Ever wondered how companies measure how user-friendly their products really are? Enter the System Usability Scale (SUS)—a simple, yet powerful tool that reveals the truth behind user experience.
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 assessment across industries—from software and websites to medical devices and consumer electronics.
Origins and Development of SUS
The SUS was first introduced in 1986 as a quick and reliable way to assess usability without requiring extensive resources. At the time, usability testing was often complex, time-consuming, and required expert observers. Brooke aimed to create a lightweight, yet effective method that could be administered quickly and yield consistent results.
- Originally developed for internal use in tech environments.
- Published in 1996 in the book Usability Evaluation in Industry, which helped popularize it globally.
- Designed to be technology-agnostic—usable for software, hardware, apps, and more.
“The beauty of the SUS lies in its simplicity and reliability. It doesn’t tell you why something is unusable, but it tells you that it is.” — Jakob Nielsen, Nielsen Norman Group
How the SUS Questionnaire Works
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.
- Odd-numbered items are positively worded (e.g., “I thought the system was easy to use”).
- Even-numbered items are negatively worded (e.g., “I found the system unnecessarily complex”).
- After scoring, a final SUS score between 0 and 100 is calculated using a standardized formula.
The scoring process involves adjusting responses: for odd-numbered items, subtract 1 from the user response; for even-numbered items, subtract the user response from 5. These values are summed and multiplied by 2.5 to normalize the score to a 0–100 range.
Why the System Usability Scale Is Universally Trusted
Despite its age, the SUS remains a gold standard in usability evaluation. Its enduring popularity stems from its balance of simplicity, reliability, and broad applicability. Unlike more complex usability metrics, SUS doesn’t require specialized training to administer or interpret.
Reliability and Validity of SUS
Multiple studies have confirmed the SUS’s strong internal consistency and test-retest reliability. Research by James R. Lewis and others has shown that SUS has a Cronbach’s alpha typically above 0.9, indicating high reliability.
- Valid across different languages and cultures—translated into over 40 languages.
- Proven effective in both controlled lab settings and remote user testing.
- Correlates well with other usability metrics like task success rate and time-on-task.
A 2013 study published in the International Journal of Human–Computer Interaction found that SUS scores strongly correlate with user satisfaction and perceived ease of use, making it a trusted proxy for overall user experience quality.
Cost-Effectiveness and Speed
One of the biggest advantages of the System Usability Scale is its low cost and rapid deployment. Unlike observational usability testing, which can take hours per participant, SUS can be completed in under 10 minutes.
- Can be administered via email, embedded in apps, or used post-task in usability labs.
- Requires no special software—though tools like MeasuringU offer automated scoring and benchmarking.
- Minimal training needed for researchers or product teams.
This efficiency makes SUS ideal for agile development cycles, where quick feedback loops are essential.
How to Administer the System Usability Scale
Using the System Usability Scale effectively requires more than just handing out a questionnaire. Timing, context, and participant selection all play crucial roles in obtaining meaningful data.
Best Practices for Administering SUS
To get reliable results, SUS should be administered immediately after a user completes a set of representative tasks with the system. This ensures that their experience is fresh and contextually grounded.
- Use SUS after task-based usability testing, not in isolation.
- Ensure participants have interacted meaningfully with the system—browsing a website or completing a workflow.
- Avoid leading questions or influencing responses before administering the survey.
For example, if testing a mobile banking app, ask users to perform tasks like checking a balance, transferring money, and reviewing transaction history before giving them the SUS questionnaire.
Who Should Take the SUS?
The ideal SUS respondent is a representative user of the system—someone who matches the target audience in terms of experience, goals, and technical proficiency.
- Include both novice and experienced users to capture a range of perspectives.
- Aim for a minimum of 8–12 participants for reliable results (Lewis & Sauro, 2018).
- For larger studies, 30+ participants allow for more robust statistical analysis.
It’s important to note that SUS measures perceived usability, not objective performance. Therefore, combining SUS with behavioral metrics (like error rates or completion times) provides a more complete picture.
Scoring and Interpreting the System Usability Scale
One of the most powerful aspects of the System Usability Scale is its standardized scoring method. Once you collect responses, calculating the SUS score is straightforward—and interpreting it is even more valuable.
Step-by-Step SUS Scoring Guide
Here’s how to calculate a SUS score manually:
- For each odd-numbered item (1, 3, 5, 7, 9), subtract 1 from the response (e.g., a response of 4 becomes 3).
- For each even-numbered item (2, 4, 6, 8, 10), subtract the response from 5 (e.g., a response of 2 becomes 3).
- Sum all the adjusted values.
- Multiply the total by 2.5 to get the final SUS score (0–100).
For example, if the sum of adjusted responses is 36, the SUS score is 36 × 2.5 = 90.
What Does Your SUS Score Mean?
Scores are interpreted using established benchmarks:
- Below 50: Poor usability—significant redesign needed.
- 50–69: Marginal—acceptable but with notable usability issues.
- 70–79: Good—solid usability, room for improvement.
- 80–100: Excellent—top-tier user experience.
The average SUS score across thousands of studies is approximately 68, according to research by Sauro and Lewis (2016). A score above 70 is considered above average.
“A SUS score of 85 doesn’t just mean users like your product—it means they can use it efficiently, with minimal frustration.” — Dr. James R. Lewis, IBM Human Factors Researcher
Strengths and Limitations of the System Usability Scale
While the System Usability Scale is widely praised, it’s not without limitations. Understanding both its strengths and weaknesses is key to using it effectively.
