Ever wondered how to measure if a product is truly user-friendly? The System Usability Scale (SUS) is your go-to tool—a simple, reliable way to gauge usability with precision and ease.
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 the late 1980s by John Brooke at Digital Equipment Corporation, SUS has since become one of the most widely adopted usability assessment tools across industries ranging from software development to medical devices.
Origins and Development of SUS
Brooke created the System Usability Scale as a quick, reliable method to assess usability without requiring extensive resources or technical expertise. Unlike more complex usability testing methods, SUS was designed to be lightweight and scalable. It emerged during a time when human-computer interaction (HCI) was gaining traction, and companies needed a standardized way to measure user experience.
The original research was conducted in a series of usability studies where participants interacted with various software systems. From these observations, Brooke distilled ten key statements that consistently reflected users’ overall satisfaction and perceived ease of use. The result was a questionnaire that could be administered quickly and scored objectively.
- Developed in 1986 by John Brooke.
- Originally tested on software interfaces within corporate environments.
- Designed for broad applicability across different technologies.
Over the decades, the System Usability Scale has been validated across thousands of studies and translated into multiple languages. Its enduring relevance lies in its simplicity and statistical robustness. Despite the rapid evolution of digital interfaces—from desktop applications to mobile apps and voice assistants—SUS remains remarkably consistent in its ability to capture usability perceptions.
Structure and Format of the SUS Questionnaire
The System Usability Scale consists of ten statements, each rated on a five-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, item 1 (“I think that I would like to use this system frequently”) is positively worded, while item 2 (“I found the system unnecessarily complex”) is negatively worded.
Here are the ten standard SUS 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.After users complete the survey, responses are scored using a specific algorithm.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 scores are then summed and multiplied by 2.5 to yield a final SUS score between 0 and 100..
“The beauty of the System Usability Scale lies in its simplicity. You don’t need a PhD in statistics to administer it, yet it produces data that’s both meaningful and actionable.” — Jakob Nielsen, Nielsen Norman Group
Why the System Usability Scale Matters in UX Design
In the world of user experience (UX) design, making assumptions about usability can be costly. The System Usability Scale provides an empirical foundation for evaluating how real users perceive a product. It bridges the gap between subjective impressions and objective measurement, allowing teams to make data-driven decisions.
Quantifying Subjective User Experience
Usability is inherently subjective—what feels intuitive to one person might confuse another. However, the System Usability Scale transforms these subjective experiences into quantifiable data. By aggregating responses from multiple users, designers and researchers can identify trends, benchmark performance, and track improvements over time.
For instance, a SUS score below 50 typically indicates poor usability, while scores above 80 suggest excellent usability. This benchmarking capability makes SUS invaluable during iterative design processes. Teams can test prototypes, gather SUS scores, refine the design, and retest—creating a feedback loop that continuously enhances the user experience.
- SUS converts qualitative feedback into a standardized numeric score.
- It enables comparison across different versions of a product or competing products.
- It supports A/B testing and usability benchmarking.
According to research published by the Nielsen Norman Group, SUS scores correlate strongly with user satisfaction and task success rates. This means that a high SUS score often reflects not just perceived ease of use, but actual performance efficiency.
Supporting Evidence-Based Design Decisions
One of the biggest challenges in UX design is overcoming internal biases. Stakeholders may believe a feature is intuitive based on their own experience, but users might struggle with it. The System Usability Scale provides an impartial voice—users’ collective opinion—backed by data.
For example, a fintech app undergoing redesign might receive a SUS score of 62 in its initial prototype phase. After implementing changes based on user feedback, the score improves to 78. This 16-point increase provides concrete evidence that the redesign enhanced usability, which can be presented to executives, developers, and investors as proof of progress.
Moreover, SUS data can be used to prioritize design sprints. Features associated with low-scoring items (e.g., “I think I would need technical support to use this system”) highlight pain points that require immediate attention. This targeted approach ensures that limited development resources are allocated where they’ll have the most impact.
How to Administer the System Usability Scale Correctly
While the System Usability Scale is straightforward, administering it effectively requires attention to timing, context, and methodology. Poorly timed surveys or unclear instructions can skew results and lead to misleading conclusions.
Best Practices for Survey Administration
To get accurate and meaningful SUS data, follow these best practices:
- Administer the survey immediately after task completion: Users should complete the SUS right after interacting with the system while their experience is still fresh.
- Ensure anonymity: Let users know their responses are confidential to encourage honest feedback.
- Use a neutral tone: Avoid leading questions or suggestive language when introducing the survey.
- Target a representative sample: Include users with varying levels of technical expertise and backgrounds.
- Collect at least 15–20 responses: While SUS can be used with small samples, larger datasets increase reliability.
A common mistake is embedding the SUS within a longer survey. This can lead to fatigue and reduced response quality. Keep the SUS standalone or place it at the end of a usability test, ensuring users aren’t overwhelmed.
Another critical factor is clarity in instructions. Users should understand that they’re rating their experience with the system, not expressing general opinions about the company or brand. A simple instruction like: “Please rate your agreement with the following statements based on your experience using [Product Name]” sets the right context.
