Ever wondered how to measure if a product is truly user-friendly? The System Usability Scale (SUS) is your go-to tool—a simple yet powerful way to evaluate usability with precision and consistency.
What Is the System Usability Scale?
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 testing across industries—from software and websites to medical devices and consumer electronics.
Origins and Development of SUS
The System Usability Scale was first introduced in 1986 as a lightweight, reliable method for measuring usability without requiring extensive resources. Unlike more complex usability assessments, SUS was designed to be quick to administer and easy to score, making it ideal for both academic research and real-world product development.
Brooke’s original goal was to create a tool that could be applied across different types of systems, regardless of their function or interface. This universality is one of the key reasons why the system usability scale has stood the test of time and remains relevant over three decades later.
- Created by John Brooke in 1986
- Designed for cross-platform usability evaluation
- Published in usability research journals and widely cited
Its simplicity and adaptability allowed it to gain traction not only in human-computer interaction (HCI) circles but also in UX design, product management, and software engineering teams worldwide.
Structure of the SUS Questionnaire
The system usability scale 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.
Here are the standard 10 items:
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.Notice how odd-numbered questions are positively worded, while even-numbered ones are negatively worded..
This balance helps ensure that respondents pay attention and don’t just agree with everything..
“The beauty of the System Usability Scale lies in its simplicity. It doesn’t tell you *why* something is unusable, but it tells you *that* it is.” — Jakob Nielsen, UX Pioneer
How to Calculate the System Usability Scale Score
One of the biggest advantages of the system usability scale is its straightforward scoring method. Despite its simplicity, the calculation yields a reliable and meaningful score between 0 and 100.
Step-by-Step Scoring Process
To calculate the SUS score, follow these steps:
- For odd-numbered questions (positive statements), subtract 1 from the user’s response. For example, if a user answers “4,” you compute 4 – 1 = 3.
- For even-numbered questions (negative statements), subtract the user’s response from 5. So, if a user answers “2,” you compute 5 – 2 = 3.
- Sum all the converted values.
- Multiply the total by 2.5 to normalize the score to a 0–100 range.
Let’s say a user responds with the following answers: [4, 1, 5, 1, 5, 1, 5, 1, 5, 1]
Applying the formula:
- Q1: 4 – 1 = 3
- Q2: 5 – 1 = 4
- Q3: 5 – 1 = 4
- Q4: 5 – 1 = 4
- Q5: 5 – 1 = 4
- Q6: 5 – 1 = 4
- Q7: 5 – 1 = 4
- Q8: 5 – 1 = 4
- Q9: 5 – 1 = 4
- Q10: 5 – 1 = 4
Total sum: 3 + 4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 = 39
Final SUS score: 39 × 2.5 = 97.5
This is an exceptionally high score, indicating excellent perceived usability.
Understanding the SUS Score Range
The final SUS score ranges from 0 to 100, but scores rarely reach the extremes. Over the years, researchers have established benchmarks to interpret these scores:
- Below 50: Poor usability
- 50–60: Marginal usability
- 60–70: Acceptable usability
- 70–80: Good usability
- 80–100: Excellent usability
According to research by Sauro and Lewis (2016), the average SUS score across thousands of studies is approximately 68. This serves as a useful benchmark—if your product scores above 68, it’s performing better than average.
It’s important to note that the system usability scale does not diagnose specific usability problems. Instead, it provides a high-level snapshot of how users perceive the overall ease of use.
Why the System Usability Scale Is So Widely Used
The enduring popularity of the system usability scale can be attributed to several key strengths that make it a favorite among usability professionals, UX researchers, and product teams.
Reliability and Validity
Despite its brevity, the SUS has demonstrated strong psychometric properties. Numerous studies have confirmed its reliability (consistency of results) and validity (accuracy in measuring what it claims to measure).
A 2009 study by James R. Lewis and Jeff Sauro showed that the SUS has a high internal consistency, with a Cronbach’s alpha typically above 0.9, which is considered excellent in survey research. This means the 10 items are measuring the same underlying construct—perceived usability.
Additionally, the SUS correlates well with other usability metrics, such as task success rates and time-on-task, further validating its effectiveness.
Speed and Simplicity
One of the most compelling reasons for using the system usability scale is its efficiency. The entire questionnaire takes less than 5–10 minutes to complete, making it easy to integrate into usability tests, beta programs, or post-interaction surveys.
Unlike more complex usability frameworks that require observational data, eye-tracking, or cognitive walkthroughs, SUS can be administered remotely, at scale, and with minimal training. This makes it ideal for agile development environments where rapid feedback is essential.
