If you’ve ever wondered how to measure the ease of use of a product or system, the System Usability Scale (SUS) is your go-to tool. Simple, reliable, and widely trusted, it gives you real insights into user experience—fast.
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 become one of the most widely used tools in usability testing across industries—from software and websites to medical devices and consumer electronics.
Origins and Development of SUS
The System Usability Scale was first introduced in 1986 as a quick, reliable way to assess usability without requiring complex metrics or extensive user testing. Brooke aimed to create a tool that was both technology-agnostic and easy to administer. Unlike other usability models that required observational data or task completion rates, SUS relied solely on user self-reporting, making it highly scalable.
Over the decades, SUS has been validated across thousands of studies and translated into dozens of languages. Its enduring popularity stems from its simplicity and statistical robustness. Despite being over 35 years old, it remains a gold standard in UX research.
- Created in 1986 by John Brooke
- Originally designed for interactive systems
- Now used globally across digital and physical products
How SUS Differs From Other Usability Metrics
While tools like the Net Promoter Score (NPS) or the User Experience Questionnaire (UEQ) also assess user satisfaction, the System Usability Scale focuses specifically on perceived usability. It doesn’t measure emotional appeal or brand loyalty—just how usable users find a system.
Unlike task-based usability testing, which observes users completing specific actions, SUS captures subjective feedback after interaction. This makes it ideal for early prototypes or when observational testing isn’t feasible. It’s also much shorter than comprehensive surveys like the SUMI (Software Usability Measurement Inventory), which can take 20+ minutes to complete.
“SUS is not about what users do, but what they think about what they did.” — Jakob Nielsen, UX Pioneer
How the System Usability Scale Works
The System Usability Scale operates on a simple yet powerful structure: 10 statements rated on a 5-point Likert scale, ranging from “Strongly Disagree” to “Strongly Agree.” Each statement alternates between positive and negative phrasing to reduce response bias.
After users complete a task or interact with a system, they answer the following 10 questions:
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.Scoring the System Usability ScaleScoring the SUS involves a specific formula that converts raw responses into a single usability score between 0 and 100..
Here’s how it works:.
- For odd-numbered items (1, 3, 5, 7, 9), subtract 1 from the user response (so a score of 1 becomes 0, 2 becomes 1, etc.).
- For even-numbered items (2, 4, 6, 8, 10), subtract the user response from 5 (so a 5 becomes 0, 4 becomes 1, etc.).
- Sum all the converted scores.
- Multiply the total by 2.5 to get the final SUS score (out of 100).
For example, if a user’s adjusted scores sum to 36, multiplying by 2.5 gives a SUS score of 90—indicating excellent usability.
Because the scale is standardized, scores can be compared across different products, platforms, or time periods. This makes SUS ideal for benchmarking.
Interpreting SUS Scores: What Do the Numbers Mean?
One of the most powerful aspects of the System Usability Scale is its interpretability. While the score ranges from 0 to 100, average scores tend to cluster around 68. Anything above 68 is considered above average, while below 68 is below average.
Alfie Norwood (2009) and later James Lewis and Jeff Sauro established grading benchmarks based on extensive data aggregation:
- 90–100: Excellent
- 80–89: Good
- 70–79: Acceptable
- 60–69: Poor
- 50–59: Awful
- Below 50: Unacceptable
For instance, a mobile banking app scoring 85 would be considered “Good,” suggesting users find it intuitive and efficient. In contrast, a score of 45 for a hospital patient portal would signal serious usability issues needing immediate attention.
“A SUS score below 50 is a red flag. It means users are struggling, and adoption will likely fail.” — Dr. James Lewis, SUS Researcher
Why the System Usability Scale Is So Widely Used
The System Usability Scale has stood the test of time because it delivers reliable, actionable data with minimal effort. Its widespread adoption across industries—from tech startups to government agencies—speaks to its versatility and effectiveness.
Speed and Simplicity of Administration
One of the biggest advantages of the System Usability Scale is how quickly it can be administered. Most users complete the 10 questions in under 5 minutes. This makes it ideal for:
- Usability testing sessions
- Post-task feedback in user interviews
- Remote testing via email or embedded surveys
Unlike observational methods that require video recording, screen sharing, or moderated sessions, SUS can be deployed at scale with minimal overhead. Tools like MeasuringU offer automated SUS calculators and templates, further streamlining the process.