Common Pitfalls and How to Avoid Them
Despite its simplicity, the System Usability Scale is sometimes misused. One frequent error is interpreting individual item scores instead of the overall SUS score. The questionnaire is designed so that the total score reflects global usability; analyzing single items can be misleading.
For example, a low score on item 4 (“I think I would need the support of a technical person…”) might suggest users feel the system is hard to use. But without looking at the full picture, you might overlook other strengths. Always calculate the composite SUS score first, then use qualitative feedback to explore underlying reasons for low scores.
Another pitfall is applying SUS to inappropriate contexts. While it’s versatile, SUS works best for interactive systems—software, websites, apps, kiosks. It’s less effective for evaluating physical products (like furniture) or non-interactive experiences (like watching a video).
“SUS is not a diagnostic tool—it tells you *how usable* something is, not *why* it’s unusable. Always pair it with qualitative methods like interviews or observation.” — Dr. Susan Weinschenk, Human Factors International
Scoring and Interpreting the System Usability Scale
One of the greatest strengths of the System Usability Scale is its standardized scoring method. Once responses are collected, calculating the SUS score is a systematic process that ensures consistency across studies.
Step-by-Step Scoring Guide
Here’s how to compute the SUS score manually:
- For each participant, go through the ten items.
- For odd-numbered items (1, 3, 5, 7, 9): Subtract 1 from the response (so a 5 becomes 4, a 1 becomes 0).
- For even-numbered items (2, 4, 6, 8, 10): Subtract the response from 5 (so a 5 becomes 0, a 1 becomes 4).
- Sum all the adjusted scores (this will be a number between 0 and 40).
- Multiply the total by 2.5 to get the final SUS score (ranging from 0 to 100).
Let’s walk through an example. Suppose a user responds with the following answers: [4, 3, 5, 2, 4, 1, 5, 2, 4, 1]
- Item 1 (odd): 4 – 1 = 3
- Item 2 (even): 5 – 3 = 2
- Item 3 (odd): 5 – 1 = 4
- Item 4 (even): 5 – 2 = 3
- Item 5 (odd): 4 – 1 = 3
- Item 6 (even): 5 – 1 = 4
- Item 7 (odd): 5 – 1 = 4
- Item 8 (even): 5 – 2 = 3
- Item 9 (odd): 4 – 1 = 3
- Item 10 (even): 5 – 1 = 4
Total = 3+2+4+3+3+4+4+3+3+4 = 33
SUS Score = 33 × 2.5 = 82.5
This score of 82.5 falls into the “Excellent” range according to standard SUS grading benchmarks.
Understanding SUS Score Benchmarks
Interpreting SUS scores involves comparing them to established benchmarks. While there’s no universal “pass/fail” threshold, researchers have developed grading scales based on extensive data aggregation.
According to Sauro and Lewis (2009), who analyzed over 5,000 SUS scores, the average SUS score is around 68. Here’s a commonly used interpretation framework:
- Below 50: Poor usability — significant redesign needed.
- 50–59: Marginal — usability issues present but not critical.
- 60–69: Acceptable — functional but room for improvement.
- 70–79: Good — solid usability, competitive in market.
- 80–100: Excellent — best-in-class user experience.
It’s also useful to compare your product’s SUS score against industry benchmarks. For example, e-commerce platforms average around 74, while enterprise software often scores closer to 65. Knowing where you stand relative to peers helps contextualize your results.
Additionally, SUS scores can be converted into letter grades (A–F) or percentile ranks. A score of 85, for instance, places a product in the top 15% of all systems tested, according to MeasuringU, a leading resource on usability metrics.
Applications of the System Usability Scale Across Industries
The versatility of the System Usability Scale has led to its adoption in diverse fields. From healthcare to education, SUS helps organizations evaluate and improve the usability of their digital tools.
Healthcare and Medical Devices
In healthcare, usability isn’t just about convenience—it’s a matter of safety. Poorly designed medical devices can lead to user errors, misdiagnoses, or even patient harm. Regulatory bodies like the FDA now require usability testing for medical devices, and SUS is frequently used as part of this process.
For example, a new insulin pump interface might be tested with diabetic patients using SUS. A low score would trigger a redesign before the product reaches the market. Studies have shown that medical devices with higher SUS scores correlate with fewer use errors and better patient compliance.
Researchers at the University of Wisconsin conducted a study where SUS was used to evaluate electronic health record (EHR) systems. They found that physicians rated usability significantly lower than patients did, highlighting the need for role-specific evaluations. SUS provided a common metric to compare different EHR platforms objectively.
Software and Mobile Applications
In the tech industry, SUS is a staple in product development cycles. Companies like Google, Microsoft, and Spotify use SUS during beta testing to assess new features or interface changes.
For mobile apps, where screen space is limited and user attention is fleeting, even small usability improvements can have a big impact on retention and engagement. A fitness app, for instance, might use SUS to compare two onboarding flows. If Flow A yields a SUS score of 70 and Flow B scores 85, the team can confidently choose Flow B for launch.