For example, a SaaS company launching a new dashboard can deploy the SUS immediately after a user completes a demo task and gather actionable insights within minutes.
Universality Across Domains
The system usability scale is domain-agnostic. Whether you’re testing a mobile banking app, a hospital patient portal, or a smart home thermostat, SUS can be applied without modification.
This universality has led to its adoption in diverse fields:
- Healthcare: Evaluating electronic health record (EHR) systems (NCBI Study on EHR Usability)
- Automotive: Assessing in-car infotainment systems
- Education: Measuring usability of e-learning platforms
- Government: Testing citizen-facing digital services
Because it focuses on subjective perception rather than technical performance, SUS transcends interface types and technological complexity.
Common Misconceptions About the System Usability Scale
Despite its widespread use, the system usability scale is often misunderstood or misapplied. Clarifying these misconceptions is crucial for accurate interpretation and effective use.
Myth: SUS Measures Objective Usability
One of the most common misconceptions is that the SUS measures objective usability—such as task completion time or error rates. In reality, SUS measures perceived usability, which is how users feel about the system’s ease of use.
While perceived usability often correlates with objective performance, they are not the same. A user might complete a task quickly but still rate the system poorly on SUS if they felt frustrated or confused.
Therefore, SUS should be used alongside objective metrics for a complete picture. For instance, combining SUS with task success rates and qualitative feedback provides a richer understanding of the user experience.
Myth: SUS Can Diagnose Specific Problems
Another misconception is that the SUS can pinpoint exactly what’s wrong with a system. While individual item scores can offer hints, the SUS is not a diagnostic tool.
For example, if users consistently give low scores on Question 4 (“I think that I would need the support of a technical person…”), it might suggest a lack of intuitiveness. But it won’t tell you whether the issue lies in navigation, terminology, or layout.
To identify root causes, SUS should be paired with methods like user interviews, heuristic evaluation, or usability testing with think-aloud protocols.
“SUS is a thermometer for usability—it tells you if the system has a fever, but not what’s causing it.” — Dr. Aaron Bangor, Usability Researcher
Best Practices for Administering the System Usability Scale
To get the most value from the system usability scale, it’s essential to follow best practices in administration, timing, and interpretation.
When to Administer SUS
The timing of the SUS administration significantly impacts the quality of data. The optimal moment is immediately after a user completes a set of representative tasks with the system.
For example:
- After a user finishes signing up for a service
- Following a product search and purchase on an e-commerce site
- Right after navigating a new software feature
Administering SUS too early (before interaction) or too late (days after use) can lead to inaccurate or biased responses. Memory decay and external influences may distort perceptions.
For longitudinal studies, SUS can be administered at multiple points (e.g., after first use, one week later, one month later) to track changes in perceived usability over time.
How to Interpret SUS Scores Contextually
A SUS score should never be interpreted in isolation. Context is everything. A score of 72 might be excellent for a complex enterprise tool but poor for a consumer mobile app.
Always compare your results to:
- Benchmarks within your industry
- Previous versions of your product
- Competitor products (via benchmark studies)
For instance, a study by Sauro (2018) found that mobile apps average around 74 on SUS, while enterprise software averages closer to 65. Knowing these benchmarks helps set realistic goals.
Additionally, consider the demographic and experience level of your users. A system used primarily by experts may score lower on SUS simply because novices find it complex—even if it’s highly efficient for trained users.
Combining SUS With Other UX Metrics
The true power of the system usability scale emerges when it’s part of a broader UX measurement strategy. Consider combining SUS with:
- NPS (Net Promoter Score): Measures willingness to recommend
- SEQ (Single Ease Question): Assesses ease of a single task
- UMUX (Usability Metric for User Experience): A shorter alternative to SUS
- Task Success Rate: Objective measure of whether users completed tasks
- Time-on-Task: How long users take to complete actions
This multi-metric approach allows you to correlate subjective satisfaction (SUS) with objective performance, giving stakeholders a comprehensive view of usability.
Alternatives and Evolutions of the System Usability Scale
While the system usability scale remains the gold standard, several alternatives and adaptations have emerged to address specific needs or limitations.
UMUX and UMUX-Lite
The Usability Metric for User Experience (UMUX), developed by Lewis and Sauro, is based on ISO 9241-11 and consists of four items that measure effectiveness, efficiency, and satisfaction.
UMUX-Lite is an even shorter version with just two questions:
- This system’s capabilities meet my requirements.
- This system is easy to use.
These are rated on a 7-point scale and can be quickly administered. While not as robust as SUS, UMUX-Lite is useful for continuous monitoring or in-app micro-surveys.
