Ever wonder how companies measure how easy or frustrating their products are to use? Enter the System Usability Scale (SUS) — a simple yet powerful tool that turns user experience into hard data. Let’s dive into what makes it indispensable.
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 since become one of the most widely adopted usability assessment tools across industries — from software and websites to medical devices and mobile apps.
What sets SUS apart is its simplicity and reliability. Despite having only ten questions, it delivers a robust quantitative score that reflects how users feel about a system’s ease of use. The questionnaire is technology-agnostic, meaning it can be applied to virtually any interactive system, regardless of platform or complexity.
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 a growing need for a quick, standardized method to assess user satisfaction with computer systems. Traditional usability testing was time-consuming and often required specialized equipment and observers.
Brooke aimed to create a lightweight, reliable tool that could be administered quickly after a user interaction session. The result was a ten-question Likert-scale survey that could be completed in under five minutes. Its initial validation showed strong internal consistency and sensitivity to differences between systems, making it ideal for comparative studies.
Over the decades, SUS has been validated across thousands of studies and translated into dozens of languages. Its enduring relevance is a testament to its design elegance and practical utility. You can read the original research paper on SUS via ResearchGate.
How SUS Compares to Other Usability Metrics
While several usability evaluation methods exist — such as the Net Promoter Score (NPS), User Experience Questionnaire (UEQ), and the Post-Study System Usability Questionnaire (PSSUQ) — SUS stands out due to its brevity and proven psychometric properties.
- Brevity: SUS takes less than 5 minutes to complete, making it ideal for time-constrained testing sessions.
- Universality: Unlike domain-specific tools, SUS works across platforms — web, mobile, kiosks, software, even voice interfaces.
- Standardized Scoring: The scoring method is consistent and produces a single number from 0 to 100, enabling easy benchmarking.
In contrast, tools like UEQ offer richer qualitative insights but require more time and resources. SUS strikes the perfect balance between speed and insight, which explains its dominance in both academic and industry settings.
“The SUS is probably the most widely used questionnaire in usability studies.” — Sauro & Lewis, 2016, in their book ‘Quantifying the User Experience’.
How the System Usability Scale Works
The mechanics of the System Usability Scale are deceptively simple. Users answer ten statements using a five-point Likert scale ranging from “Strongly Disagree” (1) to “Strongly Agree” (5). The statements alternate between positive and negative polarity to reduce response bias.
After responses are collected, a specific scoring algorithm is applied to generate a final SUS score between 0 and 100. This score provides a standardized measure of perceived usability, allowing teams to compare products, track improvements over time, or benchmark against industry averages.
The 10 SUS Questions Explained
Each of the ten items in the SUS questionnaire targets different aspects of usability, including learnability, efficiency, and user confidence. Here’s a breakdown of each question and what it assesses:
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I think that I would like to use this system frequently.— Measures overall user satisfaction and willingness to reuse.I found the system unnecessarily complex.— Assesses perceived complexity; negatively worded to balance the survey.I thought the system was easy to use.— Directly evaluates ease of use, a core component of usability.I think that I would need the support of a technical person to be able to use this system.— Gauges self-sufficiency and intuitiveness.I found the various functions in this system were well integrated.— Evaluates consistency and cohesion of features.I thought there was too much inconsistency in this system..
— Identifies interface inconsistencies; negatively phrased.I would imagine that most people would learn to use this system very quickly.— Assesses learnability from a social perspective.I found the system very cumbersome to use.— Measures perceived effort; negatively worded.I felt very confident using the system.— Evaluates user confidence and perceived control.I needed to learn a lot of things before I could get going with this system.— Another learnability indicator; negatively phrased.Notice how odd-numbered questions are positively framed, while even-numbered ones are negatively worded.This design helps minimize acquiescence bias — the tendency for respondents to agree with statements regardless of content..
Scoring the System Usability Scale
The scoring process for the System Usability Scale follows a precise formula. While it may seem complex at first, it’s easily automated using spreadsheets or dedicated tools.
Here’s how it works step-by-step:
- For **odd-numbered items** (1, 3, 5, 7, 9), subtract 1 from the user’s response (so a score of 5 becomes 4, 1 becomes 0).
