If you’ve ever wondered how to measure the ease of use of a product or system, the System Usability Scale (SUS) is your golden ticket. Simple, reliable, and widely trusted, it’s the go-to tool for UX professionals worldwide.
What Is the System Usability Scale (SUS)?
The System Usability Scale, commonly known as SUS, is a standardized, ten-item questionnaire designed to assess the perceived usability of a system, product, or service. Developed in the late 1980s by John Brooke at Digital Equipment Corporation, the SUS has become one of the most widely used tools in usability evaluation 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 and dirty usability scale. Despite its informal nickname, it was built on solid psychometric principles. John Brooke aimed to create a tool that was both easy to administer and capable of producing reliable, quantifiable results across different systems and user groups.
- Originally developed for internal use in usability testing at Digital Equipment Corporation.
- Published in 1996 in the Usability Evaluation Methods report, gaining widespread academic and industry attention.
- Designed to be technology-agnostic, making it applicable to software, hardware, and digital services.
Over time, the SUS gained popularity due to its brevity, simplicity, and strong validity. Unlike longer usability questionnaires, the SUS requires only ten questions, making it user-friendly and less burdensome for participants.
How SUS Differs from Other Usability Metrics
While many usability assessment tools exist—such as the UMUX (Usability Metric for User Experience), SUPR-Q, and NASA-TLX—the SUS stands out for its balance of simplicity and effectiveness. It doesn’t measure specific aspects like satisfaction or efficiency in isolation but provides a holistic view of perceived usability.
- SUS is format-neutral: it can be applied pre- or post-task in usability tests.
- It doesn’t require specialized training to administer or score.
- It produces a single score from 0 to 100, allowing for easy benchmarking.
“The SUS has stood the test of time because it’s short, reliable, and gives a good approximation of how usable users think a system is.” — Jeff Sauro, MeasuringU
How the System Usability Scale Works
The SUS operates on a simple yet powerful structure: ten statements rated on a five-point Likert scale, ranging from “Strongly Disagree” to “Strongly Agree.” Each statement addresses a different aspect of usability, such as ease of use, consistency, and perceived complexity.
The 10 SUS Questions Explained
Each of the ten items in the System Usability Scale is carefully crafted to capture a dimension of usability. Here’s a breakdown of each question and what it measures:
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 system coherence and integration.6..
I thought there was too much inconsistency in this system.– Measures perceived inconsistency (reverse-scored).7.I would imagine that most people would learn to use this system very quickly.– Assesses learnability.8.I found the system very awkward to use.– Evaluates user comfort (reverse-scored).9.I felt very confident using the system.– Measures user confidence.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.This design helps reduce response bias..
Scoring the System Usability Scale
Scoring the SUS is straightforward but requires attention to detail. Each response is converted to a numerical value from 0 to 4, depending on the Likert scale position. For positively worded items (1, 3, 5, 7, 9), the score is the scale position minus 1. For negatively worded items (2, 4, 6, 8, 10), the score is 4 minus the scale position.
After converting all ten responses, the total is summed and multiplied by 2.5 to yield a final score between 0 and 100. For example:
- User answers: 4, 1, 5, 2, 4, 1, 5, 2, 5, 1
- Converted: 3, 3, 4, 2, 3, 3, 4, 2, 4, 3
- Sum: 31
- SUS Score: 31 × 2.5 = 77.5
A score of 77.5 is considered above average, indicating good usability. The scoring process is simple enough to be done manually, though many researchers use online calculators or tools like MeasuringU’s SUS Calculator for automation.
Why the System Usability Scale Is So Effective
The enduring popularity of the System Usability Scale isn’t accidental. Its effectiveness stems from a combination of psychometric rigor, practical design, and broad applicability. Let’s explore why SUS remains a gold standard in usability assessment.
Reliability and Validity of SUS
One of the key reasons SUS is trusted by researchers and practitioners is its strong psychometric properties. Numerous studies have confirmed its reliability (consistency of results) and validity (accuracy in measuring what it claims to measure).
- Internal consistency (Cronbach’s alpha) typically ranges from 0.80 to 0.90, indicating high reliability.
- It correlates well with other usability metrics, including task success rates and user satisfaction scores.
- It performs consistently across different languages, cultures, and domains.
A landmark study by James R. Lewis and Jeff Sauro confirmed that SUS scores are stable across different user populations and testing conditions, making it a robust tool for comparative analysis.
