Ever wondered how to measure if a product is truly user-friendly? The System Usability Scale (SUS) is your go-to tool—simple, reliable, and backed by decades of research. Let’s dive into why it’s a game-changer.
What Is the System Usability Scale (SUS)?
The System Usability Scale, commonly known as SUS, is a 10-item questionnaire designed to assess the perceived usability of a system, product, or service. Developed in 1986 by John Brooke at Digital Equipment Corporation, it has since become one of the most widely used tools in usability evaluation across industries—from software and websites to medical devices and consumer electronics.
What makes the SUS stand out is its simplicity and effectiveness. Despite having only ten questions, it delivers a reliable and quantifiable score that reflects how users perceive a system’s ease of use. The SUS doesn’t require extensive training to administer, and results can be calculated quickly, making it ideal for both academic research and real-world product development.
Origins and Development of SUS
The System Usability Scale was born out of the need for a quick, reliable method to compare the usability of different systems during usability testing. At the time, existing methods were either too complex or lacked consistency. John Brooke aimed to create a tool that was both practical and psychometrically sound.
Brooke’s original research was published in a 1986 conference paper titled *“SUS: A quick and dirty usability scale”*—a name that stuck due to the tool’s straightforward nature. Despite the self-deprecating title, the SUS proved to be far from “dirty.” Over time, it demonstrated strong reliability and validity across diverse contexts and user populations.
One of the key strengths of the SUS is its agnostic design. It doesn’t favor any specific type of system or interface, which allows it to be applied universally. Whether you’re testing a mobile app, a kiosk interface, or a complex enterprise software, the SUS provides a consistent benchmark.
How the SUS Questionnaire Works
The SUS consists of ten statements, each rated on a five-point Likert scale ranging from “Strongly Disagree” (1) to “Strongly Agree” (5). The statements alternate between positive and negative phrasing to reduce response bias. For example:
- 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.
After users complete the questionnaire, a specific scoring algorithm is applied. Odd-numbered items are scored by subtracting 1 from the user response (e.g., a “4” becomes “3”), while even-numbered items are scored by subtracting the user response from 5 (e.g., a “2” becomes “3”). These scores are then summed and multiplied by 2.5 to produce a final SUS score between 0 and 100.
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For example, if a user gives all “3” responses (neutral), the total raw score is 50, and after multiplying by 2.5, the final SUS score is 62.5—right around the average benchmark.
“The beauty of the SUS lies in its balance: it’s simple enough to be used by anyone, yet robust enough to stand up to scientific scrutiny.” — Jakob Nielsen, Nielsen Norman Group
Why the System Usability Scale Matters
In today’s competitive digital landscape, usability isn’t just a nice-to-have—it’s a business imperative. A poorly designed interface can lead to user frustration, increased support costs, and ultimately, customer churn. The System Usability Scale provides a standardized way to quantify usability, enabling teams to make data-driven decisions.
Unlike subjective feedback or anecdotal observations, the SUS offers a numerical score that can be tracked over time, compared across products, or benchmarked against industry standards. This makes it invaluable for UX researchers, product managers, and designers who need to demonstrate the impact of design changes.
Quantifying User Experience
One of the biggest challenges in user experience (UX) design is measuring something as abstract as “usability.” The SUS bridges this gap by translating subjective perceptions into a concrete metric. A SUS score gives stakeholders a clear, easy-to-understand number that reflects user satisfaction.
For instance, a product with a SUS score of 85 is generally considered to have excellent usability, while a score below 50 indicates significant usability problems. This allows teams to set clear goals—such as “improve SUS score from 60 to 75 after redesign”—and measure progress objectively.
Moreover, because the SUS is normalized (scores range from 0 to 100), it enables comparisons across different projects, teams, or even companies. A software company can compare the usability of its mobile app versus its desktop version, or benchmark its product against competitors using published SUS norms.
Supporting Iterative Design
Modern product development is iterative. Teams release prototypes, gather feedback, and refine designs in cycles. The System Usability Scale fits perfectly into this workflow. It can be administered after each usability test or design sprint to measure the impact of changes.
For example, a team might conduct a usability test on a prototype, collect SUS scores, make design improvements, and then retest. If the SUS score increases from 60 to 78, that’s strong evidence that the changes improved usability. This kind of data is especially persuasive when presenting results to executives or stakeholders who may not be familiar with UX jargon.
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The SUS also supports A/B testing scenarios. Two design variants can be tested with different user groups, and their SUS scores compared to determine which is perceived as more usable. This quantitative approach complements qualitative insights from user interviews or observation.
How to Administer the System Usability Scale
Administering the SUS is straightforward, but doing it correctly ensures reliable results. The process involves selecting participants, delivering the questionnaire, and calculating scores accurately. Let’s break down each step.
Choosing the Right Participants
The validity of your SUS results depends heavily on who completes the questionnaire. Ideally, participants should be representative of your target user group. This means considering factors like age, technical proficiency, familiarity with similar systems, and usage context.
While the SUS can be used with as few as five users (in line with Jakob Nielsen’s heuristic that five users uncover 85% of usability issues), larger sample sizes increase the reliability of the average score. For benchmarking or high-stakes decisions, a sample of 15–30 users is recommended.
It’s also important to time the SUS administration correctly. The questionnaire should be given immediately after the user has completed a set of representative tasks with the system. This ensures that their experience is fresh and their responses are based on actual interaction, not assumptions.
Delivering the Questionnaire
The SUS can be administered in various formats: paper, online survey tools (like Google Forms or SurveyMonkey), or integrated into usability testing software (such as UserTesting or Lookback). Regardless of the method, consistency is key.
Each participant should answer all ten questions without skipping any. The instructions should be clear: “Please indicate your level of agreement with the following statements based on your experience with the system.” Avoid leading questions or influencing responses.
One common mistake is modifying the SUS wording. While it might be tempting to rephrase items for clarity, doing so can invalidate the scoring model and make comparisons to standard benchmarks unreliable. If you must adapt the SUS (e.g., for a mobile app), keep the original wording intact and only change the word “system” to “app” or “website” where necessary.
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Calculating and Interpreting SUS Scores
Once responses are collected, the next step is scoring. As mentioned earlier, the formula involves adjusting each response, summing them, and multiplying by 2.5. While this can be done manually, tools like the MeasuringU SUS Calculator automate the process and reduce errors.
After calculating individual scores, the average SUS score for your sample is computed. This average is then interpreted using established benchmarks. According to research by Sauro and Lewis (2006), the average SUS score across thousands of studies is around 68.
- Below 50: Poor usability
- 50–60: Marginal
- 60–70: Acceptable
- 70–80: Good
- 80–90: Excellent
- 90+: Outstanding
It’s also useful to look at the distribution of scores. A high average with a large standard deviation might indicate that some users found the system easy to use while others struggled—pointing to potential inconsistencies in the design.
Advantages of the System Usability Scale
The enduring popularity of the System Usability Scale is no accident. Its widespread adoption is due to a combination of practical and scientific advantages that make it a preferred choice for usability assessment.
Simplicity and Ease of Use
One of the most cited benefits of the SUS is its simplicity. With only ten questions, it takes users less than 5–10 minutes to complete. This low burden increases response rates and reduces fatigue, especially in longer usability sessions.
For researchers and practitioners, the scoring is straightforward and can be automated. Unlike more complex usability metrics that require specialized software or statistical expertise, the SUS is accessible to teams of all sizes and skill levels.
This simplicity doesn’t come at the cost of reliability. Studies have shown that the SUS has high internal consistency (Cronbach’s alpha typically above 0.9), meaning the ten items are measuring the same underlying construct—usability.
Proven Reliability and Validity
Despite its brevity, the System Usability Scale has been rigorously tested and validated across numerous studies. Research has confirmed its reliability (consistency of results) and validity (accuracy in measuring what it claims to measure).
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A 2010 study by James R. Lewis and Jeff Sauro analyzed over 5,000 SUS scores and found that the scale performs well across different domains, including software, websites, and medical devices. The study also confirmed that the SUS is sensitive enough to detect usability improvements after design changes.
Furthermore, the SUS correlates well with other usability metrics, such as task success rates and user satisfaction scores. This means that a high SUS score generally aligns with better performance and higher user satisfaction.
“The SUS has stood the test of time because it’s both simple and scientifically sound. Few tools in UX offer that combination.” — Dr. Susan Weinschenk, Human Factors International
Flexibility Across Domains
Another major strength of the System Usability Scale is its domain independence. It doesn’t assume any specific technology, interface type, or user group. This makes it incredibly versatile.
For example, the same SUS questionnaire can be used to evaluate:
- A new e-commerce website
- A hospital patient portal
- An industrial control panel
- A mobile banking app
- A voice-activated smart assistant
This flexibility allows organizations to maintain a consistent usability metric across multiple products and teams. It also enables benchmarking against industry averages, even when comparing vastly different systems.
Additionally, the SUS has been translated into dozens of languages and validated in various cultural contexts, making it a truly global tool. Organizations with international user bases can confidently use localized versions of the SUS.
Limitations and Criticisms of the System Usability Scale
While the System Usability Scale is widely respected, it’s not without limitations. Understanding these weaknesses helps practitioners use the SUS more effectively and know when to supplement it with other methods.
Lack of Diagnostic Detail
One of the most common criticisms of the SUS is that it provides a global usability score but doesn’t explain *why* users rated the system the way they did. A low score tells you there’s a problem, but not where it lies.
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For example, if a product scores 55 on the SUS, you know usability is below average, but you don’t know if the issue is with navigation, terminology, response time, or something else. This is where qualitative methods like think-aloud protocols, interviews, or heuristic evaluations become essential.
To get actionable insights, the SUS should be used alongside observational data. Watching users struggle with a specific feature while noting their SUS responses can help pinpoint the root cause of low scores.
Sensitivity to Context and Framing
Research has shown that SUS scores can be influenced by factors outside the system itself, such as the user’s mood, the testing environment, or how tasks are framed. For instance, users who complete difficult tasks may rate the system lower, even if the system performed flawlessly.
Similarly, the way the questionnaire is introduced can affect responses. If a moderator says, “We’re testing a new system we think is really intuitive,” users may feel pressured to agree, inflating scores. Neutral, unbiased instructions are crucial.
Another issue is the lack of anchoring in the Likert scale. The terms “Strongly Disagree” to “Strongly Agree” are subjective and may be interpreted differently across cultures or individuals. Some researchers have proposed using visual analog scales or anchoring the endpoints more clearly to improve consistency.
Not a Replacement for Behavioral Metrics
The SUS measures perceived usability, not actual performance. A user might find a system “easy to use” (high SUS score) but still fail to complete tasks due to poor information architecture or misleading labels.
Conversely, a system with a low SUS score might be highly efficient for expert users. This discrepancy highlights the importance of combining SUS with behavioral metrics like task success rate, time on task, error rate, and click count.
For a complete usability assessment, the SUS should be part of a mixed-methods approach. Relying solely on SUS scores risks missing critical usability issues that only emerge through direct observation.
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Best Practices for Using the System Usability Scale
To get the most value from the System Usability Scale, it’s essential to follow best practices in administration, analysis, and interpretation. These guidelines help ensure that your results are accurate, reliable, and actionable.
Use SUS in Conjunction with Other Methods
The SUS is most powerful when combined with qualitative and behavioral data. After users complete tasks, administer the SUS, but also conduct a brief interview to ask open-ended questions like:
- What did you find most confusing?
- Was there anything that surprised you?
- If you could change one thing, what would it be?
This triangulation of data—quantitative SUS scores, qualitative feedback, and observed behavior—provides a holistic view of usability. It allows you to not only measure usability but also understand how to improve it.
Tools like Nielsen Norman Group’s usability testing templates often include the SUS as part of a broader assessment framework.
Track SUS Over Time
One of the most valuable uses of the SUS is tracking usability improvements across product iterations. Establish a baseline SUS score during early testing, then re-measure after each major design update.
For example:
- Version 1.0: SUS = 62
- After navigation redesign: SUS = 74
- After onboarding improvements: SUS = 81
This trend data demonstrates the impact of UX investments and helps justify further design work. It also builds a historical database that can inform future projects.
When tracking over time, ensure consistency in testing conditions, tasks, and participant profiles to make comparisons valid.
