Ever wonder how companies measure how easy their products are to use? Enter the System Usability Scale (SUS)—a simple yet powerful tool that’s been shaping user experience decisions for decades. It’s fast, reliable, and universally trusted.
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, it has since become one of the most widely used tools in usability assessment across industries—from software and websites to medical devices and consumer electronics.
Origins and Development of the SUS
The SUS was first introduced in 1986 as a quick and dirty usability assessment tool. Unlike other complex and time-consuming methods, SUS was designed to be lightweight, easy to administer, and effective even with small sample sizes. Its development stemmed from the need for a standardized way to compare usability across different systems without requiring extensive resources.
Brooke’s original research focused on internal usability testing within tech environments, but the simplicity and reliability of the scale quickly made it popular beyond its initial scope. Over the years, numerous studies have validated its psychometric properties, confirming its consistency and effectiveness across diverse applications.
How the SUS Works: Structure and Scoring
The SUS consists of 10 statements, each rated on a 5-point Likert scale ranging from “Strongly Disagree” (1) to “Strongly Agree” (5). The statements alternate between positive and negative phrasing to reduce response bias. After collecting responses, a specific scoring algorithm is applied to calculate a final SUS score between 0 and 100.
- Odd-numbered items are scored by subtracting 1 from the user response.
- Even-numbered items are scored by subtracting the user response from 5.
- The total of these adjusted scores is multiplied by 2.5 to convert the range to 0–100.
For example, if a user gives all “3” responses (neutral), the raw total would be 25, and after multiplying by 2.5, the final SUS score would be 62.5—considered average.
“The beauty of the System Usability Scale lies in its simplicity. You don’t need a PhD in statistics to use it, yet it delivers remarkably consistent results.” — Jakob Nielsen, Nielsen Norman Group
Why the System Usability Scale Matters in UX Design
In today’s competitive digital landscape, user experience (UX) is no longer a luxury—it’s a necessity. The System Usability Scale plays a critical role in helping designers, developers, and product managers understand how real users perceive the usability of their creations. By providing a quantifiable metric, SUS bridges the gap between subjective impressions and objective data.
Quantifying User Experience
One of the biggest challenges in UX design is measuring something inherently subjective. How do you measure “ease of use” or “user satisfaction”? The System Usability Scale solves this by converting qualitative feedback into a numerical score. This allows teams to track improvements over time, compare different versions of a product, and benchmark against industry standards.
For instance, a mobile banking app might score a 68 on SUS before a redesign. After implementing changes based on user testing, the same app scores 82—indicating a significant improvement in perceived usability.
Supporting Data-Driven Design Decisions
Design decisions often come down to opinions, preferences, and assumptions. The System Usability Scale introduces an evidence-based approach. When stakeholders debate whether a new navigation menu is easier to use, a SUS score can provide a clear answer.
Teams can use SUS scores to prioritize features, validate prototypes, and justify design investments. For example, a healthcare software company might use SUS to compare two electronic health record (EHR) interfaces. If one scores 75 and the other 52, the higher-scoring system clearly offers a better user experience, guiding the final product decision.
How to Administer the System Usability Scale
Administering the System Usability Scale is straightforward, but doing it correctly ensures reliable results. Whether you’re testing a website, app, or physical device, following best practices will maximize the validity of your findings.
When and Where to Use SUS
The System Usability Scale can be used at various stages of the design and development lifecycle:
- Early Prototypes: Test low-fidelity wireframes or clickable mockups to catch usability issues before coding begins.
- Mid-Development: Evaluate beta versions to assess whether recent changes improved usability.
- Post-Launch: Measure real-world user satisfaction after a product has been released.
- Competitive Benchmarking: Compare your product’s SUS score against competitors or industry averages.
SUS is platform-agnostic, meaning it works equally well for desktop software, mobile apps, kiosks, and even voice-based interfaces.
Best Practices for Administering SUS
To get the most accurate results, follow these guidelines:
- Use After a Task-Based Session: Administer the SUS immediately after users complete a set of realistic tasks. This ensures their feedback is based on actual interaction, not general impressions.
- Keep Sample Size in Mind: While SUS can be used with as few as 5 users, larger samples (15–20+) provide more stable and generalizable results.
- Ensure Anonymity: Let users know their responses are anonymous to encourage honest feedback.
- Avoid Leading Questions: Don’t influence responses by praising or criticizing the system before the survey.
- Translate Carefully: If using SUS in non-English languages, ensure translations preserve the original meaning and tone.
For detailed guidance, refer to the official Usability.gov resource on SUS, which provides templates and best practices.
Interpreting System Usability Scale Scores
Getting a number is only half the battle—the real value of the System Usability Scale comes from interpreting what that number means. Understanding score ranges, benchmarks, and qualitative implications helps teams make informed decisions.
Understanding the SUS Score Range
SUS scores range from 0 to 100, with no predefined “pass/fail” threshold. However, research has established general guidelines for interpretation:
- Below 50: Poor usability. Users likely struggle significantly with the system.
