Ever wondered how to measure if a product is truly user-friendly? Enter the System Usability Scale—a simple yet powerful tool that reveals the real usability of any system.
What Is the System Usability Scale?
The System Usability Scale (SUS) is a widely adopted, reliable questionnaire used to evaluate the perceived usability of a product, software, website, or application. Developed in the late 1980s by John Brooke at Digital Equipment Corporation, SUS has become a gold standard in usability testing due to its simplicity, consistency, and effectiveness.
Origins and Development of SUS
The System Usability Scale was first introduced in 1986 as a quick and practical method to assess usability across different systems without requiring extensive resources. At the time, usability testing was often complex, time-consuming, and required qualitative observations. Brooke aimed to create a lightweight, quantitative tool that could be applied universally.
Despite being developed over three decades ago, the SUS remains remarkably relevant. Its enduring popularity stems from its ability to produce consistent, comparable results across diverse technologies and user groups. The original research paper, though brief, laid the foundation for one of the most cited tools in human-computer interaction literature.
Structure of the 10-Item Questionnaire
The SUS consists of 10 statements, each rated on a 5-point Likert scale ranging from “Strongly Disagree” to “Strongly Agree.” 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.
- I think that I would need the support of a technical person to be able to use this system.
Each item is scored and then transformed using a specific formula to generate a final SUS score between 0 and 100. This score provides a standardized metric for comparing usability across different products or iterations.
Why SUS Stands Out Among Usability Metrics
Unlike other usability assessment tools that require observational studies, eye-tracking, or task success rates, the System Usability Scale is purely attitudinal. It captures users’ subjective perceptions of usability, which are critical because even a technically efficient system can fail if users perceive it as difficult or frustrating.
Its brevity makes it ideal for integration into usability tests, beta programs, or post-release feedback loops. Moreover, because it’s technology-agnostic, it can be used to evaluate everything from mobile apps to medical devices. According to research published by Taylor & Francis, SUS correlates strongly with user satisfaction and perceived ease of use.
How to Administer the System Usability Scale
Administering the System Usability Scale correctly is crucial to obtaining valid and reliable data. While the questionnaire itself is short, the context in which it’s used significantly impacts the quality of insights gained.
Best Practices for Timing and Context
The optimal time to administer the SUS is immediately after a user completes a set of representative tasks with the system. This ensures that their experience is fresh and grounded in actual interaction rather than vague impressions.
For example, in a usability test where participants complete five core tasks—such as signing up, navigating a dashboard, and making a purchase—the SUS should be presented right after the final task. Delaying the survey risks memory decay and reduces accuracy.
Additionally, the SUS should not be used in isolation. It works best when combined with qualitative feedback, behavioral metrics (like task completion time), and observational notes. This triangulation provides a fuller picture of usability.
Who Should Take the SUS Survey?
The ideal respondents are actual or representative users of the system. For consumer-facing apps, this means recruiting individuals who match the target demographic. For enterprise software, it might involve employees from relevant departments.
Research suggests that a sample size of at least 15–20 users provides a reasonably stable SUS score. While smaller samples (even 5–8 users) can offer directional insights, larger samples increase confidence in the results. A study by Sauro and Lewis (MeasuringU) shows that SUS scores stabilize with around 15 participants, making it cost-effective for most organizations.
Common Mistakes to Avoid When Using SUS
One of the most frequent errors is modifying the wording of the SUS questions. Even small changes can invalidate the scoring model and make comparisons to benchmark data unreliable. The official SUS is copyrighted, but it’s freely available for non-commercial use under certain conditions.
Another mistake is administering the SUS without any user interaction. Asking someone to rate a system they’ve only seen in a demo video or heard about secondhand leads to inaccurate results. The SUS measures post-task perception, not general opinion.
Finally, some teams misinterpret the SUS score as a diagnostic tool. While it tells you *how usable* a system is, it doesn’t explain *why*. To uncover root causes, pair SUS with follow-up interviews or open-ended questions.
Scoring and Interpreting the System Usability Scale
One of the most powerful aspects of the System Usability Scale is its standardized scoring method. Despite its simplicity, the scoring algorithm allows for meaningful comparisons across industries, platforms, and time.
Step-by-Step Scoring Process
Each of the 10 SUS items is rated from 1 (Strongly Disagree) to 5 (Strongly Agree). The scoring alternates: odd-numbered items are scored as (response – 1), while even-numbered items are reverse-scored as (5 – response).
After converting each response, the total is summed and multiplied by 2.5 to normalize the score to a 0–100 scale. For example:
- User answers all items with “3” (neutral): Total = (2+2+2+2+2) * 2.5 = 50
- User answers all positively (5s on odd, 1s on even): Total = (4+4+4+4+4) * 2.5 = 100
This formula ensures that a neutral response pattern yields a score of 50, which serves as a baseline for comparison.
Understanding SUS Score Benchmarks
While there’s no universal “passing” score, researchers have established interpretive guidelines based on extensive data collection. According to Sauro and Lewis (2006), the average SUS score across thousands of studies is approximately 68.
