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 frustration into actionable insights. Let’s dive deep into why SUS is a game-changer.
What Is the System Usability Scale (SUS)?
The System Usability Scale (SUS) is a widely used, reliable 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 stood the test of time due to its simplicity, flexibility, and robustness across various domains—from software and websites to medical devices and mobile apps.
Origins and Development of SUS
Brooke created the SUS as a quick, cost-effective way to evaluate usability without requiring extensive resources or technical expertise. It was first introduced in a 1986 conference paper and later published in 1996 in the Usability Evaluation Beyond Testing report. Despite its age, SUS remains one of the most cited and validated tools in human-computer interaction (HCI) literature.
The original goal was not to replace in-depth usability testing but to provide a standardized, high-level snapshot of usability. Its ten-item structure was carefully crafted to balance positive and negative statements, ensuring respondents engage critically with each question rather than falling into response bias.
Structure and Scoring Mechanism
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 acquiescence bias (the tendency to agree with statements regardless of content).
- Odd-numbered items are positively worded (e.g., “I think that I would like to use this system frequently.”)
- Even-numbered items are negatively worded (e.g., “I found the system unnecessarily complex.”)
Scoring follows a specific formula:
- For odd-numbered items: Subtract 1 from the user’s response (so a “1” becomes 0, “5” becomes 4).
- For even-numbered items: Subtract the user’s response from 5 (so a “1” becomes 4, “5” becomes 0).
- Sum the converted scores and multiply by 2.5 to get a final score out of 100.
This normalization ensures that the SUS score ranges from 0 to 100, with higher scores indicating better perceived usability. A score of 68 is considered average, based on extensive benchmarking studies.
“The beauty of the System Usability Scale lies in its simplicity—it’s short enough to not burden users, yet reliable enough to inform real design decisions.” — Jakob Nielsen, Nielsen Norman Group
Why SUS Is Not a Diagnostic Tool
It’s crucial to understand that while the System Usability Scale provides a quantitative measure of usability, it does not diagnose *why* a system is usable or not. It tells you *how usable* something feels to users, but not *what* needs fixing.
For example, a low SUS score might indicate user frustration, but it won’t pinpoint whether the issue lies in navigation, terminology, loading times, or error messages. That’s why SUS is best used alongside qualitative methods like think-aloud protocols, interviews, or usability testing sessions.
Why the System Usability Scale Matters in UX Design
In today’s competitive digital landscape, user experience (UX) is no longer a luxury—it’s a necessity. The System Usability Scale offers UX professionals a standardized, evidence-based way to measure and communicate usability across teams, stakeholders, and products.
Quantifying the User Experience
One of the biggest challenges in UX design is translating subjective experiences into objective data. Stakeholders often ask: “Is our app easy to use?” Without a tool like the System Usability Scale, answers tend to be anecdotal or based on gut feeling.
SUS converts user perception into a single, interpretable number. This makes it easier to compare different versions of a product (e.g., before and after a redesign), benchmark against competitors, or track progress over time.
Supporting Data-Driven Design Decisions
Designers often face pressure to make changes based on opinions rather than evidence. The System Usability Scale empowers teams to advocate for usability improvements with hard data.
For instance, if a new feature rollout causes the SUS score to drop from 78 to 52, that’s a red flag demanding investigation. Conversely, a jump from 60 to 80 post-redesign validates the team’s efforts and justifies investment in UX.
Facilitating Cross-Team Communication
SUS acts as a common language between designers, developers, product managers, and executives. A score of “75” is easier to grasp than a lengthy report on cognitive load or interaction patterns.
It also supports agile environments where quick feedback loops are essential. Because SUS takes only 5–10 minutes to complete, it can be integrated into sprint reviews, beta tests, or customer onboarding flows.
How to Administer the System Usability Scale Effectively
While the System Usability Scale is simple to use, administering it correctly is key to obtaining valid and reliable results. Poor timing, incorrect context, or flawed distribution can undermine the quality of your data.
Choosing the Right Time to Deploy SUS
The most accurate SUS scores come from users who have just completed a realistic task with the system. For example, after a participant finishes booking a flight on a travel app, that’s the ideal moment to present the SUS questionnaire.
Administering SUS too early (e.g., immediately after onboarding) or too late (e.g., days after use) can skew results. The memory of the experience fades, leading to less accurate self-reports.
Ensuring Representative Sample Size
While SUS can be used with small samples (as few as 5–8 users), larger samples increase statistical reliability. According to research by Sauro and Lewis (2009), a sample size of 30–40 provides stable mean scores with a margin of error under ±5 points.
However, even with small samples, SUS is effective for identifying major usability issues. The key is consistency: use the same methodology across tests to enable meaningful comparisons.
