Ever wondered how to measure if a product is truly user-friendly? Enter the System Usability Scale (SUS)—a simple, reliable tool that’s revolutionized how we assess usability across tech, healthcare, and beyond.
What Is the System Usability Scale (SUS)?
The System Usability Scale, commonly known as SUS, is a 10-item questionnaire designed to evaluate the perceived usability of a system, product, or service. Developed in 1986 by John Brooke at Digital Equipment Corporation, SUS has become one of the most widely adopted usability assessment tools in the world. Its strength lies in its simplicity, speed, and reliability.
Origins and Development of SUS
The System Usability Scale was born out of a need for a quick, standardized way to measure usability without requiring complex testing environments. At the time, usability testing was often qualitative and subjective. Brooke sought a quantitative method that could be applied across different systems—whether software, websites, or hardware interfaces.
He developed a 10-statement questionnaire where users rate their agreement on a 5-point Likert scale. The result is a single score from 0 to 100, providing a clear, comparable metric. Despite its age, SUS remains relevant because it’s technology-agnostic and easy to administer.
- Created in 1986 by John Brooke
- Designed for broad applicability across systems
- Published in a technical report, not a peer-reviewed journal initially
Interestingly, SUS wasn’t immediately embraced by the academic community. It gained traction slowly through practitioner adoption. Over time, its reliability and validity were confirmed through numerous studies, cementing its status as a gold standard in usability measurement.
How SUS Differs From Other Usability Metrics
Unlike task-based metrics such as success rate or time-on-task, SUS measures perceived usability. It doesn’t require observers or usability labs. Instead, it captures the user’s holistic experience after interacting with a system.
Compared to longer questionnaires like the SUMI (Software Usability Measurement Inventory) or UMUX (Usability Metric for User Experience), SUS is shorter and easier to score. It strikes a balance between brevity and accuracy, making it ideal for agile development cycles.
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“SUS is not the most sophisticated tool, but it’s the most practical.” — Jakob Nielsen, Nielsen Norman Group
Another key differentiator is its scoring algorithm. SUS uses an odd-even item scoring method that alternates between positive and negative phrasing, reducing response bias. This clever design contributes to its robustness across diverse user groups.
How the System Usability Scale Works
The mechanics of the System Usability Scale are deceptively simple. It consists of 10 statements, each rated on a 5-point scale from “Strongly Disagree” (1) to “Strongly Agree” (5). The statements alternate between positive and negative wording to prevent straight-lining—where users select the same response for all items.
The 10 SUS Questions Explained
Here are the standard SUS questions, along with an explanation of what each aims to assess:
I think that I would like to use this system frequently.– Measures perceived usefulness and willingness to adopt.I found the system unnecessarily complex.– Assesses perceived complexity (reverse-scored).I thought the system was easy to use.– Direct measure of perceived ease of use.I think that I would need the support of a technical person to be able to use this system.– Evaluates self-sufficiency and intuitiveness (reverse-scored).I found the various functions in this system were well integrated.– Looks at coherence and consistency of features.I thought there was too much inconsistency in this system.
.– Measures perceived inconsistency (reverse-scored).I would imagine that most people would learn to use this system very quickly.– Assesses learnability from a social perspective.I found the system very cumbersome to use.– Evaluates efficiency and friction (reverse-scored).I felt very confident using the system.– Measures user confidence and perceived control.I needed to learn a lot of things before I could get going with this system.– Assesses initial learning curve (reverse-scored).Each question is carefully crafted to tap into different aspects of usability: learnability, efficiency, memorability, errors, and satisfaction—aligning indirectly with Jakob Nielsen’s usability heuristics..
Scoring the System Usability Scale
Scoring SUS involves a specific algorithm. For odd-numbered items, you subtract 1 from the user response. For even-numbered items (which are negatively worded), you subtract the user response from 5. Then, you sum all the adjusted values and multiply by 2.5 to get a final score between 0 and 100.
For example, if a user responds with a 4 on item 1 (positive), the adjusted score is 4 – 1 = 3. If they respond with a 2 on item 2 (negative), the adjusted score is 5 – 2 = 3. This process continues for all 10 items.
