If you’ve ever wondered how users truly feel about a product’s ease of use, the System Usability Scale (SUS) is your golden ticket. Simple, reliable, and universally trusted, this tool turns subjective opinions into actionable data—fast.
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What Is the System Usability Scale (SUS)?
The System Usability Scale, commonly known as SUS, is a 10-item questionnaire designed to measure 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 adopted usability assessment tools across industries—from software and websites to medical devices and mobile apps.
Origins and Development of SUS
The SUS was first introduced in 1986 as a quick, reliable way to evaluate user experience without requiring extensive resources. Unlike more complex usability testing methods, SUS was designed to be technology-agnostic and easy to administer. Its development stemmed from the need for a standardized method that could provide consistent results across different platforms and user groups.
Brooke’s original research aimed to create a tool that was both efficient and effective. The result was a ten-question survey that could be completed in under 10 minutes, yet still yield statistically meaningful data. Over the decades, SUS has been validated through numerous studies and remains a benchmark in UX research.
How SUS Differs from Other Usability Metrics
While many usability tools exist—such as the User Experience Questionnaire (UEQ) or the NASA-TLX—SUS stands out due to its simplicity and robustness. It doesn’t require specialized training to administer or interpret, making it accessible even to teams with limited UX expertise.
Unlike task-specific metrics that focus on success rates or time-on-task, SUS captures the user’s overall perception of usability. This holistic approach allows organizations to compare usability across vastly different systems—from a banking app to an MRI machine interface—using the same yardstick.
The Structure of the SUS Questionnaire
The SUS consists of 10 statements, each rated on a five-point Likert scale ranging from “Strongly Disagree” (1) to “Strongly Agree” (5). 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.
After responses are collected, a specific scoring algorithm is applied: odd-numbered items are scored by subtracting 1 from the user response, while even-numbered items are scored by subtracting the user response from 5. These scores are then summed and multiplied by 2.5 to produce a final score between 0 and 100.
Why the System Usability Scale Matters in UX Design
In today’s competitive digital landscape, usability isn’t just a nice-to-have—it’s a business imperative. A poorly designed interface can lead to user frustration, abandonment, and lost revenue. The System Usability Scale provides a quantifiable way to assess how intuitive and efficient a product feels to real users.
Driving User-Centered Design Decisions
Design teams use the System Usability Scale to validate design choices before launch. By testing prototypes with SUS, they can identify pain points early and iterate quickly. For instance, if a new checkout flow scores below average on SUS, designers know they need to simplify navigation or reduce cognitive load.
Because SUS delivers a single, easy-to-understand score, it enables non-technical stakeholders—like product managers or executives—to grasp usability performance at a glance. This shared understanding fosters collaboration across departments and ensures that user experience remains a priority.
Supporting Agile and Iterative Development
In agile environments, where products evolve rapidly through sprints, having a lightweight evaluation tool like SUS is invaluable. Teams can run quick SUS tests after each iteration to track improvements over time. A rising SUS score signals that changes are positively impacting usability.
For example, a fintech startup might deploy SUS after releasing a new budgeting feature. If the score jumps from 62 to 78, the team has empirical evidence that users find the update easier to use—justifying further investment in similar features.
Enabling Benchmarking Across Products and Industries
One of the most powerful aspects of the System Usability Scale is its ability to serve as a benchmarking tool. Because it’s been used in thousands of studies, researchers have established normative data. The average SUS score across all systems is approximately 68, with scores above 80 considered excellent and below 50 deemed poor.
This allows companies to compare their product not only against previous versions but also against industry standards. A healthcare app scoring 75 may seem good until you realize competitors average 82—prompting a deeper dive into what’s causing the gap.
“The beauty of SUS lies in its simplicity. You don’t need a lab, a script, or a PhD to get meaningful insights.” — Dr. Jakob Nielsen, Nielsen Norman Group
How to Administer the System Usability Scale Correctly
While the System Usability Scale is straightforward, improper administration can skew results. To ensure accuracy and reliability, follow best practices in timing, participant selection, and data collection.
When to Use SUS in the Product Lifecycle
SUS can be used at multiple stages: during early prototyping, after major updates, or as part of post-launch evaluation. However, it’s most effective when users have had enough interaction with the system to form an informed opinion—typically after completing key tasks.
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Administering SUS too early—before users have explored core functionality—can lead to unreliable feedback. Conversely, waiting too long may delay critical improvements. The sweet spot is usually immediately after a usability test session or beta release.
Selecting the Right Participants
To get valid results, test with a representative sample of your target audience. While SUS can produce reliable data with as few as 5–10 users (due to its high sensitivity), larger samples improve statistical confidence.