Distributing the Questionnaire: Online vs. In-Person
SUS can be administered in various formats:
- In-person during usability testing: Ideal for controlled environments where moderators can ensure task completion before survey delivery.
- Online via survey tools: Platforms like SurveyMonkey or Qualtrics allow automated distribution and data collection, making large-scale studies feasible.
- In-app prompts: Some companies embed SUS directly into their apps after key user actions (e.g., completing a purchase).
Regardless of method, ensure the instructions are clear and the context is preserved. Avoid leading questions or influencing responses.
Interpreting System Usability Scale Scores Accurately
Getting a number is only half the battle—understanding what that number means is where the real value lies. Interpreting SUS scores requires context, benchmarks, and an awareness of scoring nuances.
Understanding the SUS Score Range
The SUS score ranges from 0 to 100, but not all scores are equally common. Based on decades of aggregated data:
- Below 50: Poor usability. Users are likely frustrated and may abandon the system.
- 50–67: Below average. Significant usability issues likely exist.
- 68: Average. This is the long-standing benchmark for “typical” system usability.
- 68–77: Good. Users find the system reasonably easy to use.
- 78–85: Excellent. The system is intuitive and efficient.
- 85+: Outstanding. Rare and indicative of exceptional usability.
These ranges help contextualize your results. A score of 70 might seem good, but it’s only slightly above average—there’s room for improvement.
Benchmarking Against Industry Standards
One of the most powerful uses of the System Usability Scale is benchmarking. By comparing your SUS score to industry averages, you can gauge competitiveness.
For example:
- E-commerce platforms average around 70–75.
- Banking apps often score between 65–70 due to security complexity.
- Top-tier consumer apps (like Google Maps or Spotify) can exceed 80.
Resources like the MeasuringU SUS Calculator provide access to large datasets and percentile rankings, helping you see how your product stacks up.
Avoiding Common Interpretation Pitfalls
Several pitfalls can distort SUS interpretation:
- Overemphasizing small differences: A 3-point change (e.g., 70 to 73) is rarely statistically significant unless backed by large sample sizes.
- Ignoring qualitative context: A high SUS score doesn’t mean there are no usability problems—some users might tolerate poor design due to necessity.
- Comparing across dissimilar systems: Don’t compare a mobile banking app to a video game using SUS alone—the contexts and user expectations differ too much.
Always pair SUS scores with qualitative feedback to get the full picture.
Strengths and Limitations of the System Usability Scale
No tool is perfect. While the System Usability Scale is one of the most trusted usability metrics, it has both notable strengths and important limitations that practitioners must understand.
Key Strengths of SUS
The enduring popularity of the System Usability Scale is no accident. Its strengths include:
- Reliability and Validity: Numerous studies have confirmed SUS’s internal consistency (Cronbach’s alpha typically > 0.9) and its ability to discriminate between usable and unusable systems.
- Agility: With only 10 questions, SUS minimizes respondent fatigue and can be deployed quickly.
- Universality: SUS works across cultures, languages, and technologies. It’s been translated into over 30 languages and used in fields from aviation to healthcare.
- Cost-Effectiveness: No licensing fees, no specialized software—just a simple questionnaire.
Recognized Limitations and Criticisms
Despite its strengths, SUS is not without criticism:
- Lack of diagnostic detail: As mentioned, SUS doesn’t tell you *what* to fix—only *that* something might be wrong.
- Subjectivity: SUS measures perceived usability, not objective performance (e.g., task success rate or time on task).
- Neutral midpoint bias: The 5-point scale includes a neutral option, which some users may overuse, reducing sensitivity.
- Assumption of unidimensionality: SUS treats usability as a single construct, but usability includes learnability, efficiency, memorability, error tolerance, and satisfaction—SUS blends them all.
Researchers have proposed alternatives like the UMUX (Usability Metric for User Experience) or SUPR-Q to address some of these issues, but SUS remains the gold standard due to its proven track record.
When to Use SUS vs. Other Usability Metrics
SUS excels in summative evaluations—measuring usability after a design is implemented. For formative testing (during design), consider combining SUS with:
- Task Success Rate: What percentage of users completed key tasks?
- Time on Task: How long did it take?
- Error Rate: How many mistakes were made?
- Net Promoter Score (NPS): Will users recommend the product?
A holistic UX evaluation uses SUS as one piece of a larger puzzle.
Practical Applications of the System Usability Scale Across Industries
The versatility of the System Usability Scale makes it applicable far beyond software interfaces. Organizations across sectors use SUS to improve products, services, and internal tools.
Technology and Software Development
In tech, SUS is a staple for evaluating websites, mobile apps, and enterprise software. Companies like Google, Microsoft, and Adobe use SUS internally to compare design iterations and validate usability improvements.
