Ever wondered how to measure if your users actually like using your product? Enter the System Usability Scale (SUS)—a simple, powerful tool that turns user experience into hard numbers. Let’s dive into why it matters and how you can use it effectively.
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 usability evaluation across industries—from software and websites to medical devices and mobile apps.
Origins and Development of SUS
The SUS was created during a time when usability testing was becoming increasingly important in human-computer interaction (HCI). John Brooke needed a quick, reliable way to evaluate system usability without requiring extensive resources or time. The result was a lightweight, yet robust, ten-question survey that could be administered easily after a user task.
Unlike other usability metrics that require observational data or complex analytics, SUS relies solely on user self-reporting. This makes it both cost-effective and scalable. Despite its simplicity, numerous studies have validated its reliability and internal consistency, with Cronbach’s alpha typically exceeding 0.9, indicating high reliability.
Structure of the SUS Questionnaire
The SUS consists of 10 statements, each rated on a five-point Likert scale ranging from “Strongly Disagree” (1) to “Strongly Agree” (5). The questions 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 responses are collected, 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 score between 0 and 100.
Why SUS Stands Out Among Usability Metrics
What makes the System Usability Scale so enduring? First, it’s agnostic—it doesn’t assume anything about the type of system being evaluated. Whether it’s a mobile banking app or an enterprise CRM platform, SUS works the same way. Second, it’s fast. Users can complete it in under 5 minutes, making it ideal for integration into usability tests, beta programs, or post-interaction surveys.
According to research published by Jim Lykholm and Chris Knight, SUS outperforms many proprietary scales in terms of sensitivity and consistency. Its widespread adoption has also led to the creation of benchmark databases, allowing organizations to compare their scores against industry standards.
How to Administer the System Usability Scale
Using the System Usability Scale effectively requires more than just handing out a survey. Timing, context, and methodology all play crucial roles in ensuring accurate and actionable results. Let’s explore how to properly administer SUS for maximum impact.
Best Practices for Survey Administration
To get reliable data from the System Usability Scale, follow these best practices:
- Administer immediately after task completion: Users should complete the SUS right after interacting with the system while their experience is still fresh.
- Use a representative sample: Aim for at least 8–12 users for formative testing; 15+ for summative or benchmarking purposes.
- Ensure anonymity: Allowing anonymous responses encourages honesty, especially when evaluating internal tools or sensitive applications.
- Pair with qualitative feedback: Combine SUS with open-ended questions or interviews to understand the “why” behind the score.
For example, after a user completes a checkout process on an e-commerce site, a pop-up could invite them to rate the experience using the SUS. This real-time feedback loop enhances data accuracy.
Common Mistakes to Avoid
Even experienced UX researchers sometimes misapply the System Usability Scale. Here are common pitfalls:
Changing the wording of questions: Any modification can invalidate the scoring model and prevent comparison with benchmarks.Using it too early in development: SUS measures perceived usability, so testing on wireframes or non-functional prototypes may yield misleading results.Ignoring context: A low SUS score might reflect poor training rather than poor design.Always consider external factors.”The beauty of SUS lies in its consistency.
.The moment you start tweaking it, you lose its comparative power.” — Dr.Sauro, MeasuringU
Tools and Platforms for Deploying SUS
Thanks to digital survey tools, administering the System Usability Scale has never been easier.Platforms like SurveyMonkey, Typeform, and Qualtrics allow seamless integration of SUS into user research workflows.Some even offer pre-built SUS templates with automatic scoring..
For developers, open-source libraries like SUS Calculator on GitHub enable real-time score computation. Integrating SUS into usability testing platforms such as UserTesting or Lookback ensures synchronized data collection across behavioral and attitudinal metrics.
Interpreting SUS Scores: What Do the Numbers Mean?
A SUS score is more than just a number—it’s a diagnostic tool. Understanding how to interpret the results is key to driving meaningful improvements in your product’s usability.
The SUS Scoring Mechanism Explained
Each of the 10 SUS items contributes to the final score through a standardized calculation:
- For odd-numbered items (positively worded): Subtract 1 from the response (e.g., a “4” becomes “3”).
- For even-numbered items (negatively worded): Subtract the response from 5 (e.g., a “2” becomes “3”).
- Sum all adjusted scores and multiply by 2.5 to get the final SUS score (0–100).
For instance, if a user responds with all “3s,” the total adjusted sum would be 20, multiplied by 2.5 gives a SUS score of 50—right at the average baseline.
Understanding SUS Benchmarks and Percentiles
One of the most valuable aspects of the System Usability Scale is the existence of benchmark data. According to research by Sauro and Lewis (2009), the average SUS score across thousands of studies is approximately 68.
- Below 50: Poor usability—significant redesign likely needed.
- 50–60: Marginal—users experience frustration but can complete tasks.