Avoid recruiting only power users or internal staff, as their familiarity with the system can inflate scores. Instead, include a mix of experience levels to reflect real-world usage patterns. For consumer apps, aim for demographic diversity; for enterprise software, focus on role-based relevance.
Avoiding Common Administration Mistakes
One frequent error is modifying the SUS questionnaire. While it might be tempting to rephrase questions to fit a specific context, doing so compromises validity. The power of SUS comes from its consistency—altering wording breaks comparability with existing benchmarks.
Another pitfall is failing to provide clear instructions. Users should understand that SUS measures their perception of usability, not satisfaction with features or aesthetics. Clarify that there are no right or wrong answers—only honest ones.
Scoring and Interpreting the System Usability Scale
One of the reasons SUS has endured is its simple yet powerful scoring mechanism. Once responses are collected, calculating the score is a matter of applying a consistent formula. But interpreting that number requires context and nuance.
Step-by-Step Scoring Process
Here’s how to calculate a SUS score manually:
- For each odd-numbered question (1, 3, 5, 7, 9), subtract 1 from the response (e.g., if the user answered “4,” the converted score is 3).
- For each even-numbered question (2, 4, 6, 8, 10), subtract the response from 5 (e.g., if the user answered “2,” the converted score is 3).
- Sum all converted scores.
- Multiply the total by 2.5 to get the final SUS score (range: 0–100).
For example, if the sum of converted scores is 30, multiplying by 2.5 gives a SUS score of 75.
Understanding SUS Score Ranges and Benchmarks
While the raw score is useful, interpretation depends on established benchmarks:
- 0–50: Poor usability. Users likely struggle with basic tasks.
- 51–65: Below average. Significant improvements needed.
- 66–75: Acceptable. Functional but not exceptional.
- 76–85: Good. Users find the system intuitive.
- 86–100: Excellent. Top-tier usability.
It’s important to note that a score of 68 is the historical average, so anything above that is better than typical. However, aiming for 80+ should be the goal for competitive products.
Using Grade Equivalents and Adjective Ratings
To make SUS scores more accessible, some organizations use adjective ratings or letter grades. For instance:
- 90–100 = “Excellent” (A)
- 80–89 = “Good” (B)
- 70–79 = “OK” (C)
- 60–69 = “Poor” (D)
- Below 60 = “Awful” (F)
These labels help communicate results to stakeholders who may not be familiar with SUS. However, they should be used cautiously, as they can oversimplify nuanced findings.
Advantages of the System Usability Scale
The widespread adoption of the System Usability Scale isn’t accidental. Its enduring popularity stems from a combination of practical benefits that few other usability tools can match.
Speed and Efficiency in Data Collection
SUS takes less than 10 minutes to complete, making it ideal for integration into usability testing sessions without burdening participants. This brevity increases response rates and reduces fatigue, leading to higher-quality data.
Because it’s self-administered, SUS can be deployed remotely via email or embedded in digital platforms, enabling large-scale studies with minimal overhead.
Reliability and Validity Across Contexts
Despite its simplicity, SUS has demonstrated strong psychometric properties. Numerous studies have confirmed its internal consistency (Cronbach’s alpha typically > 0.9) and test-retest reliability.
Its validity has been proven across cultures, languages, and domains. Translated versions of SUS maintain comparable performance, allowing global teams to collect comparable data. For example, a study published in International Journal of Human-Computer Interaction found that SUS performed consistently across English, German, and Japanese speakers.
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Cost-Effectiveness for Organizations
Unlike expensive eye-tracking labs or longitudinal field studies, SUS requires no special equipment or software. It’s free to use (no licensing fees), and results can be analyzed with basic spreadsheet tools.
For startups and small businesses, this low barrier to entry makes SUS an essential part of their UX toolkit. Even large enterprises use it as a first-line screening tool before investing in deeper analysis.
Limitations and Criticisms of the System Usability Scale
No tool is perfect, and the System Usability Scale is no exception. While its strengths are undeniable, understanding its limitations helps prevent misuse and misinterpretation.
Lack of Diagnostic Detail
SUS provides a global usability score but doesn’t pinpoint specific issues. A low score tells you something is wrong, but not what. For example, if users rate a mobile app poorly, SUS won’t reveal whether the problem lies in navigation, loading speed, or button placement.
To address this, SUS should be paired with qualitative methods like think-aloud protocols, interviews, or heuristic evaluations. Combining SUS with open-ended follow-up questions (e.g., “What did you find most confusing?”) can add valuable context.
