Ever wondered how users truly feel about a product’s ease of use? The System Usability Scale (SUS) cuts through the noise with a simple, reliable way to measure usability—backed by decades of research and real-world application.
What Is the System Usability Scale (SUS)?
The System Usability Scale, commonly known as SUS, is a 10-item questionnaire designed to evaluate the perceived usability of a system, product, or service. Developed in 1986 by John Brooke at Digital Equipment Corporation, it has since become one of the most widely used tools in usability assessment across industries—from software and websites to medical devices and mobile apps.
Origins and Development of SUS
Brooke created the SUS as a quick, reliable method to assess usability without requiring extensive user testing or complex metrics. It was initially developed during usability studies for voice recognition systems, where there was a need for a standardized, lightweight tool that could be administered quickly after a user interaction.
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Unlike other usability metrics that require observational data or task completion rates, the SUS relies solely on subjective user feedback. This made it especially valuable in environments where controlled testing wasn’t feasible. Over time, its simplicity and consistency led to widespread adoption in both academic research and industry practice.
- Originally developed in 1986 by John Brooke
- Designed for voice recognition system evaluation
- Published in a technical report, not a peer-reviewed journal initially
Despite its humble beginnings, the SUS gained traction due to its psychometric robustness. Researchers found that it produced reliable and valid results across different contexts, languages, and user populations. Today, it’s cited in thousands of studies and is considered a gold standard in usability measurement.
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:
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- I think that I would like to use this system frequently. (Positive)
- I found the system unnecessarily complex. (Negative)
- I thought the system was easy to use. (Positive)
After users complete the questionnaire, scores are calculated using a specific formula: for odd-numbered items, the score is the response value minus 1; for even-numbered items (which are negatively worded), the score is 5 minus the response value. These are summed and multiplied by 2.5 to yield a final score between 0 and 100.
“The beauty of the SUS lies in its simplicity—it’s short, easy to administer, and produces a single, interpretable score.” — Jeff Sauro, MeasuringU
Why SUS Stands Out Among Usability Metrics
There are many ways to measure usability—task success rates, time-on-task, error counts, Net Promoter Score (NPS), and more. But the SUS offers something unique: a standardized, subjective measure that can be compared across products, platforms, and time.
Unlike observational metrics, which tell you *what* users did, the SUS tells you *how they felt* about doing it. This emotional and cognitive dimension is crucial for understanding long-term user satisfaction and adoption. A system might allow users to complete tasks quickly, but if they find it frustrating or confusing, they’re unlikely to use it again.
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Moreover, the SUS score is normalized to a 0–100 scale, making it easy to benchmark against industry standards. According to research by Sauro and Lewis (2009), a score above 68 is considered above average, while scores above 80.3 are in the top 10% of all systems tested.
How to Administer the System Usability Scale
Administering the SUS correctly is key to obtaining valid and reliable results. While the questionnaire itself is short, the context in which it’s given can significantly affect the quality of the data.
Best Practices for Deployment
To get the most accurate feedback, the SUS should be administered immediately after a user completes a set of representative tasks with the system. This ensures that their experience is fresh and contextually grounded.
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It’s important to avoid leading questions or influencing responses. The instructions should be neutral: “Please answer the following questions based on your experience using the system.” Avoid phrases like “we hope you found it easy” or “did you enjoy using it?” which can bias responses.
The SUS can be delivered via paper, email, online survey tools (like Google Forms or SurveyMonkey), or integrated directly into usability testing software. Regardless of the method, consistency in wording and formatting is essential.
- Administer post-task or post-session
- Use neutral, non-leading instructions
- Maintain consistent formatting across all users
Another best practice is to pair the SUS with qualitative feedback. While the SUS gives you a quantitative score, open-ended follow-up questions like “What did you find most confusing?” or “What one improvement would you suggest?” provide rich context that helps interpret the score.
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Common Mistakes to Avoid
Despite its simplicity, there are several pitfalls that can compromise SUS results. One common error is modifying the wording of the questions. Even small changes—like replacing “system” with “app” or “website”—can affect how users interpret the statements and invalidate comparisons with benchmark data.
Another mistake is administering the SUS too early or too late. If users haven’t had enough interaction with the system, their responses may not reflect actual usability. Conversely, if too much time passes after use, recall bias can distort their answers.
