improved to 82.
- Customer support calls related to app usage dropped by 40%.
This demonstrated a direct link between usability improvements and reduced operational costs.
Healthcare: Evaluating Electronic Health Records (EHR)
In healthcare, SUS is used to assess EHR systems, which are often criticized for poor usability. A U.S. hospital evaluated two EHR platforms using SUS.
- System A scored 62; System B scored 78.
- Despite System A having more features, clinicians preferred System B.
- Decision was made based on usability, not just functionality.
This highlights how SUS can influence high-stakes procurement decisions.
Academic Research and SUS Validation
SUS is one of the most cited tools in human-computer interaction (HCI) research. Its use in academic studies has helped validate new methodologies and technologies.
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- Used in studies on voice assistants, AR/VR interfaces, and AI chatbots.
- Helps compare traditional vs. innovative interaction methods.
- Provides standardized data for meta-analyses.
Researchers appreciate its consistency, allowing cross-study comparisons that would otherwise be impossible.
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 score from 0 to 100, with higher scores indicating better usability. It’s widely used due to its simplicity, reliability, and ease of use.
How do you calculate a SUS score?
To calculate a SUS score: (1) For odd-numbered items, subtract 1 from the response (1–5 becomes 0–4). (2) For even-numbered items, subtract the response from 5 (5–1 becomes 0–4). (3) Sum all 10 items and multiply by 2.5 to get a score from 0 to 100.
What is a good SUS score?
A SUS score above 68 is considered above average. Scores from 70–79 are good, and 80+ are excellent. However, context matters—what’s acceptable for a complex enterprise tool may not be for a consumer app.
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Can I modify the SUS questionnaire?
While you can adapt SUS, any changes should be validated to ensure reliability. Minor wording adjustments for clarity are acceptable, but structural changes may compromise its psychometric properties.
Is the System Usability Scale free to use?
Yes, the SUS is in the public domain and free to use for both commercial and academic purposes. No permission or licensing is required.
In conclusion, the System Usability Scale remains a cornerstone of usability evaluation. Its blend of simplicity, reliability, and broad applicability makes it indispensable for UX professionals. While it has limitations—such as lack of diagnostic detail—it excels as a quick, standardized measure of user experience. When combined with qualitative insights and other metrics, SUS provides a powerful lens through which to view and improve product usability. Whether you’re refining a mobile app, selecting enterprise software, or conducting academic research, the SUS offers a proven, efficient way to quantify how users perceive your system. Its enduring relevance proves that sometimes, the simplest tools are the most powerful.
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