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, commonly known as SUS, is a 10-item questionnaire designed to assess the perceived usability of a system, product, or service. Developed in the late 1980s by John Brooke at Digital Equipment Corporation, SUS has become one of the most widely adopted tools in usability research across industries—from software and websites to medical devices and consumer electronics.
Unlike complex usability testing methods that require extensive observation or behavioral tracking, SUS offers a quick, reliable, and standardized way to gather subjective feedback from users. It doesn’t tell you why something is hard to use, but it does tell you how usable users perceive it to be.
Origins and Development of SUS
The System Usability Scale was first introduced in 1986 during usability research at Digital Equipment Corporation. At the time, there was no consistent, lightweight method to compare the usability of different systems. Brooke aimed to create a tool that was easy to administer, quick to complete, and capable of producing a single, reliable score.
He tested various versions of usability questionnaires and eventually settled on a 10-item Likert-scale format that balanced positive and negative statements. The result was a robust, language-independent tool that could be applied across platforms and cultures.
Over the decades, SUS has been validated through numerous studies and remains a cornerstone in human-computer interaction (HCI) research. Its longevity speaks volumes about its effectiveness and adaptability.
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, while even-numbered items are scored by subtracting the user response from 5. These scores are then summed and multiplied by 2.5 to yield a final SUS score between 0 and 100.
This scoring method ensures consistency and allows for easy benchmarking across different products and studies. More details on scoring can be found in the original research paper available via Usability.gov.
“The System Usability Scale is not just a metric—it’s a bridge between user experience and measurable outcomes.” — Dr. James Lewis, Human Factors Researcher
Why the System Usability Scale Matters in UX Design
In the world of user experience (UX) design, assumptions can be dangerous. Without data, teams risk building products that look good but fail in real-world use. The System Usability Scale provides an objective, quantifiable way to evaluate how users feel about a product’s usability—making it an essential tool in any UX professional’s toolkit.
Whether you’re conducting a usability test, comparing two design iterations, or validating a prototype, SUS offers a fast and reliable snapshot of user perception. It’s especially valuable in agile environments where quick feedback loops are critical.
Supporting Evidence-Based Design Decisions
One of the biggest challenges in UX is convincing stakeholders to prioritize usability improvements. Designers often rely on qualitative insights like user interviews or observational notes, which, while valuable, can be seen as subjective.
SUS introduces a numerical score that makes usability tangible. A product scoring 68 might be considered average, while one scoring 85 is deemed excellent. This allows teams to set clear goals, track progress over time, and justify design changes with hard data.
For instance, if a redesign increases the SUS score from 60 to 78, that’s a 18-point improvement—concrete evidence of enhanced usability. This kind of metric is powerful when presenting results to executives or development teams.
Enabling Cross-Product and Cross-Industry Comparisons
Because SUS is standardized, it allows for meaningful comparisons across different systems, even if they serve entirely different purposes. A mobile banking app can be compared to a hospital patient portal using the same scale.
Research has established benchmark scores for various industries. For example, the average SUS score across all products is around 68. Scores above 80 are considered excellent, while those below 50 indicate significant usability issues.
This benchmarking capability is invaluable for competitive analysis. Companies can test their product against competitors’ offerings and identify areas for improvement. More information on industry benchmarks is available through MeasuringU, a leading resource on usability metrics.
How to Administer the System Usability Scale
Administering the System Usability Scale is straightforward, but doing it correctly ensures reliable and actionable results. The process involves selecting participants, timing the survey, and ensuring clarity in instructions.
SUS is typically given immediately after a user completes a set of tasks with a system. This context ensures that their experience is fresh, leading to more accurate responses.
Best Practices for Participant Selection
To get meaningful results, it’s crucial to test with representative users. These should be individuals who match your target audience in terms of demographics, technical proficiency, and usage patterns.
While SUS can be administered with as few as 5–8 users and still yield reliable data (thanks to its high sensitivity), larger sample sizes improve statistical confidence, especially when comparing groups or tracking changes over time.
Avoid testing only internal stakeholders or highly technical users unless they are your actual target audience. Their familiarity with the system can skew results upward, giving a false sense of usability.
Timing and Context of the Survey
The ideal time to administer SUS is right after a usability test session. Once the participant has completed a series of predefined tasks—such as signing up, navigating a menu, or making a purchase—they are asked to fill out the SUS questionnaire.
Administering it too early (before interaction) or too late (days after use) reduces its validity. The goal is to capture immediate perceptions while the experience is still vivid.
Ensure that participants are not influenced by external feedback. For example, if a moderator says, “Great job on that task!” before handing out the survey, it might bias the user toward more positive responses.
Scoring and Interpreting the System Usability Scale
One of the most appealing aspects of the System Usability Scale is its simple scoring mechanism. Despite having only 10 questions, the scoring algorithm is designed to balance positive and negative items, resulting in a single, normalized score between 0 and 100.
Understanding how to calculate and interpret this score is essential for drawing accurate conclusions from your data.
Step-by-Step Scoring Methodology
Here’s how to compute a SUS score manually:
- For each odd-numbered question (1, 3, 5, 7, 9), subtract 1 from the user’s response (so a “3” becomes “2”).
- For each even-numbered question (2, 4, 6, 8, 10), subtract the user’s response from 5 (so a “4” becomes “1”).
- Sum all the adjusted scores.
- Multiply the total by 2.5 to get the final SUS score.
For example, if a user’s adjusted scores sum to 34, multiplying by 2.5 gives a SUS score of 85—well above average and indicating high perceived usability.
While manual calculation is possible, many researchers use online calculators or built-in functions in survey tools like Qualtrics or Google Forms to automate the process.
Understanding SUS Score Ranges and Benchmarks
Interpreting a SUS score requires context. While the scale runs from 0 to 100, not all scores are equally meaningful. Research has established general guidelines:
- Below 50: Poor usability. Significant redesign is likely needed.
- 50–65: Marginal usability. Some improvements are necessary.
- 65–75: Acceptable usability. Meets basic expectations.
- 75–85: Good usability. Users find it easy and efficient.
- 85–100: Excellent usability. Rare and highly desirable.
It’s also helpful to compare your score to industry averages. For instance, consumer software averages around 78, while enterprise software tends to score closer to 65. These benchmarks help contextualize your results.
“A SUS score of 68 is the median across thousands of studies—your product should aim higher.” — Sauro, J., & Lewis, J. R. (2016)
Advantages of Using the System Usability Scale
The enduring popularity of the System Usability Scale isn’t accidental. It offers several distinct advantages over other usability assessment methods, making it a go-to tool for researchers, designers, and product managers alike.
From its simplicity to its reliability, SUS has proven itself time and again as a powerful instrument in the UX ecosystem.
Simplicity and Ease of Use
One of the biggest strengths of the System Usability Scale is its simplicity. The questionnaire takes less than 5 minutes to complete, minimizing participant fatigue and increasing response rates.
Its language is straightforward and can be easily translated into other languages without losing meaning. This makes it ideal for international studies or multilingual user bases.
Moreover, the scoring system, while precise, is easy to implement with basic math or automated tools. This low barrier to entry allows even small teams with limited resources to conduct professional-grade usability evaluations.
High Reliability and Validity
Despite its brevity, SUS is remarkably reliable. Numerous studies have confirmed its internal consistency, with Cronbach’s alpha values typically above 0.9, indicating strong reliability.
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It also demonstrates good test-retest reliability—users tend to give similar scores when tested again under the same conditions. This consistency makes SUS suitable for longitudinal studies and iterative design processes.
Its validity has been supported across diverse domains, including healthcare, education, and e-commerce. Whether testing a mobile app or a medical device interface, SUS consistently correlates with actual user performance and satisfaction.
Limitations and Criticisms of the System Usability Scale
While the System Usability Scale is widely respected, it’s not without limitations. Understanding its weaknesses is crucial for using it effectively and avoiding misinterpretation of results.
No single tool can capture every aspect of usability, and SUS is no exception. It excels at measuring perceived usability but falls short in diagnosing specific problems.
Lack of Diagnostic Detail
SUS provides a global usability score but doesn’t tell you why users found a system difficult to use. For example, a low score could result from poor navigation, confusing terminology, slow performance, or any number of issues—but SUS won’t pinpoint the cause.
To address this limitation, it’s common practice to combine SUS with qualitative methods like think-aloud protocols, interviews, or observational notes. This mixed-methods approach gives both the “what” (SUS score) and the “why” (qualitative insights).
Some researchers have proposed expanded versions of SUS, such as the SUS-16 or the Post-Study System Usability Questionnaire (PSSUQ), to capture more granular data, but these come at the cost of increased length and complexity.
Sensitivity to Context and Task Design
The SUS score is highly dependent on the tasks users perform before taking the survey. If the tasks are too easy, the score may be artificially inflated. If they’re too difficult or poorly designed, the score may be unfairly low.
For example, asking a user to complete a complex financial transaction in a prototype that only supports basic navigation will likely result in a poor SUS score—not because the interface is bad, but because the task exceeded the system’s capabilities.
To mitigate this, ensure that the tasks are realistic, well-scoped, and aligned with the system’s intended functionality. Pilot testing the task flow can help identify and fix issues before正式 data collection.
Practical Applications of the System Usability Scale
The System Usability Scale isn’t just a theoretical tool—it’s actively used in real-world scenarios across industries. From tech startups to government agencies, organizations leverage SUS to improve products, validate designs, and meet regulatory requirements.
Its versatility makes it applicable in both formative (early-stage) and summative (final evaluation) testing phases.
Use in Software and App Development
In software development, SUS is often used during usability testing cycles to compare design iterations. For example, a team might test Version A of a checkout flow, collect SUS scores, implement changes, then retest with Version B.
A statistically significant improvement in SUS scores provides strong evidence that the new design is more usable. This data-driven approach supports agile development and continuous improvement.
Mobile app developers also use SUS to evaluate onboarding flows, navigation menus, and feature discoverability. Because mobile interfaces have limited screen space, even small usability improvements can have a big impact on user retention.
Application in Healthcare and Medical Devices
In healthcare, usability isn’t just about convenience—it’s a matter of safety. The FDA and other regulatory bodies encourage or require usability testing for medical devices, and SUS is frequently included in these evaluations.
For example, a glucose meter app or an infusion pump interface must be intuitive, especially under stress. A low SUS score could indicate potential use errors, which might lead to patient harm.
Studies have shown that medical devices with higher SUS scores are associated with fewer user errors and better compliance. Organizations like the Human Factors and Ergonomics Society advocate for SUS as part of a comprehensive usability validation strategy.
Alternatives and Extensions to the System Usability Scale
While the System Usability Scale remains the most popular usability questionnaire, several alternatives and extensions have been developed to address its limitations or cater to specific contexts.
These tools build on SUS’s foundation while offering additional insights or improved granularity.
The SUS-16 and Other Expanded Versions
The SUS-16 is an extended version of the original SUS, containing 16 items instead of 10. It aims to provide more detailed feedback by breaking down usability into sub-dimensions like efficiency, learnability, and satisfaction.
While more informative, the SUS-16 takes longer to complete and may increase participant burden. It’s best used when deeper insights are needed and time allows for a longer survey.
Other variations include the Modified SUS (MSUS), which rephrases items for specific domains, and the Short SUS (SSUS), a 5-item version for ultra-rapid assessments.
Comparison with Other Usability Questionnaires
Several other usability scales exist, each with its own strengths:
- UMUX (Usability Metric for User Experience): A 4-item scale based on ISO 9241-11, highly correlated with SUS but shorter.
- PSSUQ (Post-Study System Usability Questionnaire): A 16-item NASA-developed tool focusing on satisfaction, efficiency, and ease of use.
- QUIS (Questionnaire for User Interaction Satisfaction): A comprehensive, multi-section survey used in academic research.
While these tools offer more detail, SUS remains the most widely used due to its balance of brevity, reliability, and ease of scoring.
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 and satisfying a product is to use, compare design iterations, benchmark against competitors, and support evidence-based design decisions.
How is the SUS score calculated?
The SUS score is calculated by adjusting responses to odd and even-numbered questions, summing the adjusted values, and multiplying the total by 2.5. This results in a score between 0 and 100, where higher scores indicate better perceived usability.
What is a good SUS score?
A SUS score above 68 is considered above average. Scores above 80 are good, and those above 85 are excellent. A score below 50 indicates significant usability problems.
Can SUS be used for non-digital products?
Yes, although SUS was originally designed for software, it has been successfully applied to physical products, services, and hybrid systems. As long as users can interact with the system and provide feedback, SUS can be adapted accordingly.
Is the System Usability Scale free to use?
Yes, the System Usability Scale is in the public domain and free to use for both commercial and non-commercial purposes. No permission is required, though proper citation of the original work is recommended.
The System Usability Scale remains a cornerstone of usability evaluation for good reason. It’s simple, reliable, and powerful—offering a quick yet insightful measure of user experience. While it doesn’t replace qualitative research or performance metrics, it complements them perfectly, providing a standardized score that teams can track, compare, and act upon. Whether you’re a UX designer, product manager, or researcher, mastering SUS is a critical step toward building truly user-centered products.
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