Ever wondered how companies measure how easy their products are to use? Enter the System Usability Scale (SUS) — a simple yet powerful tool that reveals user experience truths in just 10 questions.
What Is the System Usability Scale (SUS)?
The System Usability Scale, commonly known as SUS, is a globally recognized 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 stood the test of time due to its simplicity, reliability, and flexibility across various domains — from software applications to medical devices and websites.
Origins and Development of SUS
The System Usability Scale was first introduced in 1986 during usability research at Digital Equipment Corporation. At the time, there was no standardized way to measure how users felt about the ease of use of interactive systems. Traditional usability testing focused heavily on task success rates and time-on-task metrics, but lacked a structured method for capturing subjective user feedback.
Brooke’s innovation was to create a lightweight, ten-item Likert-scale questionnaire that could be administered quickly after a user interaction session. The beauty of the SUS lies in its brevity and consistency. Despite having only ten statements, it delivers a reliable score between 0 and 100, offering a snapshot of perceived usability.
Over the decades, SUS has been validated across countless studies and adapted for use in diverse fields such as healthcare, finance, education, and mobile app development. Its enduring relevance is a testament to its robust design and universal applicability.
Structure of the SUS Questionnaire
The System Usability Scale consists of ten statements, each rated on a five-point scale ranging from “Strongly Disagree” to “Strongly Agree.” The odd-numbered items are positively worded (e.g., “I thought the system was easy to use”), while the even-numbered items are negatively worded (e.g., “I found the system unnecessarily complex”).
This alternating pattern helps reduce response bias, ensuring that participants engage with each question rather than falling into a rhythm of agreement or disagreement. The specific questions are:
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.Each response is scored numerically, with adjustments made for reverse-scored items..
A final calculation yields a single SUS score, making it easy to compare usability across different systems or iterations..
“The SUS is not just a metric; it’s a conversation starter about user experience.” — Jakob Nielsen, UX Pioneer
Why the System Usability Scale Matters in UX Design
In today’s competitive digital landscape, usability isn’t a luxury — it’s a necessity. Users expect intuitive interfaces, seamless navigation, and minimal friction. The System Usability Scale provides designers, developers, and product managers with a quantifiable way to assess whether their product meets these expectations.
Quantifying Subjective User Experience
One of the biggest challenges in user experience (UX) design is measuring something inherently subjective: how users feel about using a product. While analytics can tell you what users are doing (e.g., click paths, bounce rates), they don’t explain why. This is where the System Usability Scale shines.
By capturing users’ perceptions through structured self-reporting, SUS transforms abstract feelings into concrete data. A SUS score of 68 might indicate average usability, while a score above 80 suggests excellent usability. This allows teams to set benchmarks, track improvements over time, and make informed decisions based on user feedback.
For example, if a redesigned app interface increases the SUS score from 55 to 75, that’s a strong indicator of improved usability — even if other metrics remain unchanged. This kind of insight is invaluable during iterative design processes.
Benchmarking Against Industry Standards
The System Usability Scale is not just useful for internal comparisons; it also enables benchmarking against industry norms. Research by Sauro and Lewis (2006) analyzed over 500 SUS scores and established an average score of 68. This serves as a baseline for interpreting results.
Here’s a general interpretation guide:
- Below 50: Poor usability
- 50–60: Below average
- 60–70: Average
- 70–80: Good
- 80–90: Excellent
- 90+: Outstanding
This standardization allows organizations to contextualize their findings. For instance, a healthcare app scoring 72 may seem acceptable, but if competitors average 85, there’s clear room for improvement. Benchmarking helps prioritize UX investments and justify design changes to stakeholders.
Moreover, because SUS is language- and culture-neutral in its structure, it can be translated and used globally, enabling cross-regional comparisons. This makes it especially valuable for multinational companies aiming to deliver consistent user experiences worldwide.
How to Administer the System Usability Scale
One of the key strengths of the System Usability Scale is its ease of administration. It doesn’t require specialized tools or extensive training. However, to get reliable and meaningful results, certain best practices should be followed.
