If you’ve ever wondered how to measure the user-friendliness of a product, the System Usability Scale (SUS) is your go-to tool. Simple, reliable, and widely trusted, it’s the gold standard in usability evaluation.
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, it has since become one of the most widely used tools in user experience (UX) research. Despite its age, the SUS remains remarkably relevant due to its simplicity, reliability, and adaptability across platforms and industries.
Origins and Development of SUS
The SUS was first introduced in 1986 during usability studies at Digital Equipment Corporation. At the time, there was no standardized way to measure how easy or difficult users found a system to use. Brooke aimed to create a lightweight, yet effective, tool that could be applied across different technologies—whether software, websites, or hardware interfaces.
Unlike more complex usability metrics, the SUS was designed to be quick to administer and easy to score. It didn’t require extensive training or specialized equipment, making it accessible even to non-UX professionals. Its initial testing showed strong internal consistency and reliability, which helped it gain rapid acceptance in both academic and industrial settings.
How the SUS Questionnaire Works
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.
- I found the system unnecessarily complex.
- I thought the system was easy to use.
After users complete the survey, a specific scoring algorithm is applied to calculate a final SUS score between 0 and 100. A score above 68 is considered above average, while scores below 50 are generally seen as poor.
Why SUS Stands Out Among Usability Metrics
There are many usability assessment tools available today—such as the Net Promoter Score (NPS), User Experience Questionnaire (UEQ), and Post-Study System Usability Questionnaire (PSSUQ)—but the SUS remains uniquely popular. Its enduring appeal lies in its brevity, neutrality, and statistical robustness.
According to research published by The UXPA Journal, the SUS demonstrates high internal consistency (Cronbach’s alpha typically above 0.9), meaning the items reliably measure the same underlying construct: perceived usability.
“The System Usability Scale is not perfect, but it’s remarkably good for what it is: a quick, dirty, and yet surprisingly accurate way to assess perceived usability.” — Jeff Sauro, MeasuringU
Why the System Usability Scale Matters in UX Design
In the world of user experience design, intuition isn’t enough. Designers and product teams need objective data to validate their decisions. The System Usability Scale provides exactly that—a quantifiable measure of how users perceive a product’s ease of use.
Supporting Data-Driven Design Decisions
One of the biggest challenges in UX design is moving beyond subjective opinions. Stakeholders might say, “I don’t like the color,” or “This feels clunky,” but without a structured way to measure those feelings, it’s hard to prioritize changes. The SUS turns subjective impressions into numerical data, enabling teams to compare versions, track improvements over time, and justify design investments.
For instance, if a redesigned app interface increases the SUS score from 52 to 78, that’s a clear indicator of improved usability. This kind of evidence is invaluable when presenting results to executives or development teams.
Enabling Benchmarking Across Products and Industries
One of the most powerful features of the System Usability Scale is its ability to serve as a benchmarking tool. Because it’s standardized and platform-agnostic, SUS scores can be compared across different products, even in unrelated domains.
For example, a medical device manufacturer can compare the usability of their new diagnostic tool to the average SUS score of consumer mobile apps. According to Sauro and Lewis (2016), the average SUS score across thousands of studies is approximately 68, with mobile apps averaging around 70 and enterprise software often scoring lower, around 64.
This cross-industry comparability allows organizations to contextualize their results. A score of 70 might seem good, but if competitors are averaging 80, there’s room for improvement.
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Facilitating Iterative Design and Continuous Improvement
Modern product development follows an iterative cycle: design, test, learn, refine. The SUS fits perfectly into this loop. After each prototype or release, teams can administer the SUS to quickly gauge user perception.
Because it only takes users about 5–10 minutes to complete, the SUS can be integrated into usability testing sessions without adding significant time. Over multiple iterations, SUS scores can reveal trends—whether usability is improving, stagnating, or declining.
For agile teams, this rapid feedback is essential. It allows them to detect usability issues early, before they become costly to fix.
How to Administer the System Usability Scale Correctly
While the SUS is simple to use, administering it correctly is crucial for obtaining valid and reliable results. Missteps in timing, context, or scoring can lead to misleading conclusions.
Best Practices for Survey Administration
To get the most accurate data, 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 in their mind.
It’s important to avoid leading questions or influencing responses. The survey should be presented neutrally, without any commentary from the facilitator. If conducting remote testing, automated tools like Google Forms, SurveyMonkey, or dedicated UX platforms (e.g., Optimal Workshop) can help standardize delivery.
Additionally, the SUS should be used with a diverse group of users to ensure the results are representative. A sample size of at least 8–12 users is recommended for formative testing, while larger samples (30+) provide more statistically robust data.
Common Mistakes to Avoid
Despite its simplicity, several common errors can undermine the effectiveness of the SUS:
Administering it too early or too late: If users haven’t had enough interaction with the system, their responses may not reflect true usability.Conversely, if too much time passes after use, recall bias can affect accuracy.Changing the wording of questions: Even minor rephrasing can alter the meaning and impact reliability.The original SUS items should be used verbatim.Using it in isolation: While the SUS provides a great overall score, it doesn’t explain why users feel a certain way.