- 60–70: Acceptable—meets basic usability expectations.
- 70–80: Good—users find the system intuitive and efficient.
- 80–100: Excellent—top-tier usability, often seen in leading consumer products.
Moreover, percentile rankings allow you to see how your product stacks up. A score of 80 puts you in the 90th percentile—better than 90% of systems tested.
Case Study: SUS in Real-World Product Evaluation
Consider a healthcare software company that launched a new patient portal. Initial SUS testing with 15 users yielded an average score of 54. While technically “marginal,” further qualitative analysis revealed that users struggled with password recovery and appointment scheduling.
After redesigning those flows, a follow-up SUS test showed an average score of 79—an impressive jump into the “good” range. This case illustrates how SUS can track progress and validate design decisions over time.
Advantages of Using the System Usability Scale
The enduring popularity of the System Usability Scale isn’t accidental. It offers several compelling advantages that make it a go-to tool for UX professionals, product managers, and researchers alike.
Speed and Simplicity in Data Collection
One of the biggest strengths of the System Usability Scale is its brevity. With only 10 questions and a completion time of 2–5 minutes, SUS minimizes respondent fatigue and maximizes participation rates. This efficiency is especially valuable in agile environments where rapid iteration is the norm.
Unlike lengthy usability questionnaires like the SUMI (Software Usability Measurement Inventory), SUS delivers actionable insights without burdening users or researchers. Its lightweight nature also makes it ideal for remote testing, mobile studies, and large-scale deployments.
High Reliability and Validity
Despite its simplicity, the System Usability Scale is backed by strong psychometric properties. Studies consistently report high internal consistency, with Cronbach’s alpha values often above 0.9, indicating that the items reliably measure the same underlying construct—usability.
Furthermore, SUS has demonstrated strong convergent validity, meaning it correlates well with other usability metrics such as task success rate, time-on-task, and error rates. A study by Bangor, Kortum, and Miller (2008) confirmed that SUS effectively discriminates between usable and unusable systems.
Cost-Effectiveness and Scalability
Because SUS requires no specialized equipment or training, it’s incredibly cost-effective. Teams with limited budgets can still gather high-quality usability data. Whether you’re a startup running guerrilla testing or a multinational corporation conducting global UX research, SUS scales effortlessly.
Its language-neutral structure also allows for easy translation and cross-cultural application, provided proper back-translation protocols are followed. This universality has made SUS a staple in international usability studies.
Limitations and Criticisms of the System Usability Scale
While the System Usability Scale is powerful, it’s not without limitations. Understanding its weaknesses helps ensure it’s used appropriately and complemented when necessary.
Lack of Diagnostic Depth
One of the most common criticisms of the System Usability Scale is that it doesn’t tell you why a system is unusable. A low score signals a problem, but not where it lies. Was it navigation? Terminology? Performance?
For example, two users might both give a SUS score of 45—one because the interface was confusing, another because the system crashed repeatedly. Without additional qualitative data, these root causes remain hidden.
Sensitivity to Context and User Expectations
SUS measures perceived usability, which can be influenced by factors outside the interface itself. A user’s prior experience, technical literacy, or even mood can affect their responses. Additionally, users may rate a system higher if they expect it to be complex (e.g., enterprise software) versus consumer-grade apps.
Research by Tullis and Stetson (2004) found that SUS scores can vary based on the perceived difficulty of the task, even if the interface remains unchanged. This context dependency means SUS should be interpreted cautiously, especially in comparative studies.
Not Designed for Formative Iteration
While SUS is excellent for summative evaluation (assessing a finished product), it’s less effective during early design phases. Because it provides a single composite score, it doesn’t offer granular feedback on specific features or interactions.
For formative testing, tools like the Nielsen Heuristics or think-aloud protocols are more appropriate. SUS is best used after a stable version is available for user interaction.
How to Improve Your SUS Score
Getting a low SUS score isn’t the end—it’s the beginning of improvement. Here’s how to turn insights into action and boost your system’s usability.
Identify Pain Points Through Follow-Up Questions
After collecting SUS scores, always include open-ended questions like:
- What did you find most frustrating about the system?
- What one change would make this easier to use?
- Was there any part of the process that confused you?
Thematic analysis of these responses can uncover patterns that explain low scores. For instance, repeated mentions of “hard to find settings” suggest a navigation issue.
Prioritize Changes Based on Impact and Effort
Not all usability issues are equal. Use a simple matrix to prioritize fixes:
- High impact, low effort: Fix these first (e.g., renaming confusing labels).
- High impact, high effort: Plan for future sprints (e.g., redesigning the onboarding flow).
- Low impact: Consider deprioritizing unless they affect key user segments.
This ensures your team focuses on changes that will move the SUS needle most effectively.