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Avoid Common Pitfalls
Several common mistakes can undermine the effectiveness of the SUS:
- Modifying the wording: Even small changes can affect scoring and comparability.
- Administering too early: Users should have hands-on experience before answering.
- Using it in isolation: SUS alone doesn’t provide enough insight for design decisions.
- Ignoring sample size: Small samples can produce misleading averages.
By avoiding these pitfalls, teams can ensure their SUS data is trustworthy and useful.
Alternatives and Complements to the System Usability Scale
While the SUS is a gold standard, it’s not the only usability metric available. Depending on your goals, you might consider complementary or alternative tools.
UMUX and UMUX-Lite
The Usability Metric for User Experience (UMUX) is a four-item scale based on ISO 9241-11, designed to be more theoretically grounded than the SUS. UMUX-Lite, a two-item version, is even shorter and correlates highly with SUS.
UMUX focuses on two core dimensions: usability and efficiency. While less comprehensive than SUS, it’s useful when survey length is a concern.
Nielsen’s Post-Study System Usability Questionnaire (PSSUQ)
Developed by Jakob Nielsen, the PSSUQ is a 16-item questionnaire that assesses system usability, information quality, and interface quality. It’s more detailed than SUS and provides subscale scores, making it better for diagnostic purposes.
However, its length makes it less suitable for quick evaluations. It’s often used in academic research or high-fidelity usability studies.
Net Promoter Score (NPS) and Customer Satisfaction (CSAT)
While not usability-specific, NPS and CSAT are often used alongside SUS to measure overall user satisfaction and loyalty. NPS asks, “How likely are you to recommend this product to a friend?” while CSAT measures satisfaction with a recent interaction.
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Combining SUS with NPS can reveal whether good usability translates into user advocacy. For example, a high SUS but low NPS might indicate that while the system is easy to use, other factors (like pricing or customer service) are affecting loyalty.
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 organizations to improve products and services.
Technology and Software Development
Major tech companies like Google, Microsoft, and Apple use SUS internally to evaluate prototypes and released products. For example, a team at Google might test two search interface designs and use SUS scores to determine which is perceived as more usable.
Startups also leverage SUS during lean UX processes. With limited resources, a quick SUS assessment after a usability test can provide immediate feedback on design direction.
Healthcare and Medical Devices
In healthcare, usability can be a matter of life and death. Regulatory bodies like the FDA require usability testing for medical devices, and the SUS is frequently included in these evaluations.
For instance, a glucose monitoring app for diabetics might undergo SUS testing with elderly users to ensure it’s intuitive and error-resistant. A high SUS score supports regulatory submissions and reduces training costs.
Government and Public Services
Government agencies use SUS to improve citizen-facing digital services. The UK’s Government Digital Service (GDS) has used SUS to evaluate the usability of online tax filing, benefit applications, and passport renewal systems.
By setting minimum SUS thresholds (e.g., 70+), agencies ensure that digital services meet basic usability standards before launch.
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What is a good System Usability Scale score?
A score of 68 is the average across all systems. Scores above 70 are considered good, 80+ is excellent, and below 50 indicates significant usability issues.
Can I modify the SUS questionnaire?
It’s strongly advised not to modify the wording of the SUS items, as this can affect the validity of the score. You can change “system” to “website” or “app” if needed, but keep the core phrasing intact.
How many users do I need for a reliable SUS score?
As few as 5 users can provide useful insights, but for reliable averages and benchmarking, 15–30 users are recommended.
Is the SUS free to use?
Yes, the System Usability Scale is in the public domain and free for both commercial and non-commercial use. No permission is required.
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Where can I find the full SUS questionnaire?
The original SUS items are available on research repositories and UX resources like MeasuringU and the Nielsen Norman Group.
The System Usability Scale remains one of the most effective tools for measuring perceived usability. Its simplicity, reliability, and flexibility make it indispensable for UX professionals, product teams, and researchers. While it has limitations—such as lack of diagnostic detail—it shines when used as part of a broader usability strategy. By combining SUS with qualitative insights and behavioral metrics, organizations can create products that are not only easy to use but also delightful. Whether you’re launching a new app or improving a legacy system, the SUS offers a proven way to quantify usability and drive meaningful improvements.
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