Ever wondered how to measure if your app or website is truly user-friendly? Enter the System Usability Scale (SUS)—a simple, reliable tool that reveals the real usability of your digital product. Backed by decades of research, it’s trusted by designers, developers, and UX researchers worldwide.
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, SUS has become one of the most widely used tools in usability assessment due to its simplicity, reliability, and versatility across different platforms and industries.
Origins and Development of SUS
Brooke created the SUS during a time when usability testing lacked standardized, quantitative measures. Before SUS, usability assessments were often subjective, inconsistent, or overly complex. The goal was to develop a lightweight, yet effective, method to gather user feedback quickly and consistently. The result was a 10-question survey that could be administered after any usability test, regardless of the system being evaluated.
Despite not being tied to any specific theoretical model, SUS was empirically derived. It was refined through iterative testing and validation across various systems, including software interfaces, medical devices, and consumer electronics. Its agnostic nature—meaning it doesn’t favor any particular type of technology or interface—has contributed significantly to its longevity and widespread adoption.
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” (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. (Positive)
- I found the system unnecessarily complex. (Negative)
- I thought the system was easy to use. (Positive)
After users complete the survey, a specific scoring algorithm is applied: odd-numbered items are scored by subtracting 1 from the user response (e.g., a “4” becomes “3”), while even-numbered items are scored by subtracting the user response from 5 (e.g., a “4” becomes “1”). These scores are summed and multiplied by 2.5 to yield a final score between 0 and 100.
system usability scale – System usability scale menjadi aspek penting yang dibahas di sini.
“The beauty of SUS lies in its simplicity. You don’t need a PhD in statistics to administer it, yet it delivers robust, actionable data.” — Jakob Nielsen, Nielsen Norman Group
Why SUS Stands Out Among Usability Metrics
Unlike other usability tools that require extensive training or complex analysis, SUS is quick to deploy and easy to interpret. It doesn’t require video recording, eye-tracking, or moderated sessions. All you need is a working prototype or live system and a few minutes of your user’s time.
Its universal applicability makes it ideal for comparing usability across different versions of a product (e.g., before and after a redesign), competing products, or even entirely different domains. For instance, you can compare the SUS score of a mobile banking app with that of a smart home thermostat interface—something few other tools allow.
How to Administer the System Usability Scale
Administering the SUS correctly is crucial to obtaining valid and reliable results. While the survey itself is short, the context in which it’s given can significantly impact the quality of the data collected.
Best Practices for Survey Administration
To ensure accurate results, SUS should be administered immediately after a user completes a set of representative tasks with the system. This timing ensures that their experience is fresh and contextually grounded. Delaying the survey can lead to memory decay and less accurate responses.
The environment should be as neutral as possible. Whether conducted in a lab, remotely, or in the field, users should feel comfortable and free from pressure. Avoid leading questions or cues that might influence their answers. For example, saying “We worked really hard on this interface” before handing them the survey could bias responses upward.
Additionally, it’s important to collect SUS data from a representative sample of users. While SUS can technically be used with as few as five users (in line with usability testing heuristics), larger sample sizes (20+) provide more statistically reliable averages and allow for meaningful comparisons.
system usability scale – System usability scale menjadi aspek penting yang dibahas di sini.
Common Mistakes to Avoid
One of the most common errors is modifying the SUS questionnaire. Despite its age, the original wording should not be changed—even slightly. Research has shown that altering item wording, reordering questions, or changing the scale can invalidate the scoring model and make comparisons to benchmark data unreliable.
Another mistake is using SUS in isolation. While SUS provides a great high-level usability score, it doesn’t explain why users rated the system the way they did. Pairing SUS with qualitative feedback, task success rates, or think-aloud protocols provides a much richer understanding of the user experience.
Finally, some teams mistakenly treat SUS as a pass/fail test. A score below 68 (the historical average) doesn’t mean a product is “bad”—it means there’s room for improvement. Context matters. A medical device with a SUS score of 60 might still be acceptable if it’s used by trained professionals, whereas a consumer app with the same score would likely need significant redesign.
