Ever wondered how companies measure how user-friendly their products really are? Enter the System Usability Scale (SUS)—a simple yet powerful tool that reveals the truth behind user experience.
What Is the System Usability Scale (SUS)?
The System Usability Scale, commonly known as SUS, is a widely adopted 10-item questionnaire designed to evaluate the perceived usability of a system, product, or service. Developed in the late 1980s by John Brooke at Digital Equipment Corporation, SUS has since become a gold standard in usability assessment 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 quick and reliable method to assess usability without requiring complex observational studies. At the time, usability testing was often resource-intensive, involving detailed task analysis and expert evaluations. Brooke aimed to create a lightweight, yet effective, subjective measure that could be administered quickly after a user interaction.
Despite its simplicity, SUS was grounded in psychometric principles. It was designed to be technology-agnostic, meaning it could be applied to any interactive system regardless of its function or interface type. This flexibility contributed significantly to its widespread adoption.
Over the decades, SUS has been validated across numerous studies and translated into multiple languages. Its enduring relevance is a testament to its robustness and adaptability in an ever-evolving digital landscape. You can learn more about its original development in the original research paper by John Brooke.
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. For example:
- I think that I would like to use this system frequently. (Positive)
- I found the system unnecessarily complex. (Negative)
- I thought the system was easy to use. (Positive)
The scoring mechanism is unique: odd-numbered items are scored by subtracting 1 from the user response (e.g., a response of 4 becomes 3), while even-numbered items are reverse-scored by subtracting the user response from 5 (e.g., a response of 4 becomes 1). These values are then summed and multiplied by 2.5 to yield a final score between 0 and 100.
This formula ensures that the SUS score is normalized and comparable across different systems and contexts. A score of 68 is considered average, with anything above 70 generally seen as above average usability.
Why SUS Stands Out Among Usability Metrics
Among the many usability evaluation tools—such as the Net Promoter Score (NPS), User Experience Questionnaire (UEQ), and Task Success Rate—SUS remains one of the most trusted. Its strength lies in its balance of simplicity and reliability.
Unlike more complex tools that require extensive training or statistical analysis, SUS can be administered by anyone with basic research knowledge. It doesn’t require video recording, eye-tracking, or usability labs. All you need is a user who has interacted with the system and 5–10 minutes to complete the survey.
Moreover, SUS provides a single, easy-to-interpret score that stakeholders—from designers to executives—can understand. This makes it ideal for benchmarking, comparing design iterations, or presenting findings to non-technical audiences.
“The SUS is probably the most widely used questionnaire for measuring perceived usability.” — James R. Lewis, Human Factors Researcher at IBM
How to Administer the System Usability Scale
Administering the System Usability Scale correctly is crucial to obtaining valid and reliable results. While the process is straightforward, small missteps can skew data and lead to incorrect conclusions.
Best Practices for Survey Administration
To get the most accurate results, the SUS should be administered immediately after a user completes a set of representative tasks with the system. Delaying the survey can lead to memory decay, where users forget specific pain points or positive experiences.
The tasks should be realistic and reflect typical user goals—such as creating an account, making a purchase, or navigating to a specific feature. This context ensures that the SUS score reflects actual usability, not just first impressions.
It’s also important to ensure anonymity and confidentiality to encourage honest responses. Users are more likely to provide critical feedback if they feel their answers won’t be traced back to them.
Choosing the Right Sample Size
One common misconception is that SUS requires a large sample size to be meaningful. In reality, research shows that even 5–10 users can provide reliable SUS scores for formative testing.
However, for summative testing—where you’re comparing two versions of a product or reporting a final usability score—a larger sample (30+ users) is recommended to increase statistical confidence.
The key is consistency: if you’re tracking SUS scores over time, use the same task set, administration method, and sample criteria to ensure comparability.
Digital vs. Paper-Based Administration
While SUS can be administered on paper, digital tools like Google Forms, SurveyMonkey, or integrated UX platforms offer several advantages. They allow for automatic scoring, real-time data collection, and easier data export for analysis.
Many online tools now include built-in SUS templates with automatic calculation. For example, MeasuringU.com offers a free SUS calculator that instantly computes the score and provides grade interpretations.
However, when using digital forms, ensure that the interface itself doesn’t interfere with the user’s experience. A poorly designed survey form could bias responses, especially if users struggle to navigate it.
Scoring and Interpreting the System Usability Scale
One of the most powerful aspects of the System Usability Scale is its scoring system, which transforms subjective opinions into an objective, comparable metric.
Step-by-Step Scoring Process
Calculating a SUS score involves a simple but precise formula:
- For each odd-numbered question (1, 3, 5, 7, 9), subtract 1 from the response (e.g., 4 becomes 3).
- For each even-numbered question (2, 4, 6, 8, 10), subtract the response from 5 (e.g., 4 becomes 1).
- Sum all the adjusted scores.
- Multiply the total by 2.5 to get the final SUS score (range: 0–100).