Finally, some organizations try to average SUS scores across vastly different user groups or systems without considering context. A score of 75 might be excellent for a complex enterprise tool but poor for a consumer-facing mobile app. Always interpret scores within the appropriate context.
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Digital Tools and Platforms Supporting SUS
Many modern usability testing platforms now include built-in support for the SUS. Tools like UserTesting, Lookback, Maze, and Hotjar allow researchers to embed the SUS questionnaire directly into test flows, automatically calculate scores, and visualize results over time.
These integrations reduce manual effort and minimize calculation errors. They also enable longitudinal tracking—comparing SUS scores across design iterations to measure improvement. Some platforms even offer benchmarking features, comparing your score against industry averages for similar products.
For teams building custom solutions, open-source libraries and templates are available. The MeasuringU website provides a free SUS calculator and downloadable templates, while GitHub hosts several implementations in Python, JavaScript, and R for automated analysis.
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Scoring and Interpreting the System Usability Scale
One of the most powerful aspects of the SUS is its ability to produce a single, standardized score that reflects overall usability. But understanding how to calculate and interpret that score is critical.
Step-by-Step Scoring Process
Calculating a SUS score involves a straightforward but precise process:
- For each odd-numbered question (1, 3, 5, 7, 9), subtract 1 from the user’s response (so a “5” becomes “4”, a “1” becomes “0”).
- For each even-numbered question (2, 4, 6, 8, 10), subtract the user’s response from 5 (so a “1” becomes “4”, a “5” becomes “0”).
- Sum all ten transformed values.
- Multiply the total by 2.5 to convert it to a 0–100 scale.
For example, if a user responds with all 3s (neutral), the sum of transformed values would be (2+2+2+2+2) + (2+2+2+2+2) = 20. Multiply by 2.5 = 50. This is the baseline “neutral” score.
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While manual calculation is possible, tools like the SUS Calculator from MeasuringU automate this process and reduce human error, especially when dealing with large datasets.
Understanding SUS Score Ranges and Benchmarks
Interpreting a SUS score isn’t just about the number—it’s about what that number means in context. As established by Sauro and Lewis (2009), here’s a general interpretation framework:
- Below 50: Poor usability
- 50–67: Below average
- 68–76: Average
- 77–85: Good
- 85+: Excellent
However, these ranges are not absolute. A score of 70 might be acceptable for a legacy enterprise system with a steep learning curve, but unacceptable for a new consumer app aiming for mass adoption.
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Benchmarking against similar products is essential. For instance, the average SUS score for mobile banking apps is around 74, while e-commerce sites average 78. Knowing these benchmarks helps set realistic goals and prioritize improvements.
“A SUS score of 68 is the median across thousands of studies—anything above that puts you ahead of the pack.” — James Lewis, IBM Human Factors Research
Statistical Considerations and Confidence Intervals
Because the SUS is based on subjective ratings, statistical analysis can enhance its reliability. When reporting SUS scores, it’s good practice to include confidence intervals (CIs), especially with small sample sizes (n < 15).
For example, if you test 10 users and get an average SUS score of 75 with a 90% CI of [68, 82], you can be reasonably confident that the true population mean falls within that range. This helps stakeholders understand the precision of the estimate.
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Bootstrapping is a common method for calculating CIs for SUS scores, as the data often doesn’t follow a perfect normal distribution. Tools like R’s boot package or online calculators can assist with this.
Additionally, when comparing two versions of a product (e.g., before and after a redesign), a paired t-test or Wilcoxon signed-rank test can determine if the difference in SUS scores is statistically significant.
Applications of the System Usability Scale Across Industries
The versatility of the SUS is one of its greatest strengths. While originally designed for software systems, it has been successfully applied across a wide range of domains.
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Software and Web Application Testing
In software development, the SUS is routinely used during usability testing phases to evaluate interfaces before launch. It’s particularly valuable in agile environments, where rapid iterations require quick feedback loops.
Product teams use SUS scores to compare design alternatives (A/B testing), track progress across sprints, and validate whether usability improvements are actually perceived by users. For example, a team might test two navigation layouts and choose the one with the higher SUS score, even if both perform similarly on task completion.
Because the SUS is language- and platform-agnostic, it can be used to evaluate desktop apps, web portals, SaaS platforms, and internal tools with equal effectiveness.
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Healthcare and Medical Device Evaluation
In healthcare, usability isn’t just about convenience—it’s a matter of safety. The FDA recognizes the SUS as a valid tool for assessing the usability of medical devices, including infusion pumps, diagnostic equipment, and electronic health record (EHR) systems.
Studies have shown that poor usability in medical devices contributes to user errors, which can lead to patient harm. By using the SUS during formative and summative evaluations, manufacturers can identify usability issues early and demonstrate compliance with regulatory requirements.
For instance, a 2018 study published in Applied Ergonomics used the SUS to evaluate an anesthesia machine interface, finding that a redesigned version improved the SUS score from 58 to 82—indicating a shift from “poor” to “excellent” usability.
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Consumer Electronics and Mobile Apps
From smartphones to smart home devices, consumer electronics rely heavily on intuitive design. The SUS helps companies gauge how easily users can learn and operate new gadgets.
Mobile app developers, in particular, use the SUS to assess onboarding flows, menu navigation, and overall user satisfaction. Given the competitive nature of app stores, even small usability improvements can lead to better retention and higher ratings.
For example, a fitness app that scores 65 on the SUS might investigate why users perceive it as complex. Qualitative feedback might reveal that the workout logging process is too multi-step, prompting a redesign that simplifies the flow and boosts the SUS to 78.
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Advantages and Limitations of the System Usability Scale
No measurement tool is perfect, and the SUS is no exception. Understanding its strengths and weaknesses is crucial for using it effectively.
Key Advantages of Using SUS
The SUS offers several compelling benefits that explain its enduring popularity:
- Simplicity: Only 10 questions, takes 5–10 minutes to complete.
- Reliability: High internal consistency (Cronbach’s alpha typically > 0.9).
- Validity: Correlates well with other usability metrics and user behavior.
- Standardization: Enables cross-product and cross-industry comparisons.
- Cost-effective: Requires no specialized equipment or training.
Its brevity makes it ideal for situations where user time is limited, such as remote testing or large-scale surveys. And because it produces a single score, it’s easy to communicate results to stakeholders who may not be usability experts.
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“The SUS is the ‘Swiss Army knife’ of usability questionnaires—simple, versatile, and always handy.” — UX Collective
Common Criticisms and Limitations
Despite its strengths, the SUS has several limitations that users should be aware of:
- Lack of diagnostic detail: While it tells you *how usable* a system is, it doesn’t explain *why*. A low score doesn’t indicate whether the problem is navigation, terminology, or layout.
- Subjective nature: It measures perception, not objective performance. Users might rate a system highly even if they made errors, or vice versa.
- Cultural bias: Response tendencies (e.g., acquiescence bias) can vary across cultures, affecting score comparability in global studies.
- Fixed structure: You can’t modify questions without risking validity, which limits adaptability to niche domains.
Additionally, the SUS was developed before the rise of mobile and touch-based interfaces, so some researchers argue it may not fully capture modern interaction paradigms.
When to Use SUS vs. Other Usability Metrics
The SUS should be seen as one tool in a broader usability toolkit. It works best when combined with other methods:
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- Task success rate: Measures whether users can complete key actions.
- Time on task: Indicates efficiency.
- Error rate: Reveals usability flaws that lead to mistakes.
- NPS or CSAT: Captures overall satisfaction.
For example, a system might have a high SUS score but low task success—suggesting users *feel* it’s easy but actually struggle to use it. Conversely, a system with high task success but low SUS might be functional but unpleasant to use.
In formative testing, use SUS alongside think-aloud protocols. In summative testing, combine it with performance metrics for a holistic view.
Enhancing the System Usability Scale: Variants and Alternatives
While the original SUS remains popular, researchers have developed variants and complementary tools to address its limitations.
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SUS Variants: SUS-S, SUS-M, and SUS-8
To improve diagnostic value, several shortened or modified versions of the SUS have emerged:
- SUS-S (SUS-Short): A 5-item version for situations where even 10 questions are too long.
- SUS-M (Mobile SUS): Adapted for mobile contexts, with wording focused on touch interactions.
- SUS-8: An 8-item version proposed to improve factor structure and reduce redundancy.
However, these variants lack the extensive validation of the original SUS, so caution is advised when using them for benchmarking.
Alternative Usability Questionnaires
Other standardized questionnaires offer different trade-offs:
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- UMUX (Usability Metric for User Experience): A 4-item scale based on ISO 9241-11, highly correlated with SUS but more concise.
- UMUX-Lite: A 2-item version (one positive, one negative) that can be used as a quick alternative.
- Nielsen’s QUIS (Questionnaire for User Interaction Satisfaction): More detailed but longer and less portable.
- PSSUQ (Post-Study System Usability Questionnaire): Developed by IBM, focuses on satisfaction with specific system components.
Each has its place, but none have matched the SUS’s combination of brevity, reliability, and widespread adoption.
Integrating SUS with Qualitative Feedback
To overcome the SUS’s lack of diagnostic power, many researchers pair it with open-ended questions. For example:
- What did you like most about the system?
- What was the most frustrating part of your experience?
- If you could change one thing, what would it be?
This mixed-methods approach provides both a quantifiable score and actionable insights. Thematic analysis of qualitative responses can reveal patterns that explain low scores and guide design improvements.
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Best Practices for Using the System Usability Scale in Research
To maximize the value of the SUS, follow these evidence-based best practices.
Ensuring Valid and Reliable Results
Validity and reliability start with proper administration. Always use the exact wording of the SUS questions. Do not substitute “app” for “system” or change the order of items. Even minor alterations can affect psychometric properties.
Ensure your sample is representative. While the SUS can be used with as few as 5 users (common in usability testing), larger samples (n > 15) provide more stable averages and narrower confidence intervals.
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Administer the SUS in a consistent environment—same instructions, same timing, same platform. This reduces variability due to external factors.
Reporting SUS Data Effectively
When presenting SUS results, include:
- The average score and standard deviation
- Sample size (n)
- Confidence interval (if applicable)
- Benchmark comparison (e.g., “above average” or “top 10%”)
- Qualitative insights that contextualize the score
Visualizations like bar charts, box plots, or trend lines over time can make the data more accessible to non-technical stakeholders.
Longitudinal Tracking and Iterative Improvement
One of the most powerful uses of the SUS is tracking usability over time. By measuring SUS scores after each design iteration, teams can quantify the impact of changes and demonstrate progress.
For example, a product team might start with a SUS score of 60, implement usability improvements, and retest to find a score of 75—showing a 15-point gain. This kind of data is invaluable for justifying UX investments to leadership.
Set internal benchmarks and improvement goals. Aim to move from “below average” to “good” or “excellent” over a defined period. Use SUS trends as a KPI for user-centered design maturity.
What is a good System Usability Scale score?
A score above 68 is considered above average, based on extensive benchmarking. Scores above 80 are in the top 10% of all systems tested. However, what’s “good” depends on the context—consumer apps should aim higher than complex enterprise tools.
Can I modify the SUS questionnaire?
It’s strongly discouraged. Even small changes to wording or order can invalidate the score and prevent comparison with established benchmarks. If you need a customized tool, consider using a different questionnaire or supplementing SUS with open-ended questions.
How many users do I need for a reliable SUS score?
As few as 5 users can provide useful insights in formative testing. For more reliable averages and statistical analysis, aim for 15–20 users. Larger samples improve precision, especially when comparing groups or tracking changes over time.
Is the SUS suitable for mobile apps?
Yes, the SUS is widely used for mobile apps. While it wasn’t designed specifically for touch interfaces, studies have shown it performs well in mobile contexts. Some researchers use the Mobile SUS (SUS-M) variant, but the original SUS remains the gold standard.
How does SUS compare to Net Promoter Score (NPS)?
SUS measures perceived usability, while NPS measures loyalty and willingness to recommend. They correlate moderately but capture different aspects of user experience. Use SUS for usability, NPS for overall satisfaction and advocacy.
The System Usability Scale remains one of the most trusted, versatile, and practical tools in the UX researcher’s arsenal. Its ability to deliver a reliable, standardized measure of usability in just 10 questions has made it a staple across industries—from software and healthcare to consumer electronics. While it has limitations, particularly in diagnostic depth, its strengths in simplicity, reliability, and comparability are unmatched. When used correctly—paired with qualitative insights and other metrics—the SUS provides actionable data that drives meaningful design improvements. Whether you’re evaluating a new app, a medical device, or an enterprise platform, the SUS offers a powerful way to understand how users truly experience your system.
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