Speed and Simplicity in Administration
In fast-paced development environments, time is critical. The SUS shines here because it takes less than 5–10 minutes to complete. This brevity reduces participant fatigue and increases response rates, especially in remote or large-scale studies.
- Can be administered via paper, email, or integrated into digital survey platforms like Qualtrics or SurveyMonkey.
- Requires no specialized software or training to implement.
- Results are easy to interpret, even for non-experts.
Because of its simplicity, the SUS is often used in agile development cycles, where quick feedback loops are essential. Teams can run SUS assessments after each sprint to track usability improvements over time.
Interpreting System Usability Scale Scores
Getting a SUS score is just the first step. The real value lies in interpreting what that number means for your product or service. Understanding the SUS scoring scale and benchmarks is crucial for making informed decisions.
Understanding the SUS Score Range (0–100)
The SUS produces a score between 0 and 100, with higher scores indicating better perceived usability. While the scale is continuous, certain thresholds have been established based on extensive research.
- Below 50: Poor usability. Users are likely frustrated and may abandon the system.
- 50–69: Marginal or acceptable usability. Room for improvement.
- 70–79: Good usability. Meets expectations for most users.
- 80–100: Excellent usability. Competitive advantage in user experience.
According to Sauro and Lewis (2009), the average SUS score across thousands of studies is approximately 68. This provides a useful benchmark: if your product scores above 68, it’s performing better than average.
Benchmarking Against Industry Standards
Beyond the general average, industry-specific benchmarks can offer deeper insights. For example:
- Consumer software averages around 78.
- Enterprise software tends to score lower, around 65–70.
- Medical devices often score in the 60–75 range due to complexity and regulatory constraints.
By comparing your SUS score to relevant benchmarks, you can contextualize your results and set realistic improvement goals. Resources like MeasuringU’s SUS Benchmarks provide detailed comparisons across sectors.
“A SUS score of 70 doesn’t mean your product is ‘good enough’—it means it’s average. To stand out, aim for 80+.” — Dr. Jim Lewis, IBM Human Factors Engineer
Practical Applications of the System Usability Scale
The System Usability Scale isn’t just a theoretical tool—it’s actively used in real-world scenarios across industries. From tech startups to healthcare systems, SUS helps teams make data-driven decisions about user experience.
Using SUS in Software and App Development
In software development, SUS is often integrated into usability testing phases. After users complete a set of tasks, they’re asked to fill out the SUS questionnaire. This provides immediate feedback on how intuitive and efficient the interface feels.
- Used in A/B testing to compare two design versions.
- Helps prioritize UX improvements based on quantitative data.
- Supports stakeholder communication by providing a clear, numerical metric.
For example, a fintech app team might run SUS assessments before and after a redesign. If the score jumps from 62 to 81, they have strong evidence that the new design is more usable.
SUS in Healthcare and Medical Devices
In high-stakes environments like healthcare, usability isn’t just about convenience—it’s a matter of safety. The FDA and other regulatory bodies encourage the use of SUS in human factors validation for medical devices.
- Used to evaluate infusion pumps, patient monitors, and diagnostic software.
- Helps identify design flaws that could lead to user errors.
- Supports compliance with IEC 62366-1, the international standard for medical device usability.
A study published in the Journal of Biomedical Informatics found that SUS scores correlated strongly with observed user errors in a simulated ICU environment, proving its predictive value in critical settings.
Strengths and Limitations of the System Usability Scale
No tool is perfect, and the System Usability Scale is no exception. While it’s one of the most trusted usability metrics, understanding its strengths and weaknesses is essential for proper application.
Advantages of Using SUS
The benefits of the System Usability Scale are numerous and well-documented:
- Universality: Can be applied to any interactive system, regardless of platform or domain.
- Speed: Takes only 5–10 minutes to complete, minimizing user burden.
- Quantifiable: Produces a single, easy-to-interpret score.
- Free to Use: No licensing fees or restrictions—SUS is in the public domain.
- Widely Researched: Backed by decades of academic and industry validation.
Its public domain status is particularly valuable. Unlike proprietary tools, SUS can be used freely by anyone, from solo designers to multinational corporations.
Criticisms and Common Limitations
Despite its strengths, the SUS has several limitations that users should be aware of:
- Lack of Diagnostic Detail: While SUS tells you how usable a system is, it doesn’t explain why. For root cause analysis, qualitative methods like interviews or think-aloud protocols are needed.