Sensitivity to User Expectations and Bias
User expectations can influence SUS scores. A user accustomed to sleek consumer apps may rate an enterprise system harshly—even if it’s well-designed for its intended audience. Similarly, brand loyalty or prior experience can skew perceptions.
Additionally, because SUS relies on self-reported data, it’s vulnerable to social desirability bias—users may give higher ratings to avoid sounding negative. This is why anonymity and neutral administration are crucial.
Not Designed for All Types of Systems
While SUS is versatile, it may not be ideal for highly specialized or safety-critical systems. For example, in aviation or medical device interfaces, usability flaws can have life-or-death consequences. In such cases, more rigorous, task-based assessments are necessary.
Researchers have proposed adaptations like the Modified SUS (M-SUS) or domain-specific variants to better capture nuances in complex environments.
Practical Applications of the System Usability Scale
From tech giants to government agencies, organizations worldwide use the System Usability Scale to improve products and services. Its flexibility makes it applicable in diverse settings.
Software and Mobile App Development
Development teams use SUS to compare design alternatives. For example, during A/B testing of two login flows, SUS can quantify which version feels more intuitive. Post-release, SUS helps monitor user sentiment and detect usability regressions.
Companies like Spotify and Airbnb have reportedly used SUS internally to evaluate new features before global rollout.
Healthcare and Medical Devices
In healthcare, where usability directly impacts patient safety, SUS plays a critical role. Regulatory bodies like the FDA encourage usability testing for medical devices, and SUS is often included as part of human factors validation.
For instance, a glucose monitoring app might undergo SUS testing with diabetic patients to ensure it’s easy to use under stress or low visibility conditions.
E-Commerce and Customer Experience
Online retailers use SUS to optimize checkout processes, product search, and account management. A high SUS score correlates with lower cart abandonment and higher conversion rates.
One case study showed that after redesigning its mobile site, an e-commerce brand improved its SUS score from 61 to 79—and saw a 22% increase in mobile sales within three months.
Future of the System Usability Scale in UX Research
As technology evolves, so too does the role of usability measurement. While newer tools and AI-driven analytics emerge, the System Usability Scale remains a cornerstone of UX evaluation—adapting rather than fading.
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Integration with Digital Analytics Platforms
Modern UX teams are embedding SUS directly into digital platforms. Tools like Hotjar, Typeform, and SurveyMonkey now support SUS templates, enabling real-time data collection and visualization.
Some platforms even correlate SUS scores with behavioral metrics (e.g., bounce rate, session duration) to provide a 360-degree view of user experience.
AI and Predictive Usability Modeling
Emerging AI tools are beginning to predict SUS scores based on design patterns. By analyzing UI elements—such as button placement, color contrast, or form length—machine learning models can estimate likely usability outcomes before human testing begins.
While not a replacement for real-user feedback, this predictive capability allows designers to prioritize high-risk areas for early SUS validation.
Global Standardization and Open Research
The open nature of SUS continues to fuel academic and industry research. Public repositories of SUS scores, such as those maintained by the MeasuringU team, provide valuable benchmarks.
As UX becomes more data-driven, SUS is likely to remain a reference point—much like IQ tests in psychology. Its longevity is a testament to its balance of simplicity and scientific rigor.
What is the System Usability Scale?
The System Usability Scale (SUS) is a 10-item questionnaire used to assess the perceived usability of a system. It produces a single score from 0 to 100, with higher scores indicating better usability.
How reliable is the SUS score?
SUS is highly reliable, with studies showing strong internal consistency (Cronbach’s alpha > 0.9) and good test-retest reliability. It’s been validated across cultures, languages, and domains.
Can I modify the SUS questions?
No. Modifying the wording or structure of the SUS compromises its validity and breaks comparability with established benchmarks. Always use the original 10 items.
How many users do I need for a valid SUS result?
As few as 5–10 users can provide reliable insights due to SUS’s high sensitivity. However, larger samples (20+) improve statistical confidence, especially when comparing groups.
Is the System Usability Scale free to use?
Yes. SUS is in the public domain and free for anyone to use without licensing fees or restrictions.
The System Usability Scale remains one of the most powerful, accessible tools in the UX researcher’s arsenal. Its ability to deliver quick, reliable, and comparable usability scores has made it a global standard for over three decades. While it has limitations—such as lack of diagnostic detail—it excels as a high-level indicator of user experience. When used correctly, alongside qualitative insights and behavioral data, SUS empowers teams to build products that are not just functional, but truly user-friendly. As technology advances, SUS will likely continue evolving, maintaining its role as a cornerstone of evidence-based design.
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