When to Use SUS in the Design Process
The System Usability Scale can be used at multiple stages of the product development lifecycle:
- Early Prototypes: Even low-fidelity wireframes can be evaluated using SUS after users complete basic tasks. This helps identify major usability issues before investing in full development.
- Mid-Development: During iterative testing, SUS scores can track progress and validate design changes.
- Post-Launch: After a product release, SUS can assess real-world usability and inform future updates.
- Competitive Analysis: SUS can be administered to users interacting with competing products, providing comparative insights.
The ideal time to administer SUS is immediately after a user completes a set of representative tasks. This ensures that their experience is fresh in memory, leading to more accurate responses. Delaying the survey risks recall bias and diminished reliability.
Best Practices for Accurate Results
To maximize the validity of your SUS findings, consider the following guidelines:
Use a Representative Sample: Aim for at least 15–20 users to achieve statistically reliable results.While SUS can be used with smaller samples, larger groups reduce variability.Ensure Task Completion First: SUS should follow hands-on interaction.Don’t administer it based on impressions alone.Maintain Neutrality: Avoid leading questions or cues that might influence responses.
.The administrator should remain neutral throughout.Randomize Question Order (Optional): While the standard order is recommended, some researchers randomize to reduce order effects — though this may affect comparability with norms.Collect Qualitative Feedback: Pair SUS with open-ended questions to understand the ‘why’ behind the scores.Additionally, ensure that the testing environment mirrors real-world conditions as closely as possible.For example, if evaluating a mobile banking app, conduct tests on actual devices rather than emulators..
“A SUS score without context is just a number. Combine it with observation and interviews for deeper insights.” — Dr. James Lewis, Human Factors Researcher
Calculating and Interpreting the System Usability Scale Score
While administering the SUS is straightforward, correctly calculating and interpreting the score requires attention to detail. Fortunately, the process is well-documented and repeatable.
Step-by-Step SUS Scoring Method
The calculation involves the following steps:
- For each participant, go through the ten responses.
- For odd-numbered items (1, 3, 5, 7, 9), subtract 1 from the user’s scale position (which ranges from 1 to 5). This gives a score from 0 to 4.
- For even-numbered items (2, 4, 6, 8, 10), subtract the user’s scale position from 5. This reverses the scoring so that higher values always indicate better usability.
- Sum all ten adjusted scores.
- Multiply the total by 2.5 to convert the range from 0–100.
For example, if a user’s adjusted sum is 32, multiplying by 2.5 gives a SUS score of 80 — indicating excellent usability.
While manual calculation is possible, many researchers use online calculators or spreadsheets to automate the process. Tools like the MeasuringU SUS Calculator allow instant scoring and even provide percentile rankings based on benchmark data.
Understanding SUS Score Interpretation
Interpreting a SUS score goes beyond just looking at the number. Context matters. A score of 70 might be excellent for a complex enterprise software but disappointing for a consumer-facing mobile app.
Consider these factors when interpreting results:
- User Expertise: Novice users may give lower scores than experts, even if the system is well-designed.
- Task Difficulty: If the tasks were inherently challenging, SUS scores may reflect task complexity rather than system usability.
- Emotional State: Frustration from external factors (e.g., time pressure) can skew responses.
- Comparison Over Time: Tracking SUS scores across versions is often more insightful than a single snapshot.
Additionally, SUS does not diagnose specific usability problems — it only indicates the overall level of perceived usability. To identify root causes, combine SUS with usability testing observations, heatmaps, or think-aloud protocols.
For instance, a low score on item 4 (“I think that I would need the support of a technical person…”) might suggest poor onboarding or unclear error messages. Follow-up qualitative data can help pinpoint the exact issue.
Advantages of Using the System Usability Scale
The widespread adoption of the System Usability Scale is no accident. Its popularity stems from a combination of practical benefits that make it accessible and effective for both researchers and practitioners.
Simplicity and Speed
One of the most compelling advantages of the System Usability Scale is its simplicity. With only ten questions, it takes users less than 5–10 minutes to complete. This brevity increases participation rates and reduces survey fatigue, especially in usability test sessions that already demand cognitive effort.
Unlike lengthy questionnaires that risk abandonment, SUS strikes the perfect balance between depth and efficiency. It captures essential insights without overwhelming users. This makes it ideal for agile environments where rapid feedback loops are critical.
Furthermore, the scoring algorithm is straightforward and can be automated, allowing teams to generate results instantly and integrate them into dashboards or reports.
Reliability and Validity
Despite its simplicity, the System Usability Scale is remarkably reliable. Numerous studies have confirmed its internal consistency, with Cronbach’s alpha typically exceeding 0.9, indicating high reliability.
Research has also demonstrated strong correlations between SUS scores and other usability metrics, such as task completion rates, time-on-task, and error rates. This construct validity means that SUS isn’t just measuring opinion — it’s closely tied to actual performance.
A landmark study by Bangor, Kortum, and Miller (2008) confirmed that SUS produces consistent results across different languages, cultures, and technologies. This cross-cultural reliability makes it a trusted tool in global UX research.
Moreover, because SUS has been used for decades, there’s a wealth of comparative data available. This historical continuity allows researchers to place new findings in context and track trends over time.
“The SUS has become the de facto standard for subjective usability measurement.” — Jim Lewis, IBM Research
Limitations and Criticisms of the System Usability Scale
While the System Usability Scale is widely praised, it is not without limitations. Understanding these weaknesses is crucial for using SUS effectively and avoiding misinterpretation.
Lack of Diagnostic Detail
One of the most frequently cited limitations of the System Usability Scale is that it doesn’t tell you *why* a system is usable or not. A low score signals a problem, but it doesn’t pinpoint where the issue lies.
For example, a SUS score of 50 could result from poor navigation, confusing terminology, slow performance, or any combination of factors. Without additional qualitative data, teams may struggle to prioritize fixes.
This is why SUS is best used as a complement to other methods, such as cognitive walkthroughs, heuristic evaluations, or user interviews. It provides the ‘what,’ but other techniques are needed to uncover the ‘why.’
Sensitivity to Context and Wording
Although SUS is designed to be generic, its effectiveness can be influenced by how it’s administered. Subtle changes in wording, translation, or administration context can affect responses.
For instance, translating SUS into another language requires careful back-translation to preserve meaning. Cultural differences in response tendencies (e.g., some cultures avoid extreme ratings) can also impact scores.
Additionally, the term “system” in the questionnaire may confuse users when applied to non-traditional interfaces like voice assistants or augmented reality. In such cases, researchers sometimes adapt the wording (e.g., “this product” instead of “this system”), though this may affect comparability with standard benchmarks.
Another concern is that SUS assumes a single, unified usability experience. However, modern systems often have multiple user roles (e.g., admin vs. end-user), each with different needs and perceptions. A single SUS score may mask significant disparities between user groups.
Alternatives and Complements to the System Usability Scale
While the System Usability Scale remains the gold standard for subjective usability measurement, several alternative and complementary tools exist to address its limitations or suit specific contexts.
Alternative Usability Questionnaires
Several other validated questionnaires offer different approaches to measuring usability:
- UMUX (Usability Metric for User Experience): A shorter, four-item scale based on ISO 9241-11, designed to be more concise than SUS while maintaining strong correlation.
- UMUX-Lite: A two-item version of UMUX, ideal for quick assessments in mobile or in-the-moment surveys.
- Nielsen’s Subjective Satisfaction Scale: A simple single-question metric asking users to rate satisfaction on a 1–7 scale.
- PSSUQ (Post-Study System Usability Questionnaire): A more detailed 16-item questionnaire developed by IBM, focusing on system usefulness, information quality, and interface quality.