- 50–69: Average. The system is usable but has noticeable pain points.
- 70–79: Good. Most users find the system easy to use.
- 80–89: Excellent. The system is highly usable and intuitive.
- 90–100: Outstanding. Rare and typically seen in well-established, user-centered designs.
It’s important to note that these ranges are not absolute. Context matters—what’s “good” for a complex enterprise tool might differ from a consumer app.
Benchmarking Against Industry Standards
One of the most powerful uses of the System Usability Scale is benchmarking. According to data compiled by the Nielsen Norman Group and other researchers, the average SUS score across all products is around 68.
Some industry-specific benchmarks include:
- E-commerce websites: 70–75
- Mobile banking apps: 72–78
- Healthcare software: 55–65 (often lower due to complexity)
- Consumer electronics: 75–80
By comparing your product’s SUS score to these benchmarks, you can gauge how it stacks up against competitors and identify areas for improvement.
“A SUS score of 70 doesn’t mean your product is ‘okay’—it means there’s room for significant improvement. Aim for 80+ to truly stand out.” — Dr. James R. Lewis, IBM Senior Usability Researcher
Advantages of Using the System Usability Scale
The enduring popularity of the System Usability Scale isn’t accidental. Its widespread adoption is due to a combination of practical benefits that make it ideal for both academic research and real-world product development.
Simplicity and Ease of Use
One of the biggest advantages of the System Usability Scale is its simplicity. The questionnaire takes less than 5 minutes to complete, making it easy to integrate into usability tests without burdening participants. It requires no specialized training to administer or score, allowing even small teams with limited UX resources to gather meaningful insights.
Because it’s so lightweight, SUS can be used frequently—before, during, and after design iterations—to track progress over time.
Reliability and Validity
Despite its brevity, the System Usability Scale is remarkably reliable. Numerous studies have confirmed its internal consistency (Cronbach’s alpha typically > 0.9), meaning the 10 items collectively measure a single underlying construct: perceived usability.
It also demonstrates strong test-retest reliability and construct validity, meaning it consistently produces similar results under similar conditions and correlates well with other usability metrics like task success rate and time-on-task.
A landmark study by Bangor, Kortum, and Miller (2008) validated the SUS across multiple domains and confirmed its robustness, cementing its status as a gold standard in usability measurement.
Limitations and Criticisms of the System Usability Scale
While the System Usability Scale is a powerful tool, it’s not without limitations. Understanding its weaknesses helps practitioners use it more effectively and know when to supplement it with other methods.
What SUS Doesn’t Measure
The System Usability Scale measures perceived usability, not objective performance. It won’t tell you how many errors users made, how long tasks took, or whether users could complete a task at all. It also doesn’t provide diagnostic insights—while a low score indicates poor usability, it doesn’t explain why.
For example, if users rate a website with a SUS score of 54, you know they found it difficult to use, but you won’t know if the problem was navigation, layout, terminology, or loading speed. To uncover root causes, SUS should be paired with qualitative methods like think-aloud protocols or interviews.
Contextual and Cultural Biases
Responses to the SUS can be influenced by cultural norms, user expectations, and prior experience. In some cultures, respondents may avoid extreme ratings, leading to artificially compressed scores. Similarly, tech-savvy users might rate a system lower than novices because they expect higher performance.
Additionally, the SUS was developed in a Western, English-speaking context. While it has been translated into dozens of languages, subtle nuances in translation can affect response patterns. Researchers should exercise caution when comparing scores across cultures without proper validation.
Alternatives and Complements to the System Usability Scale
While the System Usability Scale remains the most popular usability questionnaire, several alternatives and complementary tools exist for different research needs.
Alternative Usability Questionnaires
Several other validated scales offer different approaches to measuring usability:
- UMUX (Usability Metric for User Experience): A 4-item scale based on ISO 9241-11, designed to be even shorter than SUS while maintaining strong correlation.
- UMUX-Lite: A 2-item version of UMUX, ideal for quick feedback in live environments.
- Nielsen’s Subjective Satisfaction Scale: A simple 7-point scale asking users to rate overall satisfaction.
- PSSUQ (Post-Study System Usability Questionnaire): A more detailed 16-item questionnaire focusing on system quality, information quality, and interface quality.
Each of these has its strengths and trade-offs. For example, PSSUQ provides richer data but takes longer to complete, making it less suitable for rapid testing.
Combining SUS with Other UX Metrics
The most effective usability evaluations combine SUS with both behavioral and qualitative data. A comprehensive approach might include:
- Task Success Rate: Percentage of users who complete key tasks.
- Time on Task: How long users take to complete tasks.
- Error Rate: Number of mistakes made during testing.
- Net Promoter Score (NPS): Willingness to recommend the product.
- Think-Aloud Protocols: Real-time verbal feedback during interaction.
When combined, these metrics create a holistic picture of usability. For instance, a high SUS score paired with low task success might indicate users feel confident but are actually struggling—highlighting a disconnect between perception and performance.