.It should be paired with qualitative methods like interviews or think-aloud protocols.Misinterpreting the score: A SUS score of 70 doesn’t mean 70% of users found the system usable.It’s a normalized score derived from a specific formula, not a percentage.Scoring the SUS: Step-by-Step Guide
Scoring the SUS involves a straightforward but precise calculation:.
- For odd-numbered questions (1, 3, 5, 7, 9), subtract 1 from the user’s response (so a 1 becomes 0, a 5 becomes 4).
- For even-numbered questions (2, 4, 6, 8, 10), subtract the user’s response from 5 (so a 1 becomes 4, a 5 becomes 0).
- Sum the converted values across all 10 items.
- Multiply the total by 2.5 to convert it to a 0–100 scale.
For example, if a user’s converted sum is 28, the final SUS score is 28 × 2.5 = 70.
While manual calculation is possible, tools like MeasuringU’s SUS Calculator automate this process and reduce errors.
Interpreting System Usability Scale Scores
Getting a number is only half the battle. The real value of the System Usability Scale comes from interpreting that score in context.
Understanding the SUS Score Range
The SUS produces a score between 0 and 100, but not all scores are equally meaningful. As mentioned earlier, the average score across thousands of assessments is about 68. Here’s a general interpretation framework:
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- Below 50: Poor usability. Users likely struggle significantly with the system.
- 50–67: Below average. There are noticeable usability issues that should be addressed.
- 68–76: Good. The system is reasonably usable, but there’s room for improvement.
- 77–85: Excellent. Users find the system intuitive and efficient.
- 86–100: Outstanding. Very few usability problems are expected.
It’s worth noting that scores above 90 are rare and often indicate either an exceptionally well-designed system or potential response bias (e.g., users being overly generous).
Using SUS Percentiles for Contextual Analysis
Beyond absolute scores, SUS results can be interpreted using percentile rankings. For example, a score of 75 places a product in the 60th percentile, meaning it’s more usable than 60% of systems tested.
This percentile data, based on large-scale studies by Sauro and Lewis, allows teams to answer questions like: “Is our app more usable than the average website?” or “How does our software stack up against industry standards?”
Percentile analysis is especially useful for competitive benchmarking. If your product scores in the 40th percentile, you know you’re behind most others in usability, even if your raw score seems decent.
Comparing SUS Scores Across Time and Versions
One of the most practical uses of the System Usability Scale is tracking usability over time. By administering the SUS after each major update, teams can visualize progress.
For example, a fintech app might have launched with a SUS score of 58. After three rounds of UX improvements—simplifying navigation, reducing form fields, and improving error messages—the score rose to 74. This 16-point increase signals a significant improvement in user experience.
When comparing scores across versions, it’s important to use the same user tasks and testing conditions to ensure consistency. Otherwise, changes in score might reflect differences in methodology rather than actual usability improvements.
Advantages and Limitations of the System Usability Scale
No tool is perfect, and the System Usability Scale is no exception. While it offers many benefits, understanding its limitations is key to using it effectively.
Key Advantages of Using SUS
The SUS has endured for decades because it delivers real value with minimal effort. Its main advantages include:
- Brevity: Only 10 questions mean low respondent burden and high completion rates.
- Reliability: High internal consistency and test-retest reliability make it a trustworthy metric.
- Flexibility: Can be used for websites, apps, kiosks, medical devices, and even voice interfaces.
- Cost-effective: No licensing fees or specialized software required.
- Well-documented: Decades of research provide rich context for interpretation.
These qualities make the SUS ideal for both small startups and large enterprises.
Common Criticisms and Limitations
Despite its strengths, the SUS has several well-known limitations:
- Lack of diagnostic detail: The SUS tells you how usable a system is, but not why. It won’t pinpoint whether navigation, terminology, or layout is causing issues.
- Subjective nature: It measures perceived usability, not objective performance (e.g., task success rate or time on task).
- No emotional or aesthetic dimensions: The SUS focuses on efficiency and learnability but doesn’t capture delight, trust, or visual appeal.
- Language and cultural sensitivity: Translations must be carefully validated, as subtle differences in phrasing can affect responses.
To overcome these limitations, the SUS is best used alongside other methods, such as usability testing, interviews, or behavioral analytics.
When to Use SUS vs. Other Usability Metrics
The SUS is not a one-size-fits-all solution. Depending on your goals, other tools might be more appropriate:
- For in-depth emotional feedback: Consider the User Experience Questionnaire (UEQ), which measures attractiveness, perspicuity, efficiency, and other dimensions.
- For customer loyalty: The Net Promoter Score (NPS) can complement SUS by measuring willingness to recommend.
- For task-specific performance: Combine SUS with metrics like task success rate, error rate, and time-on-task.