Key Advantages of SUS
The SUS offers several compelling benefits that explain its longevity and widespread adoption.
- Simplicity: Easy to administer, score, and understand—even for non-experts.
- Flexibility: Applicable to any interactive system, regardless of platform or complexity.
- Comparability: Enables benchmarking across products, versions, or competitors.
- Speed: Delivers actionable insights in minutes, not weeks.
For instance, a company rolling out a new CRM system can use SUS to compare the old and new versions, quantifying improvements in usability.
Common Criticisms and Limitations
Despite its strengths, the SUS has some drawbacks:
- No diagnostic insight: It tells you that a system is usable, but not why or where problems exist.
- Subjective nature: Relies on self-reported perceptions, which can be influenced by mood, expectations, or brand loyalty.
- Limited granularity: A single score may mask usability issues in specific features or workflows.
- Acquiescence bias: Some users tend to agree with statements regardless of content.
To mitigate these, experts recommend pairing SUS with qualitative methods like interviews or think-aloud protocols.
System Usability Scale vs. Other Usability Metrics
The SUS isn’t the only usability metric available. Understanding how it compares to alternatives helps teams choose the right tool for the job.
Comparison with SUPR-Q and UMUX
The SUPR-Q (Standardized User Experience Percentile Rank Questionnaire) builds on SUS by adding dimensions like trust, loyalty, and appearance. It’s particularly useful for websites.
- SUPR-Q includes SUS as a component but adds brand-specific metrics.
- Requires a license, unlike the free-to-use SUS.
- Provides percentile rankings based on a large benchmark database.
The UMUX (Usability Metric for User Experience) is a shorter, 4-item scale based on ISO 9241-11. It’s simpler than SUS but less robust.
- UMUX-Lite is just 2 items—ideal for quick mobile surveys.
- Highly correlated with SUS (r = 0.85), but less sensitive to change.
- Best used when survey length is a constraint.
For more details, visit MeasuringU’s guide to UMUX.
Task Success Rate and Time-on-Task
While SUS measures perception, objective metrics like task success rate and time-on-task measure actual performance.
- Task success rate: Percentage of users who complete a task without assistance.
- Time-on-task: How long it takes users to complete a task—shorter is usually better.
- These metrics are often used alongside SUS to triangulate findings.
For example, a system might have a high SUS score but poor task success—indicating users feel confident but struggle in practice.
Real-World Applications of the System Usability Scale
The System Usability Scale isn’t just a theoretical tool—it’s used daily by leading companies and institutions to improve real products.
Case Study: SUS in Healthcare Technology
In a 2020 study, a team evaluated a new electronic health record (EHR) system using SUS. After training, clinicians scored the system at 62—below the average benchmark.
- Follow-up interviews revealed confusion around navigation and data entry.
- Designers simplified the interface and retested—SUS score rose to 81.
- Improved usability led to faster documentation and fewer errors.
This case shows how SUS can drive measurable improvements in high-stakes environments.
SUS in E-Commerce and Digital Products
Major e-commerce platforms use SUS to evaluate checkout flows, search functionality, and mobile app experiences.
- A/B testing different checkout designs with SUS helps identify the most user-friendly version.
- Scores below 70 often trigger UX redesigns.
- Used in conjunction with conversion rate data to link usability to business outcomes.
For example, Nielsen Norman Group found that improving SUS scores in e-commerce correlates with higher sales and lower cart abandonment.
How to Improve Your System Usability Scale Score
Getting a low SUS score isn’t the end—it’s the beginning of a journey toward better design. Here’s how to turn insights into action.
Identify Problem Areas with Follow-Up Research
Since SUS doesn’t diagnose issues, use it as a starting point for deeper investigation.
- Conduct usability testing to observe where users struggle.
- Run card sorting or tree testing to evaluate information architecture.
- Use heatmaps and session recordings to analyze behavior.
For instance, if users rate “I found the system unnecessarily complex” highly, investigate whether navigation, terminology, or feature overload is the culprit.
Iterate and Retest
Usability is not a one-time fix. The real power of SUS lies in tracking changes over time.
- Retest after every major design update.
- Set internal benchmarks (e.g., “We aim for SUS > 80 by Q3”).
- Share scores with stakeholders to demonstrate UX progress.
Teams using agile methodologies can integrate SUS into sprint reviews, ensuring continuous improvement.
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?
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 technologies.
Can I use SUS for free?
Yes, the SUS is in the public domain and free to use. No permission is required, though proper citation is recommended. You can find the full questionnaire and scoring guide on Usability.gov.
What is a good SUS score?
A score above 70 is considered good, above 80 is excellent. The average SUS score across industries is around 68. Scores below 50 indicate significant usability problems.
How many users do I need for a SUS test?
For a reliable estimate, test with at least 8–12 users. For more precise benchmarking, 30+ users are recommended. Even 5 users can reveal major usability issues.
The System Usability Scale remains one of the most powerful, accessible, and proven tools in the UX researcher’s toolkit. Its simplicity belies its depth—offering a quick, standardized way to measure usability across any interactive system. While it doesn’t replace qualitative research, it provides a critical quantitative benchmark that teams can track, compare, and improve over time. Whether you’re designing a mobile app, a medical device, or a government website, SUS offers a clear, actionable snapshot of how users truly experience your product. By understanding its strengths, limitations, and best practices, you can harness its full potential to build more usable, satisfying, and successful systems.
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