Scoring and Interpreting the System Usability Scale
One of the most powerful aspects of the System Usability Scale is its standardized scoring method, which allows for consistent interpretation across studies and industries.
Step-by-Step Scoring Process
Scoring SUS involves a straightforward but precise calculation:
- For each odd-numbered item (1, 3, 5, 7, 9), subtract 1 from the response (e.g., if the user answered “4”, the score is 3).
- For each even-numbered item (2, 4, 6, 8, 10), subtract the response from 5 (e.g., if the user answered “4”, the score is 1).
- Sum all ten adjusted scores.
- Multiply the total by 2.5 to convert it to a 0–100 scale.
For example, if a user’s adjusted scores sum to 36, multiplying by 2.5 gives a final SUS score of 90—a very high score indicating excellent perceived usability.
system usability scale – System usability scale menjadi aspek penting yang dibahas di sini.
Understanding SUS Score Ranges and Benchmarks
The average SUS score across thousands of studies is approximately 68. This serves as a useful benchmark. Scores above 68 are considered above average, while those below are below average.
More granular interpretations have been proposed by researchers like Sauro and Lewis, who developed a grading scale based on extensive meta-analysis:
- 90–100: Excellent
- 80–89: Good
- 70–79: Acceptable
- 60–69: Poor
- Below 60: Awful
These grades help teams contextualize their results. A score of 85 suggests a highly usable system, while a 55 indicates serious usability issues that likely impact user satisfaction and efficiency.
Comparing SUS Scores Across Products and Studies
Because SUS is standardized, it enables direct comparisons. For instance, if Version A of an app scores 62 and Version B scores 78 after a redesign, the 16-point improvement suggests a meaningful enhancement in perceived usability.
However, comparisons should be made cautiously. Differences of less than 5–10 points may not be statistically significant, especially with small sample sizes. Always consider confidence intervals and conduct statistical tests (e.g., t-tests) when making claims about improvements or differences.
Organizations like the MeasuringU have compiled extensive benchmark databases, allowing teams to compare their SUS scores against industry standards for similar products.
system usability scale – System usability scale menjadi aspek penting yang dibahas di sini.
Advantages of Using the System Usability Scale
The enduring popularity of the System Usability Scale is no accident. Its widespread adoption is rooted in a combination of practical benefits that make it an indispensable tool in the UX researcher’s toolkit.
Simplicity and Ease of Use
SUS is remarkably easy to administer and score. It takes users less than 5 minutes to complete and requires no specialized software. The scoring formula, while precise, can be automated using spreadsheets or online calculators, making it accessible even to teams without dedicated UX researchers.
This simplicity lowers the barrier to entry for startups, small businesses, and educational institutions that may lack extensive usability testing resources. Even a solo developer can integrate SUS into their testing process with minimal effort.
Reliability and Validity
Despite its brevity, SUS has demonstrated strong psychometric properties. Numerous studies have confirmed its internal consistency (Cronbach’s alpha typically > 0.9), test-retest reliability, and construct validity.
It reliably distinguishes between usable and unusable systems and correlates well with other usability metrics such as task completion time, error rates, and user satisfaction. This makes it a trustworthy indicator of overall usability, even when used as a standalone measure.
Versatility Across Domains
One of SUS’s greatest strengths is its domain independence. It has been successfully applied in fields as diverse as healthcare, automotive interfaces, e-commerce, mobile apps, enterprise software, and even voice assistants.
system usability scale – System usability scale menjadi aspek penting yang dibahas di sini.
For example, a study published in the Journal of Biomedical Informatics used SUS to evaluate electronic health record (EHR) systems, while another in Human Factors applied it to in-vehicle infotainment systems. This cross-industry applicability makes SUS a universal language for usability.
Limitations and Criticisms of the System Usability Scale
While the System Usability Scale is a powerful tool, it is not without its limitations. Understanding these weaknesses is essential for using SUS effectively and avoiding misinterpretation of results.