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The formula: (Sum of adjusted scores) × 2.5
While manual calculation is possible, many researchers use online SUS calculators or integrate scoring into survey tools like Google Forms or Qualtrics. Tools like the MeasuringU SUS Calculator automate the process and provide instant results.
“The SUS score is not a percentage, but it’s scaled to look like one.” — James Lewis, IBM Research
It’s important to note that SUS scores are relative, not absolute. A score of 68 is considered average, based on decades of aggregated data. Anything above 70 is generally seen as above average, while scores below 50 indicate significant usability issues.
Why the System Usability Scale Is So Widely Used
The enduring popularity of the System Usability Scale can be attributed to its unique combination of simplicity, reliability, and versatility. It’s used by UX researchers, product managers, and developers across industries—from fintech to healthcare apps.
Speed and Simplicity of Administration
One of the biggest advantages of SUS is how fast it is to administer. The questionnaire takes users less than 5 minutes to complete. This makes it ideal for usability testing sessions, beta releases, or even post-purchase surveys.
Because it’s short, users are more likely to complete it, reducing dropout rates. It can be embedded in digital platforms, sent via email, or administered on paper. Its low cognitive load ensures that feedback is collected without burdening participants.
Compared to in-depth interviews or cognitive walkthroughs, SUS offers a lightweight alternative that still delivers actionable insights. It’s especially useful in iterative design, where quick feedback loops are essential.
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Proven Reliability and Validity
Despite its brevity, SUS has strong psychometric properties. Numerous studies have confirmed its internal consistency, with Cronbach’s alpha typically ranging from 0.85 to 0.90—indicating high reliability.
Research by James R. Lewis and others has shown that SUS scores correlate well with other usability metrics, including task success, time-on-task, and user satisfaction. This construct validity makes SUS a trustworthy tool for both formative and summative evaluations.
A 2013 meta-analysis published in the International Journal of Human-Computer Interaction confirmed that SUS is valid across languages, cultures, and domains. It has been translated into over 30 languages, including Chinese, Arabic, and Russian, with consistent results.
- High internal consistency (Cronbach’s alpha > 0.85)
- Strong correlation with objective usability metrics
- Valid across cultures and languages
Its robustness has been tested in everything from medical devices to e-learning platforms, proving its adaptability.
Applications of the System Usability Scale Across Industries
The System Usability Scale is not limited to software or websites. Its flexibility allows it to be applied in diverse fields where user interaction matters. From healthcare to automotive interfaces, SUS provides a common language for measuring usability.
Technology and Software Development
In tech companies, SUS is often used during usability testing phases to compare design iterations. For example, a team might test two versions of a mobile app and use SUS scores to determine which one feels more intuitive.
Startups use SUS to validate MVPs (Minimum Viable Products) before scaling. Enterprise software providers use it to benchmark their products against competitors. Because SUS produces a single number, it’s easy to track usability improvements over time.
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Companies like Google, Microsoft, and IBM have incorporated SUS into their UX research protocols. It’s also used in academic research to evaluate new interaction paradigms, such as voice interfaces or augmented reality.
Healthcare and Medical Devices
In healthcare, usability can be a matter of life and death. The FDA recommends usability testing for medical devices, and SUS is frequently used to assess the ease of use of equipment like infusion pumps, EHR (Electronic Health Record) systems, and diagnostic tools.
A study published in Applied Ergonomics found that SUS scores correlated with error rates among nurses using medical software. Lower SUS scores predicted higher likelihood of user errors, highlighting its predictive power.
Hospitals use SUS to evaluate patient portals and telehealth platforms. A high SUS score indicates that patients can navigate the system independently, reducing support costs and improving care outcomes.
“In healthcare, a 10-point difference in SUS can mean the difference between safe and risky use.” — Dr. Ayse Gurses, Johns Hopkins Medicine
Education and E-Learning Platforms
E-learning platforms like Coursera, Khan Academy, and corporate LMS (Learning Management Systems) use SUS to assess the usability of their interfaces. A high SUS score suggests that learners can focus on content rather than struggling with navigation.