For example, a SaaS company might run SUS before and after a UI overhaul. If the score jumps from 62 to 79, that’s strong evidence the redesign improved user perception.
Healthcare and Medical Devices
In healthcare, usability isn’t just about convenience—it’s a safety issue. The FDA recommends usability testing for medical devices, and SUS is frequently used to assess systems like infusion pumps, EHRs (Electronic Health Records), and patient portals.
A study published in BMJ Quality & Safety found that SUS scores correlated strongly with error rates in clinical software—lower SUS scores predicted higher risk of user-induced errors.
Education and E-Learning Platforms
E-learning platforms use SUS to evaluate course interfaces, learning management systems (LMS), and student engagement tools. A high SUS score suggests students can focus on learning rather than struggling with navigation.
Universities often administer SUS after online exam sessions or module completions to identify pain points in digital learning environments.
How to Improve Your System Usability Scale Score
A low SUS score isn’t a death sentence—it’s a starting point for improvement. Once you identify usability issues, you can take targeted actions to boost your score.
Conduct Follow-Up Interviews
After collecting SUS data, conduct brief interviews with participants to explore their ratings. Ask open-ended questions like:
- “What made the system difficult to use?”
- “Was there a moment when you felt stuck?”
- “If you could change one thing, what would it be?”
This qualitative insight helps decode the “why” behind the numbers.
Run Comparative Usability Testing
Use SUS as a baseline, then test alternative designs. For example, if users rate navigation poorly, create two prototype menus and run A/B testing with SUS as the outcome metric.
This approach isolates variables and identifies which changes have the biggest impact on perceived usability.
Iterate and Re-Test Regularly
Usability isn’t a one-time fix. As features evolve, new issues emerge. Establish a cadence for regular SUS assessments—quarterly, post-release, or after major updates.
Tracking SUS over time reveals trends and helps build a culture of continuous improvement.
Future of the System Usability Scale: Evolution and Alternatives
While the System Usability Scale remains dominant, the field of usability measurement is evolving. New tools and methodologies are emerging to complement—or potentially replace—SUS in specific contexts.
Modern Alternatives to SUS
Several newer questionnaires aim to refine or expand on SUS:
- UMUX-Lite: A 2-item version of the Usability Metric for User Experience, highly correlated with SUS but even shorter.
- SUPR-Q: Measures usability, trust, appearance, and loyalty—ideal for websites.
- PSSUQ (Post-Study System Usability Questionnaire): A more detailed 16-item NASA-developed survey for in-depth analysis.
While these tools offer advantages, none have matched SUS’s widespread adoption and validation history.
Integration with Behavioral Analytics
The future lies in combining SUS with behavioral data. Tools like Hotjar, FullStory, or Google Analytics can track user clicks, scroll depth, and drop-off points—data that, when paired with SUS scores, creates a powerful mixed-methods approach.
For example, a user with a low SUS score who also abandoned a checkout flow three times provides both attitudinal and behavioral evidence of a problem.
AI and Predictive Usability Modeling
Emerging AI tools can now predict SUS scores based on design patterns. By analyzing UI elements—button placement, color contrast, form length—machine learning models estimate likely usability outcomes before user testing even begins.
While not a replacement for real-user feedback, these tools can prioritize which designs to test first, saving time and resources.
What is the System Usability Scale?
The System Usability Scale (SUS) is a 10-item questionnaire used to measure the perceived usability of a system. It produces a score from 0 to 100, with higher scores indicating better usability. Developed by John Brooke in 1986, it’s widely used due to its simplicity and reliability.
How do you calculate a SUS score?
To calculate a SUS score: for odd-numbered items, subtract 1 from the response; for even-numbered items, subtract the response from 5. Sum all adjusted scores and multiply by 2.5. The result is a score out of 100.
What is a good SUS score?
A SUS score of 68 is average. Scores above 77 are considered good, 78–85 is excellent, and 85+ is outstanding. Below 68 suggests room for improvement.
Can SUS be used for mobile apps?
Yes, the System Usability Scale is platform-agnostic and widely used for mobile apps, websites, software, and even physical devices with digital interfaces.
Is the SUS questionnaire free to use?
Yes, the System Usability Scale is in the public domain and free for anyone to use without licensing fees or restrictions.
The System Usability Scale remains one of the most effective, accessible tools for measuring user experience. Its blend of simplicity, reliability, and broad applicability makes it indispensable for UX researchers, designers, and product teams. While it has limitations—particularly its lack of diagnostic detail—its value in providing a quick, standardized usability benchmark is unmatched. By understanding how to administer, interpret, and act on SUS data, organizations can make smarter design decisions, improve user satisfaction, and ultimately build better products. As technology evolves, so too will usability measurement—but for now, the System Usability Scale stands strong as a cornerstone of user-centered design.
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