Ever wondered how users truly feel about a product’s ease of use? The System Usability Scale (SUS) is a proven, lightning-fast tool that delivers reliable usability scores with just 10 questions. Simple, effective, and universally trusted—it’s no wonder designers and researchers swear by it.
What Is the System Usability Scale (SUS)?
The System Usability Scale, commonly known as SUS, is a widely adopted, standardized questionnaire designed to evaluate the perceived usability of a system, product, or service. Developed in the late 1980s by John Brooke at Digital Equipment Corporation, the SUS has since become one of the most popular tools in user experience (UX) research due to its simplicity, reliability, and versatility.
Origins and Development of SUS
The SUS was first introduced in 1986 as a quick and dirty usability scale during usability testing for interactive systems. John Brooke aimed to create a tool that could be administered quickly after a user task without requiring extensive training or interpretation. Despite its informal nickname, the SUS proved to be remarkably robust and accurate in measuring perceived usability across a wide range of technologies—from software and websites to medical devices and mobile apps.
Over the decades, the SUS has been validated through numerous studies and is now considered a gold standard in usability assessment. Its enduring relevance stems from its ability to produce a single, reliable usability score using only ten questions, making it efficient for both researchers and practitioners.
How SUS Differs From Other Usability Metrics
Unlike more complex usability evaluation methods such as cognitive walkthroughs or heuristic evaluations, the SUS focuses solely on the user’s subjective perception of usability. It doesn’t require observers, video recordings, or performance metrics. Instead, it captures the user’s post-task reflections through a structured Likert-scale questionnaire.
Compared to other tools like the User Experience Questionnaire (UEQ) or the Software Usability Assessment Scale (SUSAS), the SUS stands out for its brevity and ease of scoring. While these other instruments may offer richer qualitative insights or assess additional dimensions like attractiveness or efficiency, the SUS remains unmatched in its balance of speed and statistical reliability.
“The System Usability Scale is not the most comprehensive usability questionnaire, but it’s the most widely used because it’s quick, reliable, and easy to analyze.” — Jakob Nielsen, Nielsen Norman Group
The Structure of the SUS Questionnaire
The SUS consists of exactly ten statements, each rated on a five-point Likert scale ranging from “Strongly Disagree” (1) to “Strongly Agree” (5). The statements alternate between positive and negative phrasing to reduce response bias. Here are the ten standard items:
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..
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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 items are positively worded (e.g., “easy to use”), while even-numbered ones are negatively worded (e.g., “unnecessarily complex”).This counterbalancing helps prevent users from mechanically agreeing or disagreeing throughout the survey..
Why the System Usability Scale Is So Powerful
The enduring popularity of the System Usability Scale isn’t accidental. Its power lies in a rare combination of scientific rigor and practical simplicity. Whether you’re a UX researcher, product manager, or developer, SUS offers actionable insights without overwhelming complexity.
Speed and Efficiency in Usability Testing
One of the biggest advantages of the SUS is its speed. Administering the questionnaire takes less than five minutes, making it ideal for integration into usability tests, beta programs, or post-release feedback loops. Because it doesn’t require extensive setup or interpretation, teams can collect usability data rapidly across multiple iterations or user groups.
This efficiency is especially valuable in agile environments where quick feedback cycles are essential. Instead of waiting weeks for detailed usability reports, teams can get a snapshot of usability performance immediately after a user completes a task.
Reliability and Validity Across Domains
Despite its brevity, the SUS has demonstrated strong internal consistency and test-retest reliability. Studies have shown Cronbach’s alpha values typically above 0.9, indicating high reliability. This means the SUS produces consistent results when administered under similar conditions.
Moreover, the SUS has been successfully applied across diverse domains—including healthcare, finance, education, and consumer technology. For example, a study published in International Journal of Human–Computer Interaction found that SUS scores correlated strongly with actual user performance metrics in medical device testing.
Its cross-platform applicability makes the SUS a go-to tool for comparing usability across different versions of a product, competing products, or even entirely different types of interfaces.
Universality and Language Neutrality
The SUS has been translated into over 40 languages and used globally, from Japan to Brazil to Germany. Its simple language and neutral structure make it adaptable across cultures without losing its psychometric integrity.