Here’s a commonly used grading scale:
- 90–100: Excellent
- 80–89: Good
- 70–79: Acceptable
- 60–69: Poor
- Below 60: Awful
A score above 68 is considered above average, while anything below 60 indicates significant usability issues. However, context matters: a medical device may require a higher threshold than an internal tool with limited users.
Comparing SUS Scores Across Products
One of the most valuable uses of the System Usability Scale is comparative testing. By administering SUS before and after a redesign, or to competing products, teams can quantify improvements or identify weaknesses.
For instance, if Product A scores 62 and Product B scores 78 in the same user group, the 16-point difference suggests a meaningful usability advantage. This kind of data is especially persuasive when presenting findings to stakeholders or justifying design investments.
Organizations like the Nielsen Norman Group have compiled benchmark databases showing typical SUS scores for e-commerce sites, mobile banking apps, and enterprise software, enabling contextual interpretation.
Advantages of Using the System Usability Scale
The enduring popularity of the System Usability Scale isn’t accidental. Its widespread adoption stems from a combination of practical, methodological, and psychological advantages that make it uniquely suited for modern usability evaluation.
Simplicity and Ease of Use
One of the biggest strengths of the System Usability Scale is its simplicity. The 10-item questionnaire takes less than 5 minutes to complete, minimizing respondent fatigue and increasing completion rates.
Unlike complex usability frameworks that require specialized training, SUS can be administered by designers, product managers, or researchers with minimal instruction. This democratizes usability assessment, allowing even small teams to gather meaningful data.
Reliability and Validity
Despite its brevity, the SUS demonstrates high internal consistency and test-retest reliability. Studies have shown Cronbach’s alpha values typically above 0.9, indicating strong reliability.
Its validity has been confirmed across cultures, languages, and domains. Translations of SUS exist in over 30 languages, and cross-cultural research shows consistent performance. For example, a study in Japan and the U.S. found nearly identical SUS score distributions for the same software, suggesting cultural neutrality.
Cost-Effectiveness for Teams of All Sizes
For startups and small businesses, the System Usability Scale offers a low-cost way to validate design decisions. There’s no licensing fee for non-commercial use, and no need for expensive tools or labs.
Even large enterprises benefit from its scalability. SUS can be embedded in automated user testing platforms, integrated into customer feedback systems, or used in global usability studies with minimal overhead.
Limitations and Criticisms of the System Usability Scale
While the System Usability Scale is a powerful tool, it’s not without limitations. Understanding these weaknesses is essential for using SUS responsibly and avoiding misinterpretation of results.
Lack of Diagnostic Detail
The SUS provides a global usability score but doesn’t pinpoint specific problems. A low score tells you something is wrong, but not what. For example, a score of 50 could result from poor navigation, confusing terminology, or slow performance—each requiring different fixes.
To address this, many practitioners supplement SUS with qualitative methods. After collecting the SUS score, they ask follow-up questions like, “What was the most frustrating part of using the system?” or “Where did you feel lost?”
Sensitivity to User Expectations and Context
User expectations heavily influence SUS responses. A tech-savvy audience may rate a simple app lower because they expect advanced features, while novice users might give high scores due to low expectations.
Similarly, context affects scores. A user completing tasks under time pressure or in a noisy environment may perceive the system as less usable, even if the interface is sound. This situational bias must be considered when interpreting results.
Subjectivity and Response Bias
Because SUS relies on self-reported data, it’s vulnerable to various biases. Social desirability bias may lead users to give higher scores to please the researcher. Acquiescence bias—where users tend to agree with statements—can also distort results, especially if the questionnaire isn’t properly counter-balanced.
While the alternating positive/negative phrasing in SUS helps mitigate this, it doesn’t eliminate it entirely. Researchers should remain cautious when drawing strong conclusions from SUS alone.
Practical Applications of the System Usability Scale
The System Usability Scale isn’t just a theoretical tool—it’s actively used across industries to improve products, guide design decisions, and measure success. Its versatility makes it applicable in both research and real-world business environments.
Using SUS in Product Design and Iteration
Design teams use SUS to validate prototypes and track usability improvements over time. For example, a UX team might test a low-fidelity prototype with 10 users, collect SUS scores, make design changes, and retest.
A rising SUS score across iterations indicates progress. If the score jumps from 58 to 75 after simplifying the checkout flow, the team has quantitative evidence that the change improved usability.
Companies like Spotify and Airbnb have reportedly used SUS in their design sprints to prioritize features and validate user experience enhancements.
SUS in Academic and Industry Research
The System Usability Scale is one of the most cited usability instruments in academic literature. Researchers use it to compare interface designs, evaluate new technologies (like VR or voice assistants), and study human factors in complex systems.
In healthcare, SUS has been used to assess electronic health record (EHR) systems, telemedicine platforms, and patient portals. A study published in the Journal of Medical Internet Research found that SUS effectively differentiated between usable and problematic health apps.
Its presence in peer-reviewed journals underscores its credibility and methodological rigor.
Integrating SUS into Agile and UX Workflows
In agile environments, where speed and iteration are key, the System Usability Scale fits seamlessly into sprint reviews and usability testing cycles. Teams can run quick 5-user tests at the end of each sprint and track SUS trends over time.