- For **even-numbered items** (2, 4, 6, 8, 10), subtract the user’s response from 5 (so a 1 becomes 4, a 5 becomes 0).
- Sum the converted scores across all ten items.
- Multiply the total by 2.5 to convert the maximum possible score from 40 to 100.
For example, if a user’s adjusted sum is 32, multiplying by 2.5 gives a SUS score of 80 — which is considered excellent.
This scoring method ensures that every response contributes meaningfully to the final score, and the resulting number is normalized to a 0–100 scale, making interpretation intuitive.
“The SUS score is not a percentage, but it’s scaled to look like one.” — Jeff Sauro, MeasuringU.
Interpreting System Usability Scale Scores
One of the greatest strengths of the System Usability Scale is its interpretability. A score between 0 and 100 is easy to grasp, but understanding what those numbers mean in practice requires context. Over the years, researchers have established benchmarks and grading systems to help interpret SUS results.
Knowing how to interpret a SUS score allows teams to make informed decisions — whether to release a product, iterate on a design, or compare against competitors.
SUS Score Ranges and Grade Equivalents
Based on extensive research, particularly by Jeff Sauro and James R. Lewis, SUS scores can be categorized into qualitative grades. These ranges help translate numerical scores into actionable insights:
- 90–100: Excellent — Top-tier usability. Users find the system intuitive and efficient.
- 80–89: Good — Solid usability with minor friction points.
- 70–79: Acceptable — Average; may require improvements for broader adoption.
- 60–69: Poor — Significant usability issues; likely to frustrate users.
- 50–59: Awful — Serious problems; high risk of user abandonment.
- Below 50: Unacceptable — The system is likely unusable without major redesign.
These ranges are not arbitrary. They are based on empirical data from thousands of real-world SUS assessments. For instance, the average SUS score across all systems is approximately 68, placing “average” in the “Poor to Acceptable” range — a sobering benchmark for many product teams.
Benchmarking Against Industry Standards
Beyond internal grading, SUS allows for external benchmarking. Organizations can compare their product’s SUS score against industry averages or direct competitors.
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For example:
- Consumer software averages around 78.
- Enterprise software tends to score lower, around 65–70.
- Medical devices, due to regulatory complexity, often score in the 60s.
- Top-performing websites like Google or Amazon typically exceed 85.
This comparative power makes SUS invaluable during competitive analysis or stakeholder reporting. A score of 75 might seem decent in isolation, but if the industry average is 82, it signals room for improvement.
Resources like MeasuringU provide detailed benchmark databases and percentile calculators to help contextualize your results.
“A SUS score of 68 is the median — half of all products score below this, half above.” — Sauro & Lewis, 2016.
Advantages of Using the System Usability Scale
The enduring popularity of the System Usability Scale isn’t accidental. Its widespread adoption stems from a combination of practical, statistical, and methodological advantages that make it a go-to tool for UX professionals, product managers, and researchers.
From rapid deployment to reliable results, SUS offers benefits that few other usability instruments can match.
Speed and Simplicity
One of the most compelling reasons to use SUS is its speed. The entire survey takes users less than five minutes to complete, minimizing participant fatigue and maximizing response rates.
In fast-paced development environments — such as agile sprints or usability labs with tight schedules — this efficiency is critical. Teams can gather usability feedback immediately after a task without disrupting the flow of testing.
Moreover, the simplicity of the questionnaire reduces cognitive load on participants. Users don’t need training to understand the questions, and administrators don’t need advanced statistical knowledge to interpret the results — especially with automated scoring tools available.
Proven Reliability and Validity
Despite its brevity, the System Usability Scale demonstrates strong psychometric properties. Studies have consistently shown high internal consistency, with Cronbach’s alpha typically above 0.9, indicating excellent reliability.
It also exhibits good test-retest reliability and construct validity, meaning it accurately measures what it claims to: perceived usability. This makes SUS suitable not just for formative evaluations but also for summative assessments used in regulatory submissions or academic research.
Its validity has been confirmed across diverse populations, languages, and cultures, reinforcing its status as a universal usability metric.