Reliability and Validity Across Contexts
Despite its brevity, the System Usability Scale has been shown to be both reliable (consistent results over time) and valid (measures what it claims to measure). Numerous studies have confirmed its internal consistency, with Cronbach’s alpha typically above 0.9, indicating high reliability.
It’s also been validated across diverse domains:
- Healthcare: Used to assess electronic health records (EHRs) and telehealth platforms
- Education: Evaluated learning management systems (LMS) like Moodle or Canvas
- Automotive: Tested infotainment systems in vehicles
- E-commerce: Measured checkout flow usability on retail websites
Because SUS is technology-neutral, it can be applied to almost any interactive system, making it a universal benchmark in UX research.
Practical Applications of the System Usability Scale
The real power of the System Usability Scale lies in its practical applications. Whether you’re a UX designer, product manager, or researcher, SUS provides actionable insights that drive design decisions.
Using SUS in Product Development Cycles
Integrating the System Usability Scale into your product development lifecycle allows for continuous usability evaluation. For example:
- During prototyping: Test low-fidelity wireframes with SUS to catch usability issues early
- In beta testing: Collect SUS scores from real users to validate design changes
- Post-launch: Monitor SUS trends over time to assess the impact of updates
By tracking SUS scores across versions, teams can quantify improvements. A redesign that increases SUS from 60 to 78, for instance, represents a significant usability gain—moving from “Poor” to “Acceptable.”
Companies like Google and Microsoft have used SUS internally to compare interface iterations, ensuring that new features don’t degrade the overall user experience.
Comparative Usability Testing with SUS
One of the most powerful uses of the System Usability Scale is comparative testing—evaluating two or more versions of a system to determine which performs better in terms of usability.
For example, a fintech company might test two different onboarding flows:
- Version A: Step-by-step form with progress bar
- Version B: Single-page form with dynamic fields
After users complete each version, they’re given the SUS questionnaire. If Version A averages 82 and Version B averages 65, the data clearly favors the first design.
This method is also used in A/B testing, competitive analysis, and benchmarking against industry standards. According to Nielsen Norman Group, SUS is one of the most effective tools for making data-driven design decisions.
Strengths and Limitations of the System Usability Scale
While the System Usability Scale is a powerful tool, it’s not without limitations. Understanding both its strengths and weaknesses is crucial for using it effectively.
Key Strengths of SUS
The System Usability Scale offers several compelling advantages:
- Brevity: Only 10 questions, taking less than 5 minutes to complete.
- Standardization: Consistent scoring allows for cross-product and cross-industry comparisons.
- Proven Reliability: High internal consistency and test-retest reliability.
- Free to Use: No licensing fees or restrictions—SUS is in the public domain.
- Language Flexibility: Translated into over 30 languages with validated versions.
Its simplicity makes it accessible even to non-UX professionals, enabling broader adoption within organizations.
Common Criticisms and Limitations
Despite its strengths, the System Usability Scale has some drawbacks:
- Lack of Diagnostic Detail: SUS tells you how usable a system is, but not why. A low score doesn’t pinpoint specific issues.
- Subjective Nature: Relies on self-reported data, which can be influenced by mood, expectations, or social desirability bias.
- No Emotional or Aesthetic Measurement: Doesn’t capture delight, beauty, or emotional engagement—only perceived ease of use.
- Context Dependency: A score must be interpreted in context. A 70 might be great for a complex enterprise tool but poor for a consumer app.
To overcome these limitations, many researchers pair SUS with qualitative methods like interviews, think-aloud protocols, or behavioral analytics.
“SUS is a thermometer for usability. It tells you if the system has a fever, but not what’s causing it.” — UX Best Practices Guide, 2022
How to Administer the System Usability Scale Effectively
Getting accurate and meaningful results from the System Usability Scale depends on proper administration. Follow these best practices to ensure reliable data.
Best Practices for Survey Design and Delivery
To maximize response quality, consider the following:
- Use the Exact Wording: Even small changes to SUS items can affect validity. Stick to the original phrasing.
- Randomize Item Order (Cautiously): While some suggest randomizing to reduce bias, research shows it may affect reliability. It’s safer to keep the standard order.
- Use a Neutral Scale: Label the scale clearly: 1 = Strongly Disagree, 2 = Disagree, 3 = Neutral, 4 = Agree, 5 = Strongly Agree.
- Administer Post-Task: Give the SUS immediately after users complete a realistic task, not days later.
Tools like Google Forms, SurveyMonkey, or dedicated UX platforms can be used to distribute SUS. Ensure mobile responsiveness, as many users will take it on smartphones.