Real-World 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 and services. From tech giants to startups, SUS helps organizations make better decisions based on real user feedback.
SUS in Software and Web Development
Software companies routinely use SUS to evaluate new features, compare design alternatives, and track usability over time. For example, a SaaS platform might test two versions of a dashboard interface. Version A scores 64, while Version B scores 81. The team can confidently move forward with Version B, knowing it offers a significantly better user experience.
Web developers also use SUS during A/B testing. After launching a redesigned checkout process, an e-commerce site administers SUS to a sample of users. A jump from 67 to 79 confirms the redesign improved usability, potentially leading to higher conversion rates.
SUS in Healthcare and Medical Devices
In high-stakes environments like healthcare, usability isn’t just about convenience—it’s a matter of safety. Regulatory bodies like the FDA encourage the use of validated usability measures, including the System Usability Scale, in human factors testing for medical devices.
For example, a company developing an insulin pump interface might use SUS to ensure nurses and patients can operate it correctly under stress. A score below 70 would trigger a redesign to reduce the risk of user error, which could have life-threatening consequences.
Research published in JMIR (Journal of Medical Internet Research) highlights how SUS is used to evaluate telehealth platforms, ensuring they are accessible and easy to use for both clinicians and patients.
How to Improve Your System Usability Scale Score
Getting a low SUS score isn’t the end—it’s the beginning of improvement. Once you identify usability issues, you can take targeted actions to enhance the user experience and boost your score.
Identify Pain Points from SUS Feedback
While the SUS score itself is quantitative, the context in which it’s collected often includes qualitative feedback. Pay close attention to open-ended comments or follow-up questions asked after the SUS. Users might mention specific frustrations like “I couldn’t find the save button” or “The menu was confusing.”
These insights help pinpoint exactly where the system falls short. For example, if multiple users comment on navigation issues, that’s a clear signal to simplify the information architecture.
Iterative Design and Retesting
Usability improvement is an iterative process. After making changes based on SUS results, retest with a new group of users to measure the impact. This cycle of test → analyze → redesign → retest is central to user-centered design.
For instance, a government portal might start with a SUS score of 58. After simplifying forms, improving labeling, and adding help text, a retest yields a score of 76—demonstrating tangible progress.
“The goal isn’t to get a perfect 100 on the System Usability Scale. The goal is to use it as a compass—guiding continuous improvement.” — Whitney Quesenbery, Usability Expert and Author
Future of the System Usability Scale
More than three decades after its creation, the System Usability Scale remains relevant. But as technology evolves, so do the ways we measure and interpret usability. The future of SUS lies in adaptation, integration, and broader application.
Integration with Automated UX Testing Tools
Modern UX research tools are incorporating SUS into automated testing platforms. Tools like UserTesting, Lookback, and Maze allow researchers to embed the SUS questionnaire directly into remote usability tests, automatically calculate scores, and visualize trends over time.
Some platforms are even exploring AI-driven analysis of SUS responses, identifying patterns and suggesting potential fixes based on historical data.
Expanding Use in Emerging Technologies
As new technologies like augmented reality (AR), virtual reality (VR), voice assistants, and AI-powered interfaces become mainstream, the need for reliable usability metrics grows. The System Usability Scale is being adapted to assess these novel interaction paradigms.
For example, researchers are using SUS to evaluate the usability of voice-controlled smart home devices. While the core questionnaire remains the same, the context of use—such as speaking commands in a noisy environment—adds new dimensions to interpretation.
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. It’s widely used due to its simplicity, reliability, and ease of administration.
How is the SUS score calculated?
Each of the 10 SUS items is rated on a 5-point scale. Odd-numbered items are scored by subtracting 1 from the response; even-numbered items by subtracting the response from 5. The sum of these adjusted scores is multiplied by 2.5 to yield a final score between 0 and 100.
What is a good SUS score?
A score above 70 is generally considered good, 80+ is excellent, and 90+ is outstanding. The average SUS score across all systems is about 68. However, context matters—what’s good for one type of product may not be sufficient for another.
Can SUS be used for non-digital products?
Yes, the System Usability Scale can be used for any interactive system, including physical devices like ATMs, medical equipment, or household appliances. As long as users interact with the system to achieve goals, SUS can measure their perception of its usability.
Is the System Usability Scale free to use?
Yes, the System Usability Scale is in the public domain and free for anyone to use without permission or cost. It was never copyrighted, making it accessible to researchers, companies, and educators worldwide.
The System Usability Scale remains one of the most trusted and widely used tools in the field of user experience. Its simplicity, reliability, and versatility make it indispensable for anyone looking to measure and improve usability. While it has limitations—such as not diagnosing specific issues—it excels as a quick, quantifiable benchmark. When combined with qualitative insights and behavioral metrics, SUS becomes part of a powerful toolkit for creating user-centered designs. Whether you’re building a mobile app, a medical device, or a smart home system, the SUS offers a proven way to ensure your product is not just functional, but truly usable.
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