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Organizations like the International Organization for Standardization (ISO) have recognized the importance of standardized usability measurement, and while SUS itself isn’t an ISO standard, it aligns well with ISO 9241-11 guidelines on usability.
This universality allows multinational companies to benchmark usability consistently across regions, ensuring that design improvements are data-driven and globally relevant.
How to Administer the System Usability Scale
Using the SUS effectively requires more than just handing out a questionnaire. Proper administration ensures accurate, meaningful results that can guide real design decisions.
When to Use the SUS in Your Research
The best time to administer the SUS is immediately after a user completes a set of representative tasks with your system. This ensures their responses are based on recent, concrete experience rather than vague impressions.
Common use cases include:
- Formative usability testing (during design iterations)
- Summative testing (benchmarking final designs)
- Comparative studies (A/B testing two interfaces)
- Post-launch user feedback collection
- Evaluation of competitor products
It’s important to note that SUS should not be used in isolation. Pairing it with qualitative methods—such as think-aloud protocols, interviews, or observation—provides deeper context for the numerical score.
Best Practices for Administering SUS
To get the most out of the SUS, follow these best practices:
- Use a representative sample: Aim for at least 8–12 users for reliable results. Larger samples improve confidence, especially when comparing groups.
- Ensure task completion first: Users should have interacted meaningfully with the system before answering.
- Maintain neutrality: Avoid leading questions or influencing responses during administration.
- Collect demographic data: Age, tech proficiency, and prior experience can help interpret SUS scores.
- Randomize question order (optional): While not standard, some researchers randomize to reduce order effects.
For remote testing, digital tools like Google Forms, SurveyMonkey, or specialized UX platforms (e.g., Optimal Workshop) can automate distribution and scoring.
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Common Pitfalls to Avoid
Despite its simplicity, the SUS can be misused. Common mistakes include:
- Administering it without any user interaction (e.g., showing only a screenshot)
- Modifying the wording of questions, which invalidates established scoring norms
- Using it as a standalone metric without qualitative backup
- Expecting it to diagnose specific usability problems (it measures overall perception, not root causes)
Remember: SUS tells you how usable a system feels, not why it feels that way. That’s where follow-up questions or interviews come in.
How to Calculate and Interpret SUS Scores
One of the most appealing aspects of the System Usability Scale is its straightforward scoring method. With a little arithmetic, you can turn ten responses into a single, interpretable number between 0 and 100.
Step-by-Step SUS Scoring Guide
Here’s how to calculate a SUS score manually:
- For each odd-numbered item (1, 3, 5, 7, 9), subtract 1 from the user’s response (so a “1” becomes 0, a “5” becomes 4).
- For each even-numbered item (2, 4, 6, 8, 10), subtract the user’s response from 5 (so a “1” becomes 4, a “5” becomes 0).
- Sum all the converted values.
- Multiply the total by 2.5 to convert it to a 0–100 scale.
Example: If a user’s adjusted scores sum to 36, then 36 × 2.5 = 90. This gives a SUS score of 90.
Because this process is repetitive, many researchers use automated calculators. Tools like the MeasuringU SUS Calculator or Excel templates can streamline scoring for large datasets.
What Do SUS Scores Mean?
SUS scores range from 0 to 100, with higher scores indicating better perceived usability. But what counts as “good”? Research by Sauro and Lewis (2016) provides a widely accepted grading scale:
- Below 50: Poor (frustrating, difficult to use)
- 50–59: OK (below average, needs improvement)
- 60–69: Acceptable (average, functional but not great)
- 70–79: Good (above average, solid usability)
- 80–100: Excellent (intuitive, highly usable)
A score of 68 is considered the global average across thousands of studies. So if your product scores above 68, it’s performing better than average.