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Cost-Effectiveness for Teams of All Sizes
Unlike complex usability studies that require eye-tracking equipment, moderated sessions, or large sample sizes, SUS can be deployed with minimal resources.
Startups, small design teams, or solo researchers can use SUS effectively without a big budget. It integrates seamlessly into remote testing platforms like UserTesting, Lookback, or Maze, where it can be appended to task-based sessions.
Because it requires as few as five users to generate meaningful insights (based on the “five users rule” in usability testing), SUS is both statistically efficient and cost-effective.
“You don’t need a big sample to get a reliable SUS score. Five to ten users often suffice for identifying major usability issues.” — Jakob Nielsen.
Limitations and Criticisms of the System Usability Scale
While the System Usability Scale is a powerful tool, it’s not without limitations. Understanding its weaknesses is crucial for using it appropriately and avoiding misinterpretation of results.
No single metric can capture every aspect of user experience, and SUS is no exception. Recognizing its constraints helps teams complement it with other methods for a more holistic view.
Lack of Diagnostic Detail
One of the most common criticisms of SUS is that it provides a score without explaining why users rated a system the way they did. A low score indicates poor usability, but it doesn’t pinpoint which features or interactions are problematic.
For example, a SUS score of 55 tells you the system is struggling, but not whether the issue lies in navigation, terminology, responsiveness, or error recovery. This lack of diagnostic power means SUS should not be used in isolation.
To overcome this, UX researchers often pair SUS with qualitative methods like think-aloud protocols, interviews, or observational notes. Combining quantitative scores with qualitative insights creates a richer understanding of user experience.
Sensitivity to Context and Task Design
The SUS score can be influenced by factors outside the system’s actual usability. For instance, if users are given poorly designed tasks during testing, they may rate the system lower — not because it’s unusable, but because the task was confusing.
Similarly, the timing of the SUS administration matters. If users complete the survey immediately after encountering a major roadblock, their frustration may skew the score downward, even if the rest of the system is intuitive.
To mitigate this, best practices recommend using SUS after a series of representative tasks and ensuring those tasks are well-scoped and realistic.
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Language and Cultural Biases
Although SUS has been translated into over 30 languages, direct translations can sometimes miss cultural nuances in how people interpret agreement scales.
For example, some cultures exhibit “acquiescence bias,” where respondents tend to agree with statements regardless of content. Others may avoid extreme responses, clustering around the middle of the scale. These tendencies can affect SUS scores and reduce cross-cultural comparability.
To address this, researchers recommend using culturally adapted versions of SUS and validating translations through pilot testing.
“SUS is robust, but not immune to cultural response styles.” — Bangor, Kortum, & Miller, 2008.
Best Practices for Administering the System Usability Scale
To get the most value from the System Usability Scale, it’s essential to follow best practices in administration, timing, and analysis. Misuse can lead to misleading results, undermining the very insights you’re trying to gain.
From selecting the right participants to interpreting scores correctly, attention to detail ensures reliable and actionable outcomes.
When and How to Administer SUS
The ideal time to administer SUS is immediately after users complete a set of representative tasks with the system. This ensures their experience is fresh and contextually grounded.
It should not be given at the beginning of a session or long after interaction, as recall bias can distort responses. For longitudinal studies, SUS can be administered at multiple touchpoints to track usability improvements over time.
When delivering SUS, ensure the environment is distraction-free. In remote testing, use embedded surveys within the testing platform. In moderated sessions, provide the questionnaire digitally or on paper, depending on the setup.
Recommended Sample Sizes
A common misconception is that SUS requires large sample sizes. In reality, due to its high reliability, meaningful insights can be drawn from as few as five users — especially in formative testing.
For summative evaluations or benchmarking, a sample size of 15–20 users is often sufficient to achieve stable mean scores. Larger samples (30+) are recommended when comparing two systems statistically or when high precision is required.
Statistical power analysis tools can help determine the optimal sample size based on expected effect size and confidence level.
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Combining SUS with Other UX Metrics
SUS is most powerful when used as part of a broader usability evaluation strategy. Pairing it with other metrics provides a more complete picture of user experience.
- Task Success Rate: Measures whether users can complete key actions.
- Time on Task: Quantifies efficiency.