Interestingly, UMUX-Lite scores can be rescaled to approximate SUS scores, allowing for direct comparison.
Nielsen Norman Group’s SUS Benchmarking
The Nielsen Norman Group (NN/g) has conducted extensive research using the system usability scale across hundreds of usability tests. They’ve published benchmark data that helps organizations contextualize their scores.
For example, NN/g reports that:
- E-commerce websites average a SUS score of 67
- Consumer software averages 72
- Internal enterprise tools average 63
Access to such benchmarks allows teams to answer: “Is a 70 good enough?” The answer depends on your domain.
Learn more about NN/g’s usability benchmarks at Nielsen Norman Group – Usability Testing 101.
SUS in Non-English and Cross-Cultural Contexts
The system usability scale has been translated into over 30 languages and used globally. However, direct translation isn’t always sufficient—cultural differences in response styles (e.g., tendency to agree or avoid extremes) can affect scores.
Researchers recommend:
- Using back-translation to verify accuracy
- Conducting pilot tests with native speakers
- Adjusting for cultural bias in survey responses
Studies have shown that SUS maintains good reliability across cultures, but norms may vary. For instance, users in some Asian countries tend to give higher average scores than Western users, even for the same system.
Real-World Applications of the System Usability Scale
The system usability scale isn’t just a theoretical tool—it’s actively used by leading companies and organizations to improve products and services.
Case Study: Improving a Healthcare App
A U.S.-based telehealth company used SUS to evaluate the usability of its patient portal. Initial testing with 20 users yielded an average SUS score of 58—below the acceptable threshold.
Qualitative feedback revealed confusion around appointment scheduling and medication tracking. The design team simplified the navigation, added tooltips, and improved error messages.
After redesign, a follow-up test showed an average SUS score of 79—a dramatic improvement indicating good usability. This increase correlated with a 30% rise in user engagement and a 25% reduction in support calls.
Case Study: Evaluating Enterprise Software
A global logistics firm introduced a new warehouse management system. Field tests with 15 employees resulted in an average SUS score of 52, signaling poor usability.
Further investigation found that workers struggled with the touchscreen interface in low-light conditions and found the menu structure illogical. Based on these insights, the company redesigned the UI with larger buttons, high-contrast colors, and a flatter menu hierarchy.
In the second round, the SUS score improved to 74, and training time decreased by 40%.
Using SUS in Academic Research
The system usability scale is a staple in academic research on human-computer interaction. Its reliability and ease of use make it ideal for controlled experiments, comparative studies, and longitudinal research.
For example, a 2020 study published in the International Journal of Human-Computer Interaction used SUS to compare the usability of voice assistants (Alexa, Google Assistant, Siri). Results showed Google Assistant scoring highest (SUS 82), followed by Alexa (78), and Siri (74).
Such studies help identify trends and inform future design decisions across the tech industry.
What is the ideal sample size for SUS?
A minimum of 15–20 users is recommended to achieve reliable SUS results. While SUS can be calculated with as few as 5 users, larger samples reduce variability and increase confidence in the findings. For high-stakes decisions, aim for 30+ participants.
Can SUS be used for websites and mobile apps?
Absolutely. The system usability scale is platform-agnostic and widely used for websites, mobile apps, desktop software, and even physical-digital hybrids like kiosks or wearables. Its flexibility makes it one of the most versatile usability tools available.
Is the System Usability Scale free to use?
Yes, the System Usability Scale is in the public domain and free for both commercial and non-commercial use. No permission is required to administer or publish SUS results. However, proper citation of the original work by John Brooke is encouraged.
How often should SUS be administered?
SUS should be administered whenever a significant change is made to the user interface or after key user interactions. For ongoing products, quarterly SUS assessments can help track usability trends over time. In agile environments, it can be used after each major sprint involving UX changes.
Can SUS predict user satisfaction?
Yes, SUS has a strong correlation with overall user satisfaction. While it doesn’t measure satisfaction directly, high SUS scores consistently align with positive user attitudes, loyalty, and willingness to recommend. It’s often used as a proxy for user satisfaction in UX research.
The System Usability Scale remains a cornerstone of usability evaluation because it strikes the perfect balance between simplicity and insight. While it doesn’t replace in-depth user research, it provides a fast, reliable, and standardized way to measure perceived usability across any system. By understanding how to properly administer, score, and interpret SUS, teams can make data-driven decisions that enhance user experience and drive product success. Whether you’re a UX designer, product manager, or researcher, mastering the system usability scale is a powerful step toward building more usable, enjoyable, and effective products.
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