Sampling and Data Collection Tips
While SUS can be used with small samples (as few as 5–8 users), larger samples improve statistical confidence. Here’s how to approach sampling:
- Minimum Sample Size: 8–12 users for formative testing; 30+ for summative or benchmarking.
- Representative Users: Ensure participants reflect your actual user base in terms of age, tech literacy, and experience.
- Avoid Leading Questions: Don’t influence responses by praising the system beforehand.
- Collect Demographics: Note user characteristics to analyze subgroup differences (e.g., age, experience level).
For longitudinal studies, track SUS scores over time to identify trends and measure the impact of design changes.
Advanced Insights and Interpretation Techniques for the System Usability Scale
While the basic SUS score is valuable, advanced analysis techniques can extract deeper insights from the data. These methods go beyond the overall score to uncover patterns and drivers of usability.
Item-Level Analysis and Diagnostic Profiling
Instead of just looking at the total SUS score, examine individual item responses to identify problem areas. For example:
- If users strongly agree with “I found the system unnecessarily complex,” it suggests cognitive overload.
- If many disagree with “I felt very confident using the system,” it points to a lack of feedback or guidance.
By plotting average responses per item, you can create a “diagnostic profile” that highlights strengths and weaknesses. This is especially useful when comparing versions or identifying design priorities.
Some teams use color-coded dashboards to visualize item-level SUS data, making it easier for stakeholders to understand where improvements are needed.
Using SUS in Combination With Other Metrics
The System Usability Scale is most powerful when combined with other usability metrics. Common pairings include:
- Task Success Rate: Measure how many users complete a task successfully.
- Time on Task: Track how long it takes to complete key actions.
- Error Rate: Count mistakes made during interaction.
- NPS (Net Promoter Score): Assess user loyalty and satisfaction.
Together, these metrics provide a holistic view of usability. For instance, a high SUS score with low task success might indicate users feel confident but are actually failing—perhaps due to poor error messages.
According to UX Matters, integrating SUS with behavioral data creates a more complete picture of user experience.
Future of the System Usability Scale: Evolution and Alternatives
As technology evolves, so do usability assessment methods. While the System Usability Scale remains dominant, new tools and adaptations are emerging to address its limitations.
Modern Adaptations of SUS
Researchers have developed several variants of the original SUS to better fit specific contexts:
- mSUS (Mobile SUS): Slightly reworded for mobile apps, though studies show standard SUS works well on mobile too.
- UMUX (Usability Metric for User Experience): A 4-item alternative based on ISO definitions of usability, highly correlated with SUS.
- SUS-8: An 8-item short form that maintains reliability while reducing respondent burden.
These adaptations aim to preserve SUS’s strengths while improving relevance in modern UX landscapes.
Emerging Alternatives and Complementary Tools
Newer tools like the Post-Study System Usability Questionnaire (PSSUQ) and the User Experience Questionnaire (UEQ) offer more granular insights into satisfaction, efficiency, and aesthetics. However, they are longer and less portable than SUS.
AI-driven analytics, such as eye-tracking, facial expression analysis, and session replay tools, are also gaining traction. While these provide behavioral insights, they lack the standardized scoring and simplicity of SUS.
Ultimately, the System Usability Scale is likely to remain a cornerstone of usability evaluation due to its balance of simplicity, reliability, and universality.
What is the System Usability Scale?
The System Usability Scale (SUS) is a 10-item questionnaire used to measure the perceived usability of a system. It produces a single score from 0 to 100, with higher scores indicating better 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 licensing fees or restrictions.
What is a good SUS score?
A score above 68 is considered above average. Scores from 80–100 are “Good” to “Excellent,” while below 50 is generally considered unacceptable.
Can SUS be used for mobile apps?
Yes, SUS is technology-agnostic and widely used for mobile apps, websites, software, and even physical systems like medical devices.
How many users do I need for a reliable SUS score?
For early testing, 8–12 users can provide useful insights. For benchmarking or publication, 30+ users are recommended for greater statistical confidence.
The System Usability Scale remains one of the most trusted tools in the UX professional’s toolkit. Its simplicity, reliability, and universal applicability make it indispensable for measuring perceived usability. While it doesn’t replace observational testing or qualitative feedback, it provides a fast, standardized way to quantify user experience. When used correctly—and ideally combined with other metrics—SUS empowers teams to make data-driven decisions that improve products and delight users.
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