- Subjective Nature: SUS measures perceived usability, not objective performance (e.g., task completion time).
- Context Dependency: Scores can be influenced by factors like user mood, prior experience, or testing environment.
- No Emotional Dimension: It doesn’t capture emotional responses like delight or frustration beyond confidence and awkwardness.
To overcome these limitations, many researchers combine SUS with other methods, such as the Net Promoter Score (NPS) or the User Experience Questionnaire (UEQ), to get a more complete picture.
How to Administer the System Usability Scale Effectively
Getting accurate and meaningful results from the System Usability Scale depends on how well it’s administered. Following best practices ensures data integrity and actionable insights.
Best Practices for SUS Administration
To maximize the reliability of your SUS results, consider the following guidelines:
- Administer after task completion: Users should have hands-on experience with the system before answering.
- Use a neutral tone: Avoid leading questions or biased instructions.
- Ensure anonymity: Encourage honest feedback by guaranteeing confidentiality.
- Collect demographic data: Age, tech proficiency, and prior experience can help interpret results.
- Test with at least 12–15 users: While SUS can be used with small samples, larger groups increase statistical confidence.
For remote testing, tools like Optimal Workshop’s SUS Calculator can streamline data collection and analysis.
Common Mistakes to Avoid
Even experienced researchers can make errors when using SUS. Here are some common pitfalls:
- Scoring errors: Forgetting to reverse-score even-numbered items is the most frequent mistake.
- Poor timing: Administering SUS before users have meaningful interaction skews results.
- Ignoring context: Not documenting the testing conditions (e.g., device type, task difficulty) limits comparability.
- Over-relying on the score: Treating the SUS score as the sole measure of usability without qualitative backup.
Always pair SUS with observational data and user comments to enrich interpretation.
Alternatives and Complements to the System Usability Scale
While the System Usability Scale is powerful, it’s not the only tool in the usability toolkit. Depending on your goals, you might consider alternatives or complementary metrics.
Popular SUS Alternatives
Several questionnaires have been developed to address perceived gaps in SUS:
- UMUX (Usability Metric for User Experience): A four-item scale based on ISO 9241, often used as a shorter alternative.
- SUPR-Q (Standardized User Experience Percentile Rank Questionnaire): Measures usability, trust, loyalty, and appearance across websites.
- NASA-TLX (Task Load Index): Focuses on cognitive workload rather than usability.
- UEQ (User Experience Questionnaire): A more detailed 26-item tool that assesses attractiveness, perspicuity, efficiency, and more.
Each has its strengths, but none match the SUS in terms of simplicity and widespread adoption.
How to Combine SUS with Other Methods
The most effective usability evaluations use a mixed-methods approach. For example:
- Use SUS alongside task success rates and time-on-task for a balanced view.
- Pair SUS with the System Confidence Scale (SCS) to assess user trust.
- Combine with NPS to understand both usability and loyalty.
- Use SUS before and after a redesign to measure improvement quantitatively.
By triangulating data from multiple sources, you gain deeper insights than any single metric can provide.
What is a good System Usability Scale score?
A score of 68 is considered average. Anything above 70 is good, 80+ is excellent, and below 50 indicates significant usability issues.
Can I use the System Usability Scale for free?
Yes, the System Usability Scale is in the public domain and can be used freely for research, commercial, or educational purposes without permission.
How many users do I need for a reliable SUS score?
While SUS can be used with as few as 5 users, a sample size of 12–15 is recommended for more stable and reliable results.
Is the SUS suitable for mobile apps?
Absolutely. The SUS is platform-agnostic and widely used for evaluating mobile apps, websites, and software interfaces.
Does SUS measure user satisfaction?
Not directly. SUS measures perceived usability, which is closely related to satisfaction but focuses more on ease of use, learnability, and efficiency.
The System Usability Scale remains one of the most trusted, practical, and scientifically validated tools for measuring usability. Its simplicity, reliability, and flexibility make it indispensable for UX professionals, product teams, and researchers. While it has limitations—such as its lack of diagnostic depth—it excels as a quick, quantitative snapshot of user experience. When used correctly and in combination with other methods, the SUS provides actionable insights that drive better design decisions. Whether you’re evaluating a new app, a medical device, or an enterprise software platform, the System Usability Scale offers a proven way to measure, compare, and improve usability over time.
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