Startups often rely on SUS because it’s low-cost and fast to implement. With minimal setup, they can gather usability data from early adopters and iterate rapidly. This agility is crucial in competitive markets where first impressions determine success.
Advantages and Limitations of the System Usability Scale
No measurement tool is perfect, and the System Usability Scale is no exception. Understanding its strengths and weaknesses is essential for using it effectively.
Key Advantages of Using SUS
The System Usability Scale offers several compelling benefits:
- Simplicity: Easy to administer and understand, even for non-experts.
- Reliability: High internal consistency and test-retest reliability.
- Speed: Takes less than 10 minutes to complete.
- Cost-effective: Requires no specialized equipment or software.
- Comparability: Enables benchmarking across products, versions, and industries.
Its brevity makes SUS ideal for agile environments where time is limited. Unlike lengthy usability questionnaires, SUS minimizes respondent fatigue while still delivering actionable insights.
Furthermore, SUS has been extensively validated through peer-reviewed research. Its psychometric properties—such as Cronbach’s alpha (typically above 0.9) — confirm that it’s a statistically sound instrument.
Known Limitations and Criticisms
Despite its popularity, the System Usability Scale has some drawbacks:
- Lack of diagnostic detail: SUS tells you *how usable* a system is, but not *why* users struggled.
- Subjective nature: Relies on self-reported perceptions, which can be influenced by mood, expectations, or recent experiences.
- Not suitable for all contexts: Less effective for evaluating aesthetic appeal, emotional engagement, or long-term user loyalty.
- Language and cultural sensitivity: Translations must be carefully validated to maintain meaning across cultures.
To overcome these limitations, experts recommend combining SUS with other methods. For example, pairing SUS with the NASA-TLX (Task Load Index) provides insight into cognitive workload, while adding open-ended questions helps uncover root causes of low scores.
“SUS is like a thermometer—it tells you if the patient has a fever, but you still need a doctor to diagnose the illness.” — Jeff Sauro, MeasuringU
Alternatives and Complements to the System Usability Scale
While the System Usability Scale remains the gold standard for usability measurement, several alternative and complementary tools exist to address its limitations.
Alternative Usability Questionnaires
Several other validated scales offer different perspectives on usability:
- UMUX (Usability Metric for User Experience): A shorter, 4-item scale based on ISO definitions of usability. It’s highly correlated with SUS but more concise.
- UMUX-Lite: A two-item version of UMUX, ideal for quick surveys. Despite its brevity, it correlates strongly with SUS (r > 0.8).
- PSSUQ (Post-Study System Usability Questionnaire): Developed by IBM, this 16-item survey dives deeper into satisfaction, interface quality, and information quality.
- QUIS (Questionnaire for User Interaction Satisfaction): A comprehensive tool with multiple sections, allowing granular analysis of specific interface components.
Each of these has its niche. For example, UMUX-Lite is perfect for in-app micro-surveys, while PSSUQ is better suited for detailed post-test evaluations in academic research.
Combining SUS with Qualitative Methods
The most powerful usability assessments combine quantitative tools like SUS with qualitative techniques. Methods such as:
- Think-aloud protocols during usability testing
- Semi-structured user interviews
- Observational studies and eye-tracking
- Heatmaps and session recordings
…provide rich context that SUS alone cannot capture. For instance, if users give a low SUS score, interview data might reveal that confusing terminology—not navigation structure—was the real issue.
A hybrid approach allows teams to answer both “How usable is it?” and “Why is it unusable?” This dual insight leads to more effective and targeted design improvements.
What is a good System Usability Scale score?
A score of 68 is considered average. Anything above 70 is good, and scores above 80 are excellent. However, what’s “good” depends on your industry and competitors. Always compare your score to relevant benchmarks.
Can I modify the System Usability Scale?
While you can adapt the wording slightly for clarity (e.g., replacing “system” with “app”), major changes can invalidate the scoring model. Stick to the original format for reliable results. If customization is needed, consider using UMUX instead.
How many people do I need to get a valid SUS score?
You can calculate a SUS score with as few as 5 users, but for reliable results, aim for 15–20. Larger samples improve statistical confidence and allow for subgroup analysis (e.g., comparing novice vs. expert users).
Is the System Usability Scale free to use?
Yes, the System Usability Scale is in the public domain and free for both academic and commercial use. No permission is required, though proper citation is appreciated.
Where can I find the official SUS questionnaire?
The original SUS items are widely published in usability literature. Reputable sources include the Nielsen Norman Group and MeasuringU. Always use the standard 10-item version for accurate scoring.
The System Usability Scale remains one of the most powerful, accessible tools in the UX researcher’s toolkit. Its ability to deliver reliable, standardized usability measurements with minimal effort has made it a global standard. While it has limitations—particularly in diagnostic depth—its value is undeniable when used appropriately. By combining SUS with qualitative insights and industry benchmarks, organizations can make smarter design decisions, improve user satisfaction, and build better products. Whether you’re designing a mobile app, a medical device, or an enterprise platform, the System Usability Scale offers a proven path to measuring and enhancing usability.
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