Ever wonder how users truly feel about a product’s ease of use? The System Usability Scale (SUS) cuts through the noise, offering a simple yet powerful way to measure usability—backed by decades of research and real-world validation.
What Is the System Usability Scale (SUS)?
The System Usability Scale, commonly known as SUS, is a globally recognized tool used 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 become a gold standard in usability testing due to its simplicity, reliability, and flexibility across various platforms—from software applications to medical devices and mobile apps.
Origins and Development of SUS
Brooke created the SUS as a quick, reliable method to assess usability without requiring complex experimental setups. It was initially designed for internal use but quickly gained traction in academic and industrial circles. The scale was first published in 1986 and has since been cited in thousands of research papers and usability studies.
One of the key reasons for its longevity is its agnostic nature—it doesn’t favor any specific technology or interface type. Whether you’re testing a website, a mobile app, or a voice-controlled assistant, SUS adapts seamlessly. Its 10-item questionnaire can be administered quickly, often taking less than 10 minutes to complete, making it ideal for iterative design processes.
How SUS Differs From Other Usability Metrics
Unlike more complex usability assessment tools that require observational data, eye-tracking, or task success rates, SUS focuses solely on the user’s subjective perception. This makes it a complementary tool rather than a replacement for objective metrics.
- Subjective vs. Objective: SUS measures perceived usability, not actual performance.
- Speed and Simplicity: Can be deployed rapidly compared to longitudinal studies.
- Scalability: Works across diverse user groups and product types.
“The SUS is probably the most widely used questionnaire for measuring perceived usability.” — Sauro, J., & Lewis, J. R. (2016), MeasuringU
How the System Usability Scale Works
At its core, the System Usability Scale is a 10-item Likert-scale questionnaire where users rate their agreement with statements about a system’s usability. Each item is scored from 1 (strongly disagree) to 5 (strongly agree), and a specific scoring algorithm converts these responses into a single usability score between 0 and 100.
The 10 SUS Questions Explained
Each of the ten questions in the SUS is carefully crafted to probe different aspects of usability, including learnability, efficiency, and user confidence. Here’s a breakdown of each item:
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1.I think that I would like to use this system frequently.– Measures user preference and willingness to adopt.2.I found the system unnecessarily complex.– Assesses perceived complexity (reverse-scored).3.I thought the system was easy to use.– Direct measure of perceived ease of use.4.I think that I would need the support of a technical person to be able to use this system.– Evaluates self-sufficiency (reverse-scored).5.I found the various functions in this system were well integrated.– Looks at coherence and consistency.6.
.I thought there was too much inconsistency in this system.– Assesses interface consistency (reverse-scored).7.I would imagine that most people would learn to use this system very quickly.– Gauges perceived learnability.8.I found the system very cumbersome to use.– Measures perceived friction (reverse-scored).9.I felt very confident using the system.– Evaluates user confidence and comfort.10.I needed to learn a lot of things before I could get going with this system.– Assesses initial learning curve (reverse-scored).Notice that odd-numbered items are positively worded, while even-numbered items are negatively worded and must be reverse-scored during analysis..
Scoring the SUS: Step-by-Step Calculation
The scoring process may seem complex at first, but it’s straightforward once broken down:
- For each item, subtract 1 from the user’s response (so responses range from 0 to 4).
- For odd-numbered items: use the score as-is.
- For even-numbered items: reverse the score (i.e., 4 becomes 0, 3 becomes 1, etc.).
- Sum all 10 adjusted scores.
- Multiply the total by 2.5 to get the final SUS score (0–100).
For example, if a user’s adjusted sum is 36, the SUS score is 36 × 2.5 = 90. A score of 68 is considered average, so 90 indicates above-average usability.
Why the System Usability Scale Is So Widely Used
The enduring popularity of the System Usability Scale isn’t accidental. Its widespread adoption across industries—from tech startups to government agencies—stems from a combination of practical advantages and strong psychometric properties.
Reliability and Validity of SUS
One of the most compelling reasons for SUS’s dominance is its proven reliability. Studies have shown that SUS produces consistent results across different populations and contexts. Its internal consistency, measured by Cronbach’s alpha, typically exceeds 0.9, indicating high reliability.