Retest to Measure Progress
Improvement isn’t real until it’s measured. After implementing changes, re-administer the SUS with a new set of users (or the same ones, if appropriate). A rising score validates your efforts and builds stakeholder confidence.
For example, a fintech app improved its SUS from 62 to 78 over three months by simplifying forms, adding tooltips, and reducing loading times. Regular retesting helped track progress and justify UX investments.
Alternatives and Complements to the System Usability Scale
While the System Usability Scale is a gold standard, it’s not the only tool in the usability toolkit. Depending on your goals, you might consider alternatives or complementary methods.
Usability Questionnaires: UMUX, SUPR-Q, and SUS Variants
Several questionnaires build on or refine the SUS framework:
- UMUX (Usability Metric for User Experience): A 4-item scale derived from SUS, focusing on usefulness and ease of use. It’s shorter and correlates highly with SUS.
- SUPR-Q (Standardized User Experience Percentile Rank Questionnaire): Measures usability, trust, loyalty, and appearance. Ideal for websites.
- mSUS (Modified SUS): Adapts SUS for mobile contexts, though purists argue it breaks comparability.
These tools offer flexibility while maintaining the rigor of SUS-based measurement.
Behavioral Metrics: Task Success, Time-on-Task, Error Rate
While SUS captures subjective perception, behavioral metrics provide objective data:
- Task success rate: Percentage of users who complete a task.
- Time-on-task: How long it takes to complete a task—shorter is usually better.
- Error rate: Number of mistakes made during task execution.
Combining SUS with these metrics gives a 360-degree view of usability. For instance, a high SUS score with low task success suggests users feel confident but are actually struggling.
Qualitative Methods: Think-Aloud, Interviews, and Cognitive Walkthroughs
To truly understand user behavior, qualitative methods are indispensable. Techniques like:
- Think-aloud protocols: Users verbalize thoughts while using the system.
- Post-test interviews: Explore motivations, frustrations, and expectations.
- Cognitive walkthroughs: Experts simulate user problem-solving to identify usability flaws.
These methods provide rich context that SUS alone cannot deliver.
Integrating the System Usability Scale into Your UX Workflow
The real power of the System Usability Scale emerges when it’s embedded into your product development lifecycle—not as a one-off test, but as a continuous feedback mechanism.
Using SUS in Agile and Lean Development
In agile environments, SUS can be deployed at the end of each sprint involving user-facing changes. For example, after releasing a new search feature, a quick SUS test with 10 users can determine if usability improved or regressed.
Teams can track SUS trends over time using dashboards, turning usability into a measurable KPI. This data-driven approach aligns UX goals with business outcomes.
SUS for Benchmarking and Competitive Analysis
Organizations can use SUS to benchmark their products against competitors. By testing users on both your product and a rival’s, you gain insight into relative strengths and weaknesses.
For instance, a study comparing three project management tools found SUS scores of 72, 78, and 85. The highest-scoring tool had superior onboarding and keyboard shortcuts—insights that informed the other teams’ roadmaps.
Training Teams to Use SUS Effectively
To maximize impact, train designers, developers, and product managers on how to interpret and act on SUS data. Workshops can cover:
- How to administer the survey correctly.
- How to calculate and interpret scores.
- How to combine SUS with other research methods.
When the entire team speaks the same usability language, collaboration improves and user-centered design becomes second nature.
What is a good System Usability Scale score?
A score of 68 is considered average. Anything above 70 is good, and 80+ is excellent. However, context matters—enterprise software may have lower expectations than consumer apps.
Can I modify the SUS questionnaire?
While you can adapt the wording for clarity, doing so risks invalidating the scoring model and preventing benchmark comparisons. It’s best to use the original version unless you have strong justification and plan to recalibrate.
How many users do I need for a reliable SUS score?
For formative testing, 8–12 users are sufficient. For summative or benchmarking studies, aim for 15–20 users to achieve statistical reliability.
Is the System Usability Scale free to use?
Yes! The System Usability Scale is in the public domain and free for both academic and commercial use. No permission is required, though proper citation is appreciated.
Can SUS be used for non-digital products?
Absolutely. While most commonly used for software, SUS has been successfully applied to physical products like medical devices, ATMs, and even household appliances, as long as users interact with a system interface.
The System Usability Scale remains one of the most trusted tools in the UX researcher’s arsenal. Its blend of simplicity, reliability, and scalability makes it indispensable for measuring perceived usability. While it has limitations—particularly in diagnostic depth—it shines when used alongside qualitative methods and behavioral metrics. By integrating SUS into your workflow, benchmarking over time, and acting on insights, you can continuously improve your product’s user experience. Whether you’re a solo designer or part of a large team, mastering the System Usability Scale is a powerful step toward building more usable, enjoyable, and successful systems.
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