Educators use SUS to compare different tools—like Zoom vs. Microsoft Teams for virtual classrooms. SUS helps institutions make data-driven decisions about which platforms to adopt.
Research shows that students are more engaged and perform better when using systems with high SUS scores. A 2020 study in Computers & Education found that SUS scores above 75 correlated with higher course completion rates.
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Interpreting System Usability Scale Scores
Understanding what a SUS score means is crucial for making informed decisions. While the calculation is straightforward, interpretation requires context and benchmarks.
The SUS Grading Scale: What Do the Numbers Mean?
There is a widely accepted grading scale for interpreting SUS scores:
- 0–25: Poor – Severe usability problems
- 25–50: Fair – Significant room for improvement
- 50–75: Good – Acceptable but could be better
- 75–100: Excellent – High usability, user-friendly
However, a more nuanced view comes from James Lewis and Jeff Sauro, who analyzed over 500 SUS scores and found that the average is around 68. This means a score of 70 is actually above average.
It’s also helpful to compare scores against industry benchmarks. For example:
- Consumer software: Average SUS ~ 73
- Enterprise software: Average SUS ~ 65
- Medical devices: Average SUS ~ 60 (due to complexity)
A score of 80+ is considered exceptional and is rare outside of highly polished consumer apps like Instagram or Spotify.
Comparing SUS Scores Over Time and Between Products
SUS is most powerful when used comparatively. For instance, if Version 1 of an app scores 60 and Version 2 scores 75, that 15-point jump indicates a significant usability improvement.
It’s also used in A/B testing. Suppose two design teams create different dashboards. SUS can help determine which one users perceive as more usable, even if both are functionally equivalent.
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However, caution is needed when comparing scores across different user groups or contexts. A SUS score from a tech-savvy audience may not reflect the experience of older adults or novices. Always consider demographic and contextual factors.
“SUS is best used as a trend indicator, not a standalone metric.” — Jeff Sauro, MeasuringU
Best Practices for Administering the System Usability Scale
To get the most value from SUS, it must be administered correctly. Poor timing, biased instructions, or incorrect scoring can invalidate results.
When and How to Deploy SUS
The optimal time to administer SUS is immediately after a user completes a set of representative tasks. This ensures their experience is fresh and contextually grounded.
For example, in a usability test, participants might be asked to complete tasks like creating an account, searching for a product, or submitting a form. Right after, they’re given the SUS questionnaire.
It should not be given too early (before meaningful interaction) or too late (after memory fades). In longitudinal studies, SUS can be administered at multiple touchpoints—onboarding, after one week, and after one month—to track evolving perceptions.
Online surveys can trigger SUS after a user completes a key action, like finishing a purchase or submitting a support ticket.
Common Pitfalls and How to Avoid Them
Despite its simplicity, SUS is often misused. Common mistakes include:
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- Altering the wording of questions: Even small changes can affect reliability. Stick to the original phrasing.
- Using it in isolation: SUS tells you how usable a system is, but not why. Always pair it with qualitative feedback.
- Administering it to non-users: SUS is for people who have actually used the system. Giving it to observers or stakeholders invalidates the data.
- Misinterpreting the score: A score of 70 isn’t “70% usable”—it’s a normalized score based on a specific algorithm.
Another pitfall is small sample sizes. While SUS can be used with as few as 5 users (as per Jakob Nielsen’s rule of thumb), larger samples (20+) provide more stable and generalizable results.
To avoid bias, ensure the survey is anonymous and voluntary. Avoid leading questions or incentives that might influence responses.
Limitations and Criticisms of the System Usability Scale
No tool is perfect, and SUS has its share of limitations. While it’s widely respected, understanding its weaknesses helps researchers use it more effectively.
What SUS Doesn’t Measure
SUS is a global usability measure, but it doesn’t break down usability into specific components. It won’t tell you if navigation is confusing, if error messages are unclear, or if performance is slow.
It also doesn’t capture emotional aspects like delight or frustration in depth. While confidence and ease are assessed, it lacks the nuance of tools like the Net Promoter Score (NPS) or the User Experience Questionnaire (UEQ).