For example, if a user responds with all 4s, the adjusted scores would be: 3,1,3,1,3,1,3,1,3,1. The sum is 18, multiplied by 2.5 gives a SUS score of 45—well below average.
This method ensures that both positive and negative responses are properly weighted, and the final score is normalized for cross-study comparison.
Understanding SUS Score Benchmarks
Interpreting a SUS score requires context. While the average score is 68, scores can be categorized as follows:
- Below 50: Poor usability
- 50–60: Marginal usability
- 60–70: Acceptable usability
- 70–80: Good usability
- 80–90: Excellent usability
- 90+: Outstanding usability
These benchmarks, established through meta-analyses of thousands of SUS scores, help teams quickly assess performance. A score of 85, for instance, suggests a highly usable system, while a 55 indicates significant usability issues that need addressing.
It’s also useful to compare SUS scores against industry standards. For example, e-commerce platforms average around 74, while enterprise software often scores closer to 65. Knowing your benchmark helps set realistic goals.
Common Misinterpretations of SUS Scores
Despite its simplicity, the System Usability Scale is often misinterpreted. One common mistake is treating the SUS score as a diagnostic tool. While it tells you how usable a system is, it doesn’t explain why users found it difficult.
For example, a low score could result from poor navigation, confusing terminology, slow performance, or lack of feedback. Without qualitative data—such as user comments or think-aloud protocols—it’s impossible to pinpoint the root cause.
Another pitfall is comparing SUS scores from different contexts. A SUS administered after a 5-minute task shouldn’t be directly compared to one given after a 30-minute session. Task complexity, user expertise, and system familiarity all influence scores.
“SUS is a yardstick, not a microscope. It tells you how far you’ve come, not what’s blocking your path.” — Dr. Sauro, MeasuringU
Advantages of Using the System Usability Scale
The enduring popularity of the System Usability Scale is no accident. Its widespread use is backed by tangible benefits that make it a go-to tool for UX professionals worldwide.
Cost-Effective and Easy to Implement
One of the biggest advantages of the System Usability Scale is its low cost and ease of deployment. Unlike moderated usability tests that require lab setups, screen recording software, and trained facilitators, SUS can be distributed via email or embedded in an app with minimal effort.
It doesn’t require specialized training to administer or score. Even non-UX team members—like product managers or customer support—can collect SUS data during user interviews or beta testing.
This accessibility makes SUS ideal for startups, small teams, or organizations with limited UX budgets. You can gather meaningful insights without investing in expensive tools or consultants.
High Reliability and Validity
Despite its brevity, the System Usability Scale has demonstrated high internal consistency and test-retest reliability. Studies have shown Cronbach’s alpha values above 0.9, indicating strong reliability.
It also correlates well with other usability metrics, such as task completion rates and time-on-task. This means that a high SUS score usually aligns with actual user performance, not just perception.
Its validity has been confirmed across diverse domains, including healthcare, aviation, and mobile apps. Whether you’re testing a hospital information system or a fitness tracker, SUS provides consistent, trustworthy results.
Universal Applicability Across Platforms
Unlike platform-specific tools, the System Usability Scale is technology-agnostic. It can be used to evaluate desktop software, mobile apps, websites, kiosks, voice assistants, and even physical devices with digital interfaces.
This universality allows teams to compare usability across different products or track improvements over time, even as technology evolves. For example, a company can use SUS to compare the usability of its legacy desktop system with a new mobile-first redesign.
Its language has also been translated into over 30 languages, making it suitable for global user testing. Cultural adaptations have been validated to ensure consistent interpretation across regions.
Limitations and Criticisms of the System Usability Scale
While the System Usability Scale is a powerful tool, it’s not without limitations. Understanding its weaknesses is essential for using it effectively and avoiding misleading conclusions.
Lack of Diagnostic Detail
As mentioned earlier, SUS provides a high-level usability score but doesn’t explain the reasons behind it. A score of 50 tells you the system is hard to use, but not whether the issue lies in navigation, terminology, or performance.
To address this, many researchers pair SUS with open-ended questions like “What did you find most difficult?” or “What one change would improve your experience?” These qualitative insights complement the quantitative score and guide design improvements.
Some teams also use follow-up interviews or heatmaps to dig deeper into problem areas identified by low SUS scores.
Sensitivity to Context and Task Design
The SUS score is highly sensitive to how the test is conducted. If users are given overly simple tasks, they may rate the system higher than if they faced real-world challenges.
Similarly, if users are rushed or distracted during testing, their scores may not reflect true usability. Even the order of questions or the phrasing of instructions can influence responses.
To mitigate this, it’s crucial to standardize the testing protocol. Use the same task scenarios, instructions, and environment across all participants to ensure consistency.
Subjectivity and Response Bias
Because SUS relies on self-reported data, it’s vulnerable to subjective biases. Users may rate a system higher if they like the brand, or lower if they’re having a bad day.
Cultural differences can also affect responses. For example, users from some cultures may avoid extreme ratings, leading to compressed scores. Others may be more critical or optimistic by nature.
