System Usability Scale: 10 Powerful Insights You Must Know

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If you’ve ever wondered how to measure the ease of use of a product or system, the System Usability Scale (SUS) is your go-to tool. Simple, reliable, and widely trusted, it’s the gold standard in usability evaluation.

Table of Contents

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 the late 1980s by John Brooke at Digital Equipment Corporation, it has since become one of the most widely used tools in usability testing across industries ranging from software and websites to medical devices and consumer electronics.

Origins and Development of SUS

The System Usability Scale was first introduced in 1986 as a quick and dirty usability assessment tool. Unlike more complex usability metrics that require extensive observation or technical analysis, SUS was designed to be lightweight and easy to administer. It emerged during a time when usability engineering was gaining traction, and companies needed a standardized way to compare user experiences across different systems.

Brooke’s original goal was to create a scale that could be applied across various technologies without requiring customization. The result was a ten-question survey that could be completed in under 10 minutes, delivering a reliable score between 0 and 100. Despite its simplicity, SUS has demonstrated strong reliability and validity across countless studies and applications.

How SUS Differs from Other Usability Metrics

While many usability assessment tools exist—such as the NASA-TLX (Task Load Index), UMUX (Usability Metric for User Experience), and SUPR-Q (Standardized User Experience Percentile Rank Questionnaire)—the System Usability Scale stands out due to its brevity, versatility, and proven track record.

Length: SUS uses only 10 questions, making it faster to administer than longer surveys.
Neutrality: It doesn’t assume prior knowledge of the system, making it ideal for both new and experienced users.
Scalability: SUS can be used across platforms—web, mobile, hardware, software—without modification.

In contrast, tools like the Post-Study System Usability Questionnaire (PSSUQ) are more detailed but require more time and context. SUS strikes a balance between depth and efficiency, which explains its enduring popularity.

“The System Usability Scale is not the most sophisticated usability metric, but it’s the most practical.” — Jakob Nielsen, Nielsen Norman Group

How the System Usability Scale Works

Understanding how the System Usability Scale functions is essential for anyone involved in user experience (UX) research, product development, or human-computer interaction. At its core, SUS is a Likert-scale questionnaire where users rate their agreement with a series of statements after interacting with a system.

The 10 Standard SUS Questions

The System Usability Scale consists of ten statements, each rated on a five-point scale from “Strongly Disagree” (1) to “Strongly Agree” (5). The questions alternate between positive and negative phrasing to reduce response bias. Here are the standard items:

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.Notice that odd-numbered questions are worded positively, while even-numbered ones are negatively framed..

This counterbalancing helps minimize the effect of acquiescence bias—where users tend to agree with statements regardless of