Additionally, SUS is not diagnostic. A low score indicates a problem but doesn’t pinpoint where it lies. That’s why it’s best paired with methods like think-aloud protocols, heatmaps, or interviews.
Alternative and Complementary Usability Metrics
To overcome SUS’s limitations, researchers often combine it with other tools:
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- UMUX (Usability Metric for User Experience): A 4-item scale based on ISO 9241, shorter than SUS but less validated.
- NPS (Net Promoter Score): Measures loyalty and willingness to recommend.
- SEQ (Single Ease Question): A single post-task question: “How easy was this task?”
- UEQ (User Experience Questionnaire): A more detailed 26-item tool covering attractiveness, perspicuity, efficiency, and more.
For example, a study might use SUS for overall usability, SEQ for individual tasks, and UEQ for emotional dimensions. This multi-method approach provides a richer picture.
Some researchers have proposed modified versions of SUS, like the SUS-8 (an 8-item short form) or the mSUS (modified SUS with clearer wording). While these aim to improve usability, they sacrifice comparability with the original SUS database.
“SUS is like a thermometer—it tells you if the user is ‘sick’ with poor usability, but you need other tools to diagnose the illness.” — Dr. Elizabeth Rosenzweig
Future of the System Usability Scale in UX Research
Despite being over 35 years old, the System Usability Scale continues to evolve. Its adaptability ensures it remains relevant in an era of AI, voice interfaces, and immersive technologies.
Integration With Modern UX Tools and Platforms
Today, SUS is integrated into popular UX research platforms like UserTesting, Lookback, and Maze. These tools automate administration, scoring, and visualization, making SUS accessible to non-researchers.
Some platforms use SUS as a default metric, allowing teams to benchmark their products against industry averages. Real-time dashboards show SUS trends across versions, user segments, and geographies.
AI-powered analytics can now correlate SUS scores with behavioral data—like click patterns or session duration—to uncover deeper insights. For instance, a low SUS score combined with high bounce rates might indicate onboarding issues.
Potential Evolutions and Adaptations
As technology changes, so must usability metrics. Researchers are exploring ways to adapt SUS for new interaction modes:
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- Voice interfaces: Can SUS measure the usability of Alexa or Google Assistant? Early studies suggest yes, with minor wording adjustments.
- Wearables and IoT: SUS has been used to evaluate smartwatches and home automation systems.
- AI-driven systems: As users interact with chatbots and recommendation engines, SUS helps assess perceived control and trust.
There’s also growing interest in making SUS more inclusive—adapting it for people with disabilities, older adults, or low-literacy populations. Simplified versions and audio administration are being tested.
While some predict SUS will eventually be replaced, its simplicity and proven track record suggest it will remain a staple for years to come—perhaps as a foundational metric in a broader UX measurement ecosystem.
What is a good System Usability Scale score?
A score of 68 is considered average. Anything above 70 is above average, and scores above 80 are considered excellent. However, what’s “good” depends on the context—consumer apps often score higher than enterprise or medical systems.
Can I modify the SUS questions?
It’s strongly advised not to modify the wording of the SUS questions. Doing so can affect the reliability and comparability of results. If modifications are necessary, clearly label it as a “SUS-inspired” scale, not SUS.
How many users do I need for a reliable SUS score?
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As few as 5 users can provide useful insights, especially in formative testing. For more reliable and generalizable results, aim for 20 or more users. Larger samples reduce the margin of error.
Is SUS free to use?
Yes, the System Usability Scale is in the public domain and free to use for both commercial and academic purposes. No permission is required, though proper citation is appreciated.
Where can I find a SUS calculator?
You can use the free SUS calculator at MeasuringU or download templates for Excel and Google Sheets from Usability.gov.
The System Usability Scale remains one of the most powerful, practical tools in the UX researcher’s toolkit. Its ability to deliver a reliable, standardized measure of perceived usability in just 10 questions is unmatched. While it has limitations, its strengths—simplicity, speed, and proven validity—make it indispensable across industries. When used correctly and in combination with qualitative insights, SUS empowers teams to build products that are not just functional, but truly user-friendly. As technology evolves, so will SUS, adapting to new challenges while maintaining its core purpose: measuring how well systems serve the people who use them.
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