Moreover, SUS has demonstrated strong construct validity—it accurately measures what it claims to: perceived usability. Research by Sauro and Lewis (2011) confirmed that SUS scores correlate well with other usability metrics, including task completion rates and user satisfaction.
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“The SUS has been shown to be a reliable and valid measure of usability across a wide range of products and user populations.” — Sauro & Lewis, Quantifying the User Experience
Flexibility Across Platforms and Industries
SUS is platform-agnostic. It has been successfully applied to:
- Web applications (e.g., e-commerce sites, SaaS platforms)
- Mobile apps (iOS and Android)
- Medical devices (FDA-approved tools use SUS in human factors testing)
- Voice assistants (Alexa, Google Assistant)
- Automotive infotainment systems
- Enterprise software (CRM, ERP systems)
This versatility makes SUS an invaluable tool for cross-product comparisons and benchmarking.
Interpreting SUS Scores: What Do the Numbers Mean?
A SUS score is more than just a number—it’s a diagnostic tool. Understanding how to interpret it is crucial for making informed design decisions.
The SUS Grading Scale and Benchmarks
While SUS scores range from 0 to 100, interpretation relies on established benchmarks:
- 0–67: Below average — indicates significant usability issues.
- 68: Average — the median score across thousands of studies.
- 68–75: Acceptable — usable but with room for improvement.
- 76–85: Good — above average, competitive in the market.
- 85–100: Excellent — top-tier usability, often seen in best-in-class products.
For instance, Apple’s iOS and Google’s search interface consistently score above 85, reflecting their intuitive design.
Comparative Analysis Using SUS
SUS is particularly powerful when used comparatively. For example:
- Testing two versions of a checkout flow (A/B testing).
- Comparing competitor products in a market analysis.
- Tracking usability improvements across product iterations.
A rise from 60 to 80 after a redesign signals a major usability win. Conversely, a drop from 75 to 65 after a UI overhaul should trigger immediate investigation.
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Best Practices for Administering the System Usability Scale
To get accurate and actionable results, how you deploy SUS matters as much as the tool itself. Poor administration can skew data and lead to misleading conclusions.
When and How to Deploy SUS
SUS should be administered immediately after a user completes a set of representative tasks. This ensures their experience is fresh and contextually relevant.
- Timing: Right after task completion, not days later.
- Context: After realistic usage scenarios (e.g., completing a purchase, setting up an account).
- Delivery: Can be paper-based, digital form, or embedded in usability testing software.
Automated tools like SurveyMonkey or Optimal Workshop can streamline data collection and scoring.
Avoiding Common Pitfalls in SUS Implementation
Even experienced researchers make mistakes with SUS. Common errors include:
- Administering SUS without task context: Asking users to rate usability without using the system leads to unreliable data.
- Mis-scoring reverse items: Forgetting to reverse even-numbered items invalidates results.
- Small sample sizes: While SUS can work with as few as 5 users, larger samples (n > 15) provide more stable averages.
- Ignoring qualitative feedback: SUS is quantitative; always pair it with open-ended questions for context.
“Numbers are only part of the story. Always ask users to explain their ratings.” — Nielsen Norman Group
Advanced Applications of the System Usability Scale
Beyond basic usability scoring, SUS can be adapted for deeper insights and specialized use cases.
Using SUS in Agile and Iterative Design
In agile environments, SUS serves as a sprint-level usability checkpoint. After each iteration, teams can test a prototype with 5–10 users and track SUS trends over time.
- Baseline SUS at sprint 1.
- Measure after each major UI change.
- Visualize progress with line charts.
This creates a data-driven feedback loop, helping teams prioritize usability fixes alongside feature development.
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SUS in Medical Devices and Regulated Industries
The FDA and other regulatory bodies recognize SUS as a valid tool for human factors validation. In medical device design, SUS helps demonstrate that a product is safe and usable by intended users.
For example, a glucose meter or infusion pump must undergo usability testing with representative users. SUS scores, combined with task success and error rates, form part of the summative evaluation required for market approval.
Organizations like Usability.gov provide guidelines on integrating SUS into regulatory submissions.
Limitations and Criticisms of the System Usability Scale
No tool is perfect. While SUS is robust, it has limitations that practitioners should understand to use it effectively.