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“A SUS score of 80 or higher is the usability equivalent of an ‘A’ grade. It suggests users find the system intuitive and efficient.” — Jeff Sauro, MeasuringU
Benchmarking and Comparative Analysis
SUS is particularly powerful for benchmarking. You can compare:
- Before vs. after a redesign
- Your product vs. a competitor’s
- Different user segments (e.g., novices vs. experts)
- Multiple features or workflows
For example, if your e-commerce app scores 62 in its current version and jumps to 78 after a redesign, that’s a significant improvement—especially if it crosses into the “Good” range.
Statistical tests like t-tests or ANOVA can determine whether differences in SUS scores are significant, adding rigor to your conclusions.
Strengths and Limitations of the System Usability Scale
No tool is perfect, and the System Usability Scale is no exception. While it’s incredibly useful, understanding its limitations helps you use it more effectively.
Key Strengths of SUS
The SUS shines in several areas:
- Brevity: Only 10 questions mean high completion rates and minimal user burden.
- Standardization: Decades of use have created a rich database of comparative scores.
- Quantifiability: Produces a single, easy-to-communicate number for stakeholders.
- Flexibility: Works for digital and physical systems, software, websites, and apps.
- No licensing fees: The SUS is in the public domain and free to use.
Its ability to deliver a reliable usability metric quickly makes it indispensable in fast-paced development cycles.
Common Criticisms and Limitations
Despite its strengths, the SUS has some drawbacks:
- Lack of diagnostic detail: It doesn’t tell you what is wrong with the interface.
- Subjective nature: Based on perception, not observed behavior or performance.
- Neutral wording: Some users may struggle with abstract statements like “I found the system unnecessarily complex.”
- No emotional or aesthetic dimensions: Doesn’t capture delight, trust, or visual appeal.
- Scoring complexity for beginners: The odd/even scoring rule can confuse new users.
These limitations don’t invalidate the SUS but highlight the need to complement it with other methods.
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When Not to Use the SUS
The SUS is not ideal in the following situations:
- When you need to identify specific usability issues (use heuristic evaluation instead).
- When testing very short interactions (e.g., a single button click).
- When users lack sufficient experience with the system.
- When you’re focused on emotional engagement or brand perception.
In these cases, consider pairing SUS with tools like the Post-Study System Usability Questionnaire (PSSUQ) or the PrEmo tool for emotional response.
Advanced Applications of the System Usability Scale
Beyond basic usability testing, the SUS has evolved into a strategic tool for product development, benchmarking, and even regulatory compliance.
Using SUS in Product Development Cycles
Integrating SUS into agile sprints allows teams to track usability improvements incrementally. For example:
- Run SUS after each usability test in a sprint.
- Set target SUS scores as acceptance criteria for new features.
- Visualize SUS trends over time using dashboards.
This data-driven approach helps prioritize design changes and justify UX investments to stakeholders.
SUS in Healthcare and Regulated Industries
In high-stakes environments like healthcare, usability isn’t just about convenience—it’s a safety issue. The FDA and other regulatory bodies encourage the use of validated usability metrics in medical device evaluation.
The SUS is frequently used in human factors engineering studies to demonstrate that a device is safe and effective for its intended users. For instance, a glucose meter with a SUS score below 50 might raise red flags during regulatory review.
Studies have shown that SUS scores correlate with error rates in clinical settings, making it a valuable proxy for risk assessment.
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Longitudinal Studies and SUS Trend Analysis
Tracking SUS scores over time reveals how usability evolves with product updates. For example, a SaaS company might measure SUS quarterly to monitor user satisfaction.
Trend analysis can uncover patterns such as:
- Declining scores after a major UI overhaul (indicating user resistance)
- Steady improvement due to iterative design refinements
- Sudden drops linked to specific feature releases
By correlating SUS trends with support tickets, churn rates, or NPS, organizations gain a holistic view of user experience health.
Alternatives and Complements to the System Usability Scale
While the SUS is powerful, it’s not the only usability metric available. Depending on your goals, you might consider alternatives or complementary tools.
Popular SUS Alternatives
Several questionnaires offer different trade-offs:
- UMUX (Usability Metric for User Experience): A 4-item scale based on ISO definitions, shorter than SUS but less proven.
- UMUX-Lite: Just 2 questions, ideal for quick surveys but less reliable.
- PSSUQ: 16 items focusing on satisfaction, more detailed but longer to administer.