While these biases are inherent in any survey-based method, they can be minimized through careful participant selection, neutral wording, and combining SUS with behavioral data.
Practical Applications of the System Usability Scale
The System Usability Scale isn’t just a theoretical tool—it’s actively used in real-world scenarios to improve products, inform design decisions, and measure ROI on UX investments.
Product Development and Iterative Design
In agile and iterative design processes, SUS is often used to compare different versions of a product. For example, a team might test a prototype (Version A) with 10 users, collect SUS scores, make design improvements, and retest (Version B).
A significant increase in the SUS score—say from 60 to 78—provides quantitative evidence that the changes improved usability. This data can be used to justify design decisions to stakeholders or prioritize future work.
Because SUS is quick to administer, it fits well into sprint cycles, allowing teams to gather feedback every 2–4 weeks.
Comparative Usability Testing
SUS is ideal for head-to-head comparisons between competing products or design alternatives. For instance, a company might use SUS to evaluate its app against a competitor’s, or to choose between two navigation layouts.
In such cases, users complete tasks on both systems, then fill out a SUS for each. The difference in scores helps determine which design is more usable.
This approach is commonly used in benchmarking studies, where organizations track their product’s usability relative to industry leaders.
Academic and Industry Research
The System Usability Scale is one of the most cited tools in human-computer interaction (HCI) research. Its standardized format allows researchers to compare findings across studies, making it a cornerstone of usability literature.
Academic papers frequently use SUS to validate new interaction techniques, evaluate assistive technologies, or assess the impact of design interventions. Its presence in peer-reviewed journals reinforces its credibility.
Industry reports, such as those from Nielsen Norman Group or Forrester, often reference SUS benchmarks to support recommendations on UX best practices.
Alternatives and Complements to the System Usability Scale
While the System Usability Scale is a powerful standalone tool, it’s often used in conjunction with other methods to provide a more complete picture of user experience.
User Experience Questionnaire (UEQ)
The User Experience Questionnaire (UEQ) is a more detailed alternative to SUS, measuring six dimensions: Attractiveness, Perspicuity, Efficiency, Dependability, Stimulation, and Novelty.
Unlike SUS, which gives one overall score, UEQ provides a profile of user experience across multiple facets. This makes it better suited for understanding emotional and aesthetic aspects of design.
However, UEQ requires 26 items and is more complex to administer and interpret. For quick assessments, SUS remains the preferred choice.
Net Promoter Score (NPS)
The Net Promoter Score measures user loyalty by asking, “How likely are you to recommend this product to a friend or colleague?” Responses range from 0 to 10, categorizing users as Detractors, Passives, or Promoters.
While NPS is not a usability metric per se, it often correlates with SUS. A highly usable product is more likely to generate promoters. Combining NPS and SUS can reveal whether usability impacts customer loyalty.
However, NPS is more attitudinal and brand-focused, whereas SUS is specifically about usability.
Task Performance Metrics
To complement the subjective nature of SUS, many teams use objective task performance metrics such as:
- Task success rate (percentage of tasks completed)
- Time on task (how long it takes to complete a task)
- Error rate (number of mistakes made)
- Click count (number of interactions required)
These metrics provide behavioral evidence of usability, which can be cross-validated with SUS scores. For example, a high SUS score with low task success might indicate users feel the system is easy but struggle to complete tasks—possibly due to overconfidence or misleading feedback.
“The best usability evaluations combine subjective (SUS) and objective (task performance) data.” — Jakob Nielsen, NN/g
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 single score from 0 to 100, with higher scores indicating better usability. It’s widely used due to its simplicity, reliability, and broad applicability.
How is the SUS score calculated?
The SUS score is calculated by adjusting responses (odd items: subtract 1; even items: subtract from 5), summing the adjusted values, and multiplying by 2.5. This results in a final score between 0 and 100. For example, a sum of 27 becomes 67.5 after multiplication.
What is a good SUS score?
A score of 68 is average. Scores above 70 are considered above average, 80+ is good, and 90+ is excellent. However, context matters—industry benchmarks vary, so it’s best to compare against similar products or track improvements over time.
Can SUS be used for mobile apps?
Yes, the System Usability Scale is platform-agnostic and can be used for mobile apps, websites, software, and even physical systems with digital interfaces. Its language is general enough to apply across different technologies.
Is the SUS free to use?
Yes, the System Usability Scale is in the public domain and free to use for any purpose—commercial or academic. No permission is required, though proper citation of the original work by John Brooke is recommended.
The System Usability Scale remains one of the most effective, reliable, and accessible tools for measuring usability. Its simplicity belies its power, offering a standardized way to quantify user experience across diverse systems and contexts. While it has limitations—particularly in diagnostic depth—it excels as a benchmarking and comparative tool. When combined with qualitative feedback and behavioral metrics, SUS provides a comprehensive view of usability. Whether you’re a UX designer, product manager, or researcher, understanding and applying the System Usability Scale can significantly enhance your ability to create user-centered products.
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