Each of these has trade-offs in length, granularity, and ease of use. For example, UMUX-Lite is faster but less reliable than SUS, while PSSUQ provides deeper insights but takes longer to administer.
Combining SUS with Behavioral Metrics
To get a complete picture of usability, many organizations combine the System Usability Scale with behavioral data. This mixed-methods approach leverages the strengths of both subjective and objective measures.
For example:
- Pair SUS scores with task success rates to see if perceived ease aligns with actual performance.
- Correlate SUS with time-on-task to identify inefficiencies.
- Use clickstream analysis or heatmaps to visualize where users struggle, then link those patterns to low SUS item scores.
- Integrate SUS with Net Promoter Score (NPS) to understand how usability impacts loyalty and advocacy.
This holistic approach enables teams to move beyond averages and uncover nuanced insights. For instance, a high SUS score with low task completion might indicate overconfidence, while a low SUS score with high success could suggest users are achieving goals but with frustration.
Tools like Optimal Workshop and Hotjar allow seamless integration of SUS with behavioral analytics, creating a powerful feedback loop for continuous improvement.
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 startups to government agencies, organizations rely on SUS to make data-driven UX decisions.
Healthcare and Medical Devices
In healthcare, usability can be a matter of life and death. Poorly designed medical devices or electronic health record (EHR) systems can lead to errors, delays, and patient harm. Regulatory bodies like the FDA now require usability testing for medical devices, and SUS is frequently used in these evaluations.
For example, a study published in the Journal of Biomedical Informatics used SUS to compare two EHR interfaces. One version scored 62, while the optimized version reached 84 — a significant improvement that correlated with reduced clinician workload and fewer prescription errors.
Hospitals also use SUS to evaluate patient portals, telehealth platforms, and mobile health apps. A high SUS score indicates that patients can easily access their records, schedule appointments, or communicate with providers — improving engagement and outcomes.
Software and Mobile App Development
Software companies routinely use the System Usability Scale during beta testing and product launches. For instance, a fintech startup might administer SUS after users complete a money transfer task to assess the clarity of the interface.
Mobile apps, in particular, benefit from SUS due to the constraints of small screens and touch-based interaction. A low SUS score can reveal issues like hidden menus, tiny buttons, or unclear navigation — all critical on mobile.
One case study from a major e-commerce platform showed that after redesigning their checkout flow, the SUS score increased from 61 to 79. This improvement was linked to a 15% increase in conversion rates, demonstrating the business value of usability.
Continuous SUS monitoring allows agile teams to track UX debt and prioritize backlog items based on user impact.
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 problems. However, what constitutes a “good” score depends on the context, industry, and user expectations.
Can I modify the System Usability Scale?
While you can adapt the wording (e.g., replacing “system” with “app”), doing so may affect comparability with standard benchmarks. It’s generally recommended to use the original version unless necessary, and to document any changes for transparency.
How many users do I need for a reliable SUS score?
As few as 5 users can reveal major usability issues, but for a reliable average score, aim for 15–20 users. Larger samples reduce variability and increase confidence in the results.
Is the System Usability Scale free to use?
Yes, the System Usability Scale is in the public domain and free for both academic and commercial use. No permission is required, though proper citation is appreciated.
Can SUS be used for non-digital products?
Absolutely. While originally designed for interactive systems, SUS has been successfully applied to physical products, services, and even processes by rephrasing “system” appropriately. Its core principles remain valid across domains.
The System Usability Scale remains one of the most trusted tools in the UX researcher’s toolkit. Its blend of simplicity, reliability, and actionable insights makes it indispensable for evaluating how users perceive a product’s ease of use. While it has limitations — particularly in diagnostic depth — its value is amplified when combined with qualitative methods and behavioral data. Whether you’re designing a mobile app, a medical device, or a government service, SUS offers a proven way to quantify usability and drive meaningful improvements. By understanding its origins, proper administration, scoring, and real-world applications, teams can harness the full power of this elegant metric to create better user experiences.
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