Lack of Diagnostic Detail
SUS provides a global usability score but does not pinpoint specific usability problems. A low score tells you that users found the system difficult to use, but not why. Was it navigation? Terminology? Layout? Performance?
To address this, SUS should always be paired with qualitative methods such as interviews, observation, or open-ended feedback. For example, if a user gives a SUS score of 50, follow-up questions like “What made the system difficult to use?” can reveal actionable insights.
Sensitivity to Context and User Expectations
SUS scores can be influenced by factors outside the system’s control, such as user expertise, prior experience with similar systems, or even mood. A novice user might rate a technically sound system poorly simply because they lack familiarity, while an expert might rate a clunky system highly if it meets their functional needs.
Additionally, user expectations play a role. A SUS score for a cutting-edge AI tool might be lower than for a traditional desktop application, not because it’s less usable, but because users expect more from “smart” technology.
system usability scale – System usability scale menjadi aspek penting yang dibahas di sini.
Scoring Ambiguities and Misinterpretations
Although the scoring algorithm is well-defined, errors are common—especially when done manually. Misapplying the formula (e.g., forgetting to reverse even-numbered items) can lead to incorrect scores and misleading conclusions.
Moreover, some practitioners misinterpret SUS scores as absolute measures of quality. In reality, SUS is a relative metric. A score of 75 might be excellent for a complex enterprise tool but mediocre for a consumer-facing mobile app.
Alternatives and Complements to the System Usability Scale
While SUS remains the gold standard for quick usability assessment, several alternative and complementary tools can enhance your evaluation strategy.
UMUX and UMUX-Lite: Modern Alternatives
The Usability Metric for User Experience (UMUX) is a 4-item questionnaire based on ISO 9241-11, designed to be a more theoretically grounded alternative to SUS. UMUX-Lite, a 2-item version, offers even greater brevity while maintaining strong correlation with SUS (r > 0.9).
UMUX focuses specifically on effectiveness, efficiency, and satisfaction—core components of usability. Because it’s based on an international standard, some organizations prefer it for compliance reasons.
NPS and CSAT: Measuring Satisfaction Beyond Usability
While SUS measures perceived usability, Net Promoter Score (NPS) and Customer Satisfaction (CSAT) assess broader user sentiment. NPS asks users how likely they are to recommend the product, while CSAT measures overall satisfaction.
system usability scale – System usability scale menjadi aspek penting yang dibahas di sini.
These metrics complement SUS by providing insight into loyalty and emotional response. A product might have a high SUS score but low NPS if users find it functional but unenjoyable.
Qualitative Methods to Enhance SUS Insights
To get the full picture, SUS should be combined with qualitative techniques:
- Think-aloud protocols: Users verbalize their thoughts while interacting with the system, revealing cognitive hurdles.
- Post-task interviews: Ask users about specific challenges they faced during tasks.
- Open-ended survey questions: Include a field like “What did you like most/least about the system?” alongside SUS.
These methods transform SUS from a diagnostic number into a springboard for deep user understanding.
Real-World Applications of the System Usability Scale
The System Usability Scale isn’t just a theoretical tool—it’s actively used across industries to drive real improvements in user experience.
SUS in Software and App Development
Software companies routinely use SUS to evaluate new features, compare design iterations, and validate usability improvements. For example, a fintech startup might run SUS after beta testing a new budgeting feature. If the score jumps from 60 to 75 post-redesign, it’s strong evidence that the changes improved usability.
Agile teams integrate SUS into sprint reviews, allowing them to track usability trends over time. Continuous monitoring helps prevent usability debt—the accumulation of small, ignored usability issues that eventually degrade the user experience.
system usability scale – System usability scale menjadi aspek penting yang dibahas di sini.
Healthcare and Medical Devices
In healthcare, usability isn’t just about convenience—it’s a matter of safety. Regulatory bodies like the FDA recommend usability testing for medical devices, and SUS is frequently used in these evaluations.