- Net Promoter Score (NPS): Assesses loyalty and satisfaction.
- System Usability Scale + Qualitative Feedback: Adds context to the score.
For example, a high SUS score combined with low task success might indicate users feel confident but are actually struggling. Conversely, a moderate SUS score with high task success suggests the system works well despite some perceived friction.
“Use SUS as a thermometer, not an X-ray. It tells you if there’s a problem, but not the exact location.” — UX best practice guideline.
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, meet regulatory requirements, and enhance user satisfaction.
From tech startups to government agencies, SUS helps organizations make data-driven decisions about design and functionality.
Software and Web Application Testing
In software development, SUS is routinely used during usability testing phases. Companies like Microsoft, Google, and Adobe incorporate SUS into their UX research pipelines to evaluate prototypes, compare design alternatives, and validate post-release updates.
For example, a fintech startup might use SUS to compare two onboarding flows. If Flow A scores 65 and Flow B scores 82, the team has clear evidence to choose the latter — even if both had similar task completion rates.
Web analytics platforms now integrate SUS into session recordings, allowing teams to correlate usability scores with behavioral data like click paths and drop-off points.
Medical Devices and Regulatory Compliance
In highly regulated industries like healthcare, usability is not just a nice-to-have — it’s a safety issue. The FDA and other regulatory bodies require human factors validation for medical devices, and SUS is frequently included in these submissions.
While SUS alone isn’t sufficient for regulatory approval, it serves as a valuable supporting metric. A high SUS score can demonstrate that a device is intuitive and reduces the risk of user error — a critical factor in preventing adverse events.
For instance, a glucose monitoring app with a SUS score of 85 provides strong evidence of usability, which can be included in a 510(k) submission to the FDA.
Academic Research and Usability Benchmarking
SUS is one of the most cited usability instruments in academic literature. Researchers use it to evaluate new interaction paradigms, compare interface designs, and validate theoretical models of user experience.
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Its standardized nature allows for meta-analyses and cross-study comparisons. For example, a 2020 literature review of 150 UX studies found that SUS was used in over 60% of them, making it the de facto standard for usability quantification.
Universities and research labs also use SUS to build benchmark databases, helping industry partners understand how their products stack up against global averages.
“SUS has become the ‘gold standard’ for subjective usability measurement.” — Journal of Usability Studies.
What is a good System Usability Scale score?
A score of 68 is the average across all systems. Anything above 70 is considered above average, and scores above 80 are generally classified as “good” or “excellent.” However, what’s “good” depends on your industry and competitors. For example, a score of 75 might be excellent for enterprise software but poor for a consumer app.
Can I modify the SUS questionnaire?
While you can adapt the wording slightly for clarity (e.g., replacing “system” with “app”), it’s strongly advised not to change the core items or scale. Doing so compromises the validity of the score and prevents comparison with established benchmarks. If you need a customized tool, consider using the SUS as inspiration but label it as a derivative.
How many people do I need to get a reliable SUS score?
For formative testing, 5–10 users are often sufficient to identify major usability issues. For summative evaluations or statistical comparisons, aim for 15–30 users to ensure reliable mean scores and adequate statistical power.
Is the System Usability Scale free to use?
Yes, the System Usability Scale is in the public domain and free for both commercial and non-commercial use. There are no licensing fees or restrictions. You can find the official questionnaire and scoring guide on resources like Usability.gov.
Can SUS be used for mobile apps?
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Absolutely. SUS is platform-agnostic and works equally well for mobile apps, websites, desktop software, and even voice-based systems. Just ensure the tasks users perform are representative of real-world usage before administering the survey.
The System Usability Scale remains one of the most effective, efficient, and reliable tools for measuring perceived usability. Its simplicity belies its power — delivering a single, interpretable score that can guide design decisions, support regulatory submissions, and benchmark performance. While it has limitations — particularly in diagnostic depth — its value is maximized when used alongside qualitative insights and other UX metrics. Whether you’re a UX researcher, product manager, or developer, understanding and applying SUS can dramatically improve how you evaluate and enhance user experience. By following best practices in administration, interpretation, and integration, you can turn subjective impressions into actionable, data-driven insights.
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