Some organizations embed SUS into their continuous integration pipelines. For example, after a new build is deployed to a staging environment, a small group of beta testers completes key tasks and submits a SUS response. This creates a real-time usability dashboard.
Tools like UserTesting, Lookback, and Maze allow automated SUS collection, making it easier than ever to integrate into modern UX workflows.
Alternatives and Complements to the System Usability Scale
While the System Usability Scale is highly effective, it’s not the only usability metric available. Depending on the goals of the evaluation, teams may choose to use alternative or complementary tools to gain deeper insights.
Nielsen’s Usability Attributes and HEART Framework
Google’s HEART framework (Happiness, Engagement, Adoption, Retention, Task Success) expands beyond usability to include broader user experience metrics. While SUS aligns closely with the “Happiness” component, HEART encourages a more holistic view.
Similarly, Jakob Nielsen’s usability heuristics provide a qualitative checklist for evaluating interfaces. When combined with SUS, heuristics can help diagnose why a score is low, offering actionable design recommendations.
UMUX and UMUX-Lite: Modern Alternatives
The Usability Metric for User Experience (UMUX) is a 4-item scale based on ISO definitions of usability. It’s designed to be more modern and conceptually aligned with current UX standards. UMUX-Lite, a 2-item version, offers even greater brevity.
Studies show UMUX correlates highly with SUS (r > 0.9), making it a viable alternative when survey length is a concern. However, SUS still has broader benchmark data and industry recognition.
Combining SUS with Behavioral Metrics
The most robust usability evaluations combine SUS with behavioral data. For example:
- Task success rate: Did users complete the task?
- Time on task: How long did it take?
- Error rate: How many mistakes were made?
- Click path analysis: What route did users take?
When SUS scores are high but task success is low, it may indicate users are satisfied despite struggling—perhaps due to perseverance or low expectations. Conversely, high task success with low SUS scores might suggest efficiency without enjoyment.
“The SUS is not a diagnostic tool, but it’s an excellent thermometer for the overall health of a user interface.” — Dr. James Lewis, IBM Research
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 in an era of AI, voice interfaces, and immersive experiences.
Adapting SUS for Emerging Technologies
Researchers are exploring how to apply the System Usability Scale to non-traditional interfaces. For voice assistants like Alexa or Google Assistant, modified versions of SUS are used to assess conversational usability.
In virtual reality (VR) and augmented reality (AR), SUS helps evaluate spatial interaction, motion sickness, and intuitiveness of gestures. While the core questions remain, context-specific instructions ensure relevance.
Some teams use SUS alongside specialized scales like the IGroup Presence Questionnaire (IPQ) or NASA-TLX (for cognitive load) to get a multi-dimensional view.
Automated SUS Collection and Real-Time Analytics
With the rise of AI-powered analytics platforms, SUS collection is becoming automated and integrated into user journeys. Chatbots can deliver SUS surveys post-interaction, and machine learning models can predict SUS scores based on behavioral patterns.
Real-time dashboards display SUS trends across user segments, geographies, or device types, enabling rapid response to usability issues. For example, if iOS users consistently score 15 points lower than Android users, the team can investigate platform-specific problems.
Potential Evolution of SUS in the Next Decade
While the core SUS formula is likely to remain unchanged, its application may evolve. We may see:
- Dynamic SUS: Adaptive questioning based on user behavior.
- Emotion-integrated SUS: Adding sentiment analysis from facial recognition or voice tone.
- Longitudinal SUS tracking: Monitoring usability over time as users gain proficiency.
Despite these innovations, the simplicity and reliability of the original SUS will likely keep it in use for years to come.
What is the System Usability Scale used for?
The System Usability Scale (SUS) is used to measure the perceived usability of a system, such as a website, app, or software. It provides a quick, reliable score that helps teams evaluate user experience, compare design alternatives, and track improvements over time.
How is the SUS score calculated?
Each of the 10 SUS questions is scored on a 5-point scale. Odd-numbered items are scored as (response – 1), even-numbered items as (5 – response). The sum of these values is multiplied by 2.5 to produce a final score between 0 and 100.
What is a good SUS score?
A score of 68 is average. Scores above 80 are considered good, while those above 90 are excellent. Below 68 is below average, and below 60 indicates significant usability problems.
Can I modify the SUS questionnaire?
It’s not recommended. Modifying the wording or structure invalidates the scoring model and prevents comparison with benchmark data. If you need a shorter version, consider UMUX-Lite instead.
Is the System Usability Scale free to use?
Yes, for non-commercial use. The SUS is in the public domain for academic and internal business purposes. However, commercial use (e.g., in a paid product) requires permission from the copyright holder.
The System Usability Scale remains one of the most trusted tools in UX research. Its blend of simplicity, reliability, and actionable insights makes it indispensable for anyone serious about user-centered design. While it has limitations, especially in diagnostic depth, its value in measuring overall usability perception is unmatched. When used correctly—paired with qualitative insights and behavioral data—SUS empowers teams to build products that are not just functional, but truly user-friendly.
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