For instance, a study on insulin pumps used SUS to compare two interface designs. The version with the higher SUS score was not only preferred by users but also associated with fewer dosing errors, demonstrating the real-world impact of usability.
Resources like the FDA’s guidance on human factors emphasize the importance of validated tools like SUS in ensuring patient safety.
E-Commerce and Customer Experience
Online retailers use SUS to optimize checkout flows, product search, and navigation. A high SUS score correlates with lower cart abandonment and higher conversion rates.
For example, an e-commerce platform that scores 85 on SUS after simplifying its checkout process can confidently attribute increased sales to improved usability. A/B testing combined with SUS provides a data-driven approach to UX optimization.
How to Improve Your System Usability Scale Score
A low SUS score isn’t a death sentence—it’s a starting point for improvement. Here’s how to turn insights into action.
system usability scale – System usability scale menjadi aspek penting yang dibahas di sini.
Identify Key Pain Points from SUS Items
While SUS gives a total score, individual item responses can highlight problem areas. For example, if users consistently disagree with “I found the system easy to use,” but agree with “I needed technical support to use it,” the issue may be poor onboarding or unclear instructions.
Break down responses by item to identify patterns. Tools like heatmaps of SUS item scores can visually highlight weak areas.
Iterative Design and Testing
Usability is not a one-time fix. Use SUS in an iterative cycle: test, redesign, retest. Each iteration should target the weakest aspects of the previous version.
For example, if SUS reveals that users find the navigation confusing, redesign the menu structure and retest with a new group. Track SUS scores over time to measure progress.
Integrate SUS into Your UX Strategy
To get the most value, embed SUS into your organization’s UX culture. Train product managers, designers, and developers to understand and use SUS. Share results transparently across teams to build shared ownership of usability.
Consider setting usability goals—for example, “All new features must achieve a minimum SUS score of 75 before launch.” This creates accountability and ensures usability is prioritized alongside functionality and performance.
system usability scale – System usability scale menjadi aspek penting yang dibahas di sini.
What is the ideal sample size for SUS?
While SUS can be used with as few as 5 users (following the “rule of five” in usability testing), larger samples (20+) provide more reliable averages and allow for statistical comparisons. For benchmarking or making high-stakes decisions, aim for at least 15–20 responses.
Can SUS be used for non-digital systems?
Yes! Although commonly used for software and websites, SUS has been successfully applied to physical products like medical devices, ATMs, and even household appliances. As long as users interact with the system in a way that involves learning, navigation, and task completion, SUS can provide valuable insights.
Is the SUS questionnaire free to use?
Yes, the System Usability Scale is in the public domain and free for both commercial and non-commercial use. No permission is required. However, proper attribution to John Brooke (1986) is recommended for academic or professional publications.
system usability scale – System usability scale menjadi aspek penting yang dibahas di sini.
How does SUS compare to other usability questionnaires?
SUS is more established and widely validated than most alternatives. While tools like UMUX, QUIS, and SUPR-Q exist, SUS remains the most cited and trusted. It strikes the best balance between brevity, reliability, and ease of use.
Can SUS predict user behavior?
While SUS measures perceived usability, high scores correlate strongly with positive user behaviors such as increased usage, higher satisfaction, and lower support costs. However, it should be combined with behavioral metrics (e.g., task success, time on task) for a complete picture.
The System Usability Scale is more than just a survey—it’s a proven, powerful tool for measuring and improving user experience. From its simple 10-item format to its robust scoring and wide applicability, SUS offers unmatched value for teams seeking to build usable, user-friendly products. While it has limitations, especially in diagnostic depth, its strengths far outweigh its weaknesses when used appropriately. By combining SUS with qualitative insights and iterative testing, organizations can make data-driven decisions that enhance usability, boost satisfaction, and drive success. Whether you’re designing a mobile app, a medical device, or an enterprise platform, the System Usability Scale remains an essential instrument in the modern UX toolkit.
system usability scale – System usability scale menjadi aspek penting yang dibahas di sini.
Further Reading:
