Ever wondered how companies measure how user-friendly their products really are? Enter the System Usability Scale—a simple yet powerful tool that’s become the gold standard in usability testing.
What Is the System Usability Scale?
The System Usability Scale (SUS) is a widely adopted, reliable questionnaire used to evaluate the perceived usability of a system, product, or service. Originally developed in 1986 by John Brooke at Digital Equipment Corporation, SUS has since become one of the most popular tools in the UX (user experience) research toolkit. It’s language-agnostic, platform-independent, and quick to administer—making it ideal for assessing everything from mobile apps to medical devices.
Origins and Development of SUS
The System Usability Scale was created during usability research at Digital Equipment Corporation (DEC) to provide a quick, standardized way to assess user perceptions of system usability. At the time, there was no quick, reliable metric that could be applied across different types of systems. Brooke designed a 10-item questionnaire that could be used regardless of the technology being tested.
Despite its simplicity, the SUS was found to be both reliable and valid across numerous studies. It wasn’t published in a peer-reviewed journal initially, but its practical value led to widespread adoption. Over time, academic research validated its psychometric properties, cementing its place in UX practice. You can read more about its original development in Brooke’s 1996 paper, which formally introduced SUS to the broader research community.
How the SUS Questionnaire Works
The SUS consists of 10 statements, each rated on a 5-point Likert scale ranging from “Strongly Disagree” to “Strongly Agree.” The statements alternate between positive and negative phrasing to reduce response bias. After users interact with a system—whether it’s a website, app, or software—they complete the SUS questionnaire.
The 10 items include statements like:
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.Each response is scored, and a formula is applied to generate a final SUS score between 0 and 100.The calculation alternates between adding and subtracting values based on whether the item is positively or negatively worded.
.We’ll dive deeper into the scoring process later..
“The SUS has stood the test of time because it’s simple, fast, and gives meaningful results.” — Jeff Sauro, MeasuringU
Why the System Usability Scale Matters in UX Design
In the world of user-centered design, understanding how users perceive a product is just as important as how it performs technically. The System Usability Scale provides a quantitative measure that helps teams make informed decisions. Unlike qualitative feedback, which can be subjective and hard to compare, SUS delivers a standardized score that can be benchmarked across projects, teams, and industries.
Quantifying User Experience
One of the biggest challenges in UX is measuring something as abstract as “usability.” The System Usability Scale turns subjective impressions into a concrete number. This allows designers, developers, and stakeholders to speak the same language. Instead of saying “users seemed confused,” you can say “the SUS score was 58, which is below average.”
This quantification supports data-driven decision-making. For example, if a redesign increases the SUS score from 60 to 78, that’s a measurable improvement. It also helps prioritize usability issues. A low score might prompt deeper investigation through interviews or usability testing.
Benchmarking Across Products and Industries
The real power of the System Usability Scale lies in its ability to be compared across different systems. Because it’s standardized, a SUS score from a banking app can be compared to one from a fitness tracker or a hospital information system.
Research has established average SUS scores across industries. According to data compiled by MeasuringU, the average SUS score is around 68. Scores above 80 are considered excellent, while those below 50 are poor. This benchmarking helps organizations understand how their product stacks up against competitors or industry standards.
For instance, if your e-learning platform scores a 72, you know it’s slightly above average. But if a competitor scores 85, there’s room for improvement. This comparative insight is invaluable for product strategy.
How to Administer the System Usability Scale
Administering the System Usability Scale is straightforward, but doing it correctly ensures reliable results. The process involves selecting participants, timing the survey, and collecting responses in a consistent format.
Choosing the Right Participants
To get meaningful SUS scores, you need participants who represent your target user base. This could be existing customers, potential users, or a mix. While SUS can work with as few as 5 users (in line with usability testing best practices), larger sample sizes increase the reliability of the average score.
It’s important to avoid bias. For example, don’t only test with tech-savvy users if your product is aimed at seniors. The more representative your sample, the more accurate your SUS score will be as a reflection of real-world usability.
When to Administer the SUS
The best time to administer the SUS is immediately after a user completes a set of tasks with the system. This ensures their experience is fresh in their mind. For example, in a usability test, participants might be asked to complete tasks like creating an account, searching for a product, or submitting a form. Right after, they’re given the SUS questionnaire.
Administering SUS too long after interaction can lead to memory decay and less accurate responses. Conversely, giving it before any interaction defeats the purpose. Timing is critical.
Delivery Methods: Paper, Digital, or Integrated Tools
The SUS can be delivered in multiple formats: paper surveys, online forms, or integrated into usability testing platforms like UserTesting, Lookback, or Maze. Digital delivery is often preferred because it allows for automatic scoring and data aggregation.
Tools like Usability.gov provide downloadable SUS templates and scoring calculators. Some platforms even offer real-time SUS scoring during remote usability tests.
“A SUS score is only as good as the context in which it’s collected.” — Jakob Nielsen, Nielsen Norman Group
Scoring and Interpreting the System Usability Scale
One of the most appealing aspects of the System Usability Scale is its simple scoring method. Despite having only 10 questions, the scoring involves a specific algorithm that ensures consistency across evaluations.
The SUS Scoring Formula Explained
Here’s how the scoring works step by step:
- For odd-numbered questions (1, 3, 5, 7, 9), subtract 1 from the user’s response (which ranges from 1 to 5).
- For even-numbered questions (2, 4, 6, 8, 10), subtract the user’s response from 5.
- Sum the adjusted values across all 10 items.
- Multiply the total by 2.5 to convert it to a 0–100 scale.
For example, if a user gives all “3” responses (neutral), the raw sum would be 25, and 25 × 2.5 = 62.5. This is close to the average score of 68, indicating neutral usability.
While manual calculation is possible, many researchers use online calculators or spreadsheets to automate the process. QuantHub’s SUS Calculator is a popular free tool that instantly computes scores from user inputs.
Understanding SUS Score Ranges and Benchmarks
Once you have a SUS score, how do you interpret it? Here’s a widely accepted grading scale:
- 90–100: Excellent
- 80–89: Good
- 70–79: Acceptable
- 60–69: Poor
- 50–59: Awful
- 0–49: Unacceptable
However, context matters. A score of 70 might be acceptable for a complex enterprise software but disappointing for a consumer app. Also, SUS scores should be viewed as part of a larger usability picture—not the sole metric.
Another way to interpret SUS is by percentile ranking. A score of 68 is roughly the 50th percentile, meaning half of all systems score below this. A score of 80.3 is the 75th percentile, placing it in the top quarter of all tested systems.
Common Misinterpretations of SUS Results
Despite its simplicity, the System Usability Scale is sometimes misused. One common mistake is treating SUS as a diagnostic tool. It tells you *how usable* a system is, but not *why* it’s usable or not. For example, a low score doesn’t tell you whether users struggled with navigation, terminology, or performance.
Another pitfall is comparing SUS scores from vastly different contexts. A SUS score from a 5-minute task shouldn’t be directly compared to one from a 2-hour session. Task complexity, user motivation, and system familiarity all influence results.
Finally, some teams treat SUS as a pass/fail test. There’s no universal “passing” score. Instead, SUS should be used formatively—to track improvements over time or compare design alternatives.
Advantages of Using the System Usability Scale
The enduring popularity of the System Usability Scale isn’t accidental. It offers several key advantages that make it a go-to tool for usability professionals worldwide.
Simplicity and Speed
One of the biggest strengths of the System Usability Scale is its brevity. With only 10 questions, it takes users less than 5 minutes to complete. This makes it easy to integrate into usability studies without adding significant time or cognitive load.
Its simplicity also means it can be used by non-experts. Product managers, developers, or even students can administer SUS without extensive training. This democratizes usability measurement across teams.
Reliability and Validity
Despite its simplicity, the SUS is both reliable and valid. Numerous studies have confirmed its internal consistency (Cronbach’s alpha typically above 0.8) and its ability to distinguish between usable and unusable systems.
It’s been tested across cultures, languages, and domains—from aviation software to e-commerce sites. Translations exist in over 30 languages, and studies show that SUS maintains its psychometric properties across cultures.
For example, a 2009 study by Bangor, Kortum, and Miller confirmed that SUS produces consistent results across different user groups and testing conditions, reinforcing its status as a robust usability metric.
Cost-Effectiveness for Teams of All Sizes
Unlike complex usability labs or eye-tracking studies, the System Usability Scale requires no special equipment. It can be administered remotely, via email, or embedded in a post-task survey. This makes it accessible to startups, nonprofits, and educational institutions with limited budgets.
Because it’s in the public domain, there’s no licensing fee. You can use it freely for commercial or academic purposes. This openness has contributed significantly to its widespread adoption.
“SUS is the Swiss Army knife of usability questionnaires—simple, versatile, and always useful.” — UX Collective
Limitations and Criticisms of the System Usability Scale
While the System Usability Scale is powerful, it’s not without limitations. Understanding these weaknesses helps teams use SUS more effectively and complement it with other methods.
Lack of Diagnostic Detail
The SUS provides a global usability score but doesn’t pinpoint specific problems. A score of 50 tells you something is wrong, but not whether it’s the interface layout, error messages, or loading times.
To address this, many researchers pair SUS with qualitative methods like think-aloud protocols, interviews, or open-ended questions. For example, after SUS, you might ask, “What was the most frustrating part of using the system?” This adds context to the numerical score.
Sensitivity to Context and Task Design
SUS scores can be influenced by factors outside the system’s design. For instance, if users are given poorly designed tasks during testing, they may rate the system lower—even if the interface itself is good.
Similarly, user expectations matter. A user testing a beta product may be more forgiving than one evaluating a polished app. Contextual factors must be controlled or documented to ensure fair comparisons.
Subjectivity and Response Bias
Like any self-reported measure, SUS is subject to response bias. Some users may tend to agree with all statements (acquiescence bias), while others may give extreme responses. Cultural differences in rating styles can also affect scores.
While the alternating positive/negative wording in SUS helps reduce bias, it doesn’t eliminate it. Researchers should be cautious when interpreting scores from small or non-representative samples.
Practical Applications of the System Usability Scale
The System Usability Scale isn’t just a theoretical tool—it’s used in real-world settings across industries to improve products and services.
Software and App Development
In software development, SUS is often used during iterative design cycles. After each prototype, teams test it with users and collect SUS scores to track progress. For example, a fintech startup might test three versions of a payment flow and choose the one with the highest SUS score.
Agile teams integrate SUS into sprint reviews or usability testing sessions. It provides a quick feedback loop, helping teams validate design decisions before investing in development.
Healthcare and Medical Devices
In healthcare, usability can be a matter of life and death. The System Usability Scale is used to evaluate medical devices, electronic health records (EHRs), and patient portals.
For example, a hospital might use SUS to compare two EHR systems before making a purchasing decision. A higher SUS score could indicate lower risk of user error, which is critical in clinical settings.
Regulatory bodies like the FDA encourage usability testing for medical devices, and SUS is often included as part of the human factors validation process.
E-Commerce and Digital Services
For e-commerce platforms, a usable interface directly impacts conversion rates and customer satisfaction. SUS helps identify friction points in the user journey—like checkout processes or product search.
A retail company might use SUS to compare the mobile and desktop versions of their site. If the mobile SUS score is significantly lower, it signals a need for mobile optimization.
Some companies even use SUS in A/B testing. For example, testing two versions of a homepage and selecting the one with the better usability score—before rolling it out to all users.
Alternatives and Complements to the System Usability Scale
While SUS is popular, it’s not the only usability questionnaire available. Other tools can be used alongside or instead of SUS, depending on the research goals.
UMUX and UMUX-Lite
The User Experience Questionnaire (UMUX) is a 4-item scale based on ISO standards for usability. It measures perceived usability in terms of effectiveness, efficiency, and satisfaction. UMUX-Lite is an even shorter 2-item version.
UMUX correlates highly with SUS (r > 0.9) but is more aligned with ISO 9241-11. It’s useful when you need a very brief questionnaire. However, it lacks the extensive benchmarking data that SUS has.
Nielsen Norman Group’s SUPR-Q
The Standardized User Experience Percentile Rank Questionnaire (SUPR-Q) goes beyond usability to measure credibility, loyalty, and appearance. It’s ideal for websites where trust and aesthetics are as important as functionality.
SUPR-Q includes SUS-like items but expands into customer experience. It also provides percentile rankings based on a large database of website scores.
Unlike SUS, SUPR-Q is not free—it requires a license. But for organizations needing a comprehensive web experience metric, it’s a strong option.
Post-Study System Usability Questionnaire (PSSUQ)
Developed by IBM, the PSSUQ is a 16-item questionnaire that assesses system usability, information quality, and interface quality. It’s more detailed than SUS and provides subscale scores.
While more comprehensive, PSSUQ is longer and less widely used. It’s best suited for in-depth studies where granular feedback is needed.
Researchers often use PSSUQ in academic settings or large-scale enterprise evaluations where time isn’t a constraint.
What is a good System Usability Scale score?
A score of 68 is considered average. Anything above 80 is good, and above 90 is excellent. However, what’s “good” depends on your context. A consumer app should aim higher than a complex internal tool.
Can I modify the System Usability Scale?
Technically, yes—but it’s not recommended. Modifying items can invalidate the scoring and make benchmarking impossible. If you need a customized tool, consider using SUS as-is and supplementing it with additional questions.
Is the System Usability Scale free to use?
Yes, the System Usability Scale is in the public domain. You can use it for free in any commercial or academic project without permission.
How many users do I need for a reliable SUS score?
As few as 5 users can provide useful insights, especially in formative testing. For more reliable averages, aim for 15–20 users. Larger samples reduce the margin of error.
Can SUS be used for non-digital products?
Absolutely. While commonly used for software, SUS has been applied to physical products like ATMs, kiosks, and even household appliances. The key is that users interact with the system in a way that allows them to form an opinion about its usability.
The System Usability Scale remains one of the most practical and widely trusted tools in usability evaluation. Its blend of simplicity, reliability, and benchmarking power makes it indispensable for UX professionals. While it has limitations—such as lack of diagnostic detail—it excels as a quick, standardized measure of perceived usability. When used alongside qualitative methods and other metrics, SUS provides a solid foundation for improving user experience across industries. Whether you’re designing a mobile app, a medical device, or an e-commerce site, the System Usability Scale offers a proven way to measure, compare, and enhance how users experience your product.
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