Subjectivity and Cultural Bias
SUS relies on self-reported data, which can be influenced by user mood, expectations, or cultural norms. For example, users from cultures that avoid negative feedback may inflate their scores.
Additionally, the Likert scale assumes that users interpret anchors (e.g., “strongly agree”) consistently, which isn’t always true across demographics.
Lack of Diagnostic Detail
SUS gives a single score but doesn’t tell you *why* a system is hard to use. A score of 50 could stem from poor navigation, confusing labels, or slow performance—SUS won’t pinpoint the cause.
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This is why SUS should always be paired with qualitative methods like think-aloud protocols, interviews, or heuristic evaluations.
Alternatives and Complements to the System Usability Scale
While SUS is powerful, it’s not the only usability metric available. Understanding alternatives helps you choose the right tool for the job.
Nielsen’s Usability Attributes and HEART Framework
Google’s HEART framework (Happiness, Engagement, Adoption, Retention, Task Success) expands beyond usability to broader user experience metrics. While SUS fits under “Happiness,” HEART provides a more holistic view.
Similarly, Jakob Nielsen’s usability heuristics offer a qualitative checklist for evaluating interfaces, which can complement SUS’s quantitative output.
UMUX and UMUX-Lite: Modern Alternatives
The Usability Metric for User Experience (UMUX) is a 4-item scale based on ISO standards. UMUX-Lite, a 2-item version, correlates highly with SUS but is even shorter.
Studies show UMUX-Lite scores can be converted to SUS equivalents with high accuracy, making it ideal for situations where brevity is critical (e.g., in-app micro-surveys).
Learn more at MeasuringU’s UMUX guide.
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How to Improve Your SUS Score: Actionable Strategies
A low SUS score isn’t a dead end—it’s a starting point for improvement. Here’s how to turn insights into action.
Identifying Pain Points from SUS Feedback
Break down the SUS items to isolate problem areas:
- If users disagree with “I found the system easy to use,” focus on simplifying workflows.
- If they agree with “I needed the support of a technical person,” improve onboarding or documentation.
- Low scores on integration suggest inconsistent UI patterns—audit your design system.
Pairing SUS with session recordings or heatmaps can reveal where users struggle.
Design and UX Improvements That Boost SUS
Proven strategies to increase SUS scores include:
- Reducing cognitive load: Simplify navigation, use clear labels.
- Improving feedback: Add loading indicators, success messages.
- Enhancing learnability: Include tooltips, progressive disclosure.
- Ensuring consistency: Follow platform conventions and design systems.
Even small changes—like renaming a confusing button—can lift SUS by 10+ points.
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 score from 0 to 100, with higher scores indicating better usability. Developed by John Brooke in 1986, it’s widely used in UX research and product development.
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How do you calculate a SUS score?
To calculate a SUS score: (1) Subtract 1 from each response (1–5 becomes 0–4). (2) Reverse the scores for even-numbered items. (3) Sum all 10 items. (4) Multiply the total by 2.5. The result is a score between 0 and 100.
What is a good SUS score?
A score of 68 is average. Scores above 75 are considered good, above 85 are excellent. Products like Google Search and iOS typically score above 85.
Can SUS be used for mobile apps?
Yes, SUS is platform-agnostic and widely used for mobile apps, websites, software, and even physical devices. Its flexibility makes it ideal for testing usability across different interfaces.
Is SUS better than other usability scales?
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SUS is not necessarily “better,” but it is the most validated and widely adopted. Alternatives like UMUX-Lite are shorter, but SUS remains the benchmark due to its reliability and extensive research backing.
The System Usability Scale remains one of the most powerful, accessible tools in the UX researcher’s toolkit. Its blend of simplicity, reliability, and actionable insights makes it indispensable for anyone serious about improving user experience. While it has limitations—such as its reliance on subjective feedback and lack of diagnostic depth—these are easily mitigated by combining SUS with qualitative methods and other metrics. Whether you’re a startup testing a prototype or a multinational corporation validating a medical device, SUS provides a common language for measuring usability. By understanding how to administer, interpret, and act on SUS data, teams can make evidence-based decisions that lead to more intuitive, satisfying, and successful products.
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