- UEQ: Assesses multiple dimensions like attractiveness, efficiency, and perspicuity.
Each has its niche. UMUX-Lite, for example, is great for in-app micro-surveys, while UEQ provides richer emotional insights.
Combining SUS With Behavioral Metrics
The most powerful usability assessments combine SUS with objective data:
- Task success rate: Did users complete the task?
- Time on task: How long did it take?
- Error rate: How many mistakes were made?
- Clickstream analysis: What paths did users take?
When SUS scores are high but task success is low, it may indicate overconfidence or misunderstanding. Conversely, low SUS with high performance might suggest users are struggling but unaware.
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Future of Usability Measurement: Beyond SUS?
As AI and behavioral analytics advance, new methods are emerging. Tools that track eye movement, facial expressions, or keystroke dynamics offer real-time usability insights without surveys.
However, SUS is likely to remain relevant due to its simplicity, low cost, and proven track record. Future adaptations may include dynamic SUS versions that adapt questions based on user behavior or integrate with voice assistants.
For now, the System Usability Scale remains a cornerstone of UX evaluation—a testament to the power of elegant simplicity.
Real-World Case Studies Using the System Usability Scale
The true value of the System Usability Scale becomes clear when we look at how it’s used in real projects. Here are three illustrative case studies.
Case Study 1: Redesigning a Banking App
A major European bank wanted to improve its mobile app’s usability. Initial testing with 15 users yielded a SUS score of 54—well below average.
After conducting interviews, the team identified key pain points: complex navigation, unclear labels, and slow load times. They redesigned the onboarding flow and simplified the main menu.
Post-redesign testing showed a SUS score of 79—a dramatic improvement. The bank reported a 30% increase in user engagement and a 20% drop in customer support calls.
Case Study 2: Evaluating Medical Device Interfaces
A medical device manufacturer used SUS to compare two versions of a portable insulin pump interface. Version A scored 61, while Version B scored 83.
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Further analysis revealed that Version B’s larger buttons and clearer feedback reduced user errors in high-stress environments. The company chose Version B for production, citing SUS data in its FDA submission.
Case Study 3: Benchmarking E-Commerce Platforms
An e-commerce consultancy used SUS to evaluate five competing shopping apps. Scores ranged from 65 (App D) to 88 (App B).
The highest-scoring app featured one-click checkout, intuitive filters, and clear error messages. The client used these insights to recommend design improvements for their own platform, which later achieved a SUS of 81.
What is the System Usability Scale used for?
The System Usability Scale is used to measure the perceived usability of a system, product, or service. It helps teams evaluate how easy or difficult users find a system to use, identify areas for improvement, benchmark against competitors, and validate design changes. It’s widely used in UX research, product development, and regulatory testing.
How many questions are in the SUS?
The System Usability Scale consists of exactly 10 questions. These are statements rated on a 5-point Likert scale, alternating between positive and negative phrasing to reduce bias.
What is a good SUS score?
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A SUS score of 68 is considered average. Scores above 70 are generally good, 80 and above are excellent, while scores below 60 indicate usability issues that should be addressed.
Is the SUS free to use?
Yes, the System Usability Scale is in the public domain and free to use for any purpose—commercial or academic—without requiring permission or licensing fees.
Can I modify the SUS questions?
It’s strongly discouraged to modify the wording of the SUS questions, as this can invalidate the scoring model and make comparisons to established benchmarks unreliable. If customization is needed, consider using a different scale like UMUX or creating a custom questionnaire.
The System Usability Scale remains one of the most effective, efficient, and widely trusted tools in the UX professional’s toolkit. Its ability to deliver a reliable, standardized usability score in just minutes makes it invaluable across industries—from tech startups to medical device manufacturers. While it has limitations, particularly in diagnosing specific issues, its strengths in speed, consistency, and universality ensure its continued relevance. When combined with qualitative insights and behavioral data, the SUS provides a powerful foundation for creating more usable, user-friendly systems. Whether you’re evaluating a website, app, or physical device, incorporating the SUS into your research process can transform subjective impressions into actionable, data-driven decisions.
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