- For website-specific insights: The Website Analysis and Measurement Inventory (WAMMI) offers a more detailed alternative.
In practice, many UX researchers use the SUS as a “top-line” metric and supplement it with deeper qualitative or behavioral data.
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Real-World Applications of the System Usability Scale
The System Usability Scale isn’t just a theoretical tool—it’s used daily by companies and researchers around the world to improve real products.
Case Study: Improving a Healthcare App’s Usability
A telehealth startup launched a mobile app for patients to schedule appointments and view medical records. Initial user feedback was mixed, so the team conducted a usability study with 15 participants using the SUS.
The average SUS score was 54—well below the benchmark. Qualitative feedback revealed that users struggled with the appointment booking flow and found medical jargon confusing.
After redesigning the interface with clearer labels, simplified steps, and a progress indicator, the team retested. The SUS score jumped to 79, indicating a dramatic improvement. This data was instrumental in securing additional funding and expanding the user base.
Application in Enterprise Software Evaluation
A global logistics company wanted to evaluate three competing warehouse management systems. Instead of relying solely on vendor demos, they invited actual warehouse staff to test each system with realistic tasks.
After each session, users completed the SUS. The scores were 62, 71, and 83. While all systems had similar features, the one with the highest SUS score was chosen for deployment because it was perceived as significantly easier to use.
This decision led to faster training times, fewer errors, and higher employee satisfaction—proving that perceived usability directly impacts operational efficiency.
Academic and Research Use of SUS
The System Usability Scale is one of the most cited tools in human-computer interaction (HCI) research. Its simplicity makes it ideal for controlled experiments, comparative studies, and longitudinal research.
For example, a 2020 study published in Computers in Human Behavior used the SUS to evaluate the usability of virtual reality learning environments. The researchers found that immersive interfaces scored significantly higher on SUS than traditional e-learning platforms, suggesting VR’s potential for engaging education.
Because the SUS is public domain and freely available, it’s especially popular in academic settings where budget constraints are common.
Future of the System Usability Scale in UX Research
As technology evolves, so do the ways we measure usability. Yet, the System Usability Scale continues to adapt and remain relevant.
Integration with Digital Analytics Tools
Modern UX teams are increasingly combining survey data with behavioral analytics. Tools like Hotjar, Mixpanel, and FullStory allow researchers to correlate SUS scores with actual user behavior—such as click paths, drop-off points, and session duration.
For example, a user might give a high SUS score but spend an unusually long time completing a task. This discrepancy suggests that while the system feels easy, it may still have hidden inefficiencies.
By integrating SUS with digital analytics, teams gain a more holistic view of usability—combining perception with performance.
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Adaptation for Emerging Technologies
The SUS was originally designed for desktop software, but it’s now being successfully applied to voice assistants, augmented reality (AR), and wearable devices.
Researchers have found that with minor contextual adjustments (e.g., changing “navigate” to “interact”), the SUS remains effective even in non-traditional interfaces. A 2022 study on smart home devices used the SUS to compare the usability of voice vs. app-based controls, finding that voice interfaces scored slightly higher on learnability but lower on reliability.
This adaptability ensures that the System Usability Scale will remain a staple in UX evaluation for years to come.
Potential Evolutions and Alternatives
While the SUS is unlikely to be replaced, researchers are exploring ways to enhance it. Proposals include:
- SUS-8: An 8-item short form that maintains reliability while reducing respondent burden.
- Modified SUS for specific domains: Tailored versions for healthcare, education, or accessibility.
- Dynamic SUS: Adaptive versions that adjust questions based on user responses.
However, any changes must preserve the SUS’s core strengths: simplicity, reliability, and comparability.
What is the System Usability Scale used for?
The System Usability Scale (SUS) 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, enabling data-driven design decisions, benchmarking, and tracking improvements over time.
How do you calculate a SUS score?
To calculate a SUS score, convert each of the 10 responses using a specific formula: subtract 1 from odd-numbered responses, subtract the response from 5 for even-numbered ones. Sum the converted values and multiply by 2.5 to get a score from 0 to 100.
What is a good SUS score?
A SUS score above 68 is considered above average. Scores between 77 and 85 are classified as excellent, while anything above 86 is outstanding. A score below 50 indicates poor usability and the need for significant improvements.
Can the SUS be used for mobile apps?
Yes, the System Usability Scale is platform-agnostic and can be used for mobile apps, websites, software, hardware, and even emerging technologies like voice interfaces and AR/VR systems.
Is the SUS free to use?
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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.
The System Usability Scale remains a cornerstone of user experience evaluation. Its simplicity, reliability, and versatility make it an indispensable tool for designers, researchers, and product teams. While it has limitations—particularly in diagnosing specific usability issues—it excels as a quick, standardized measure of perceived usability. When used correctly and in combination with other methods, the SUS provides actionable insights that drive better design decisions. As technology continues to evolve, the SUS will likely adapt alongside it, maintaining its status as the go-to metric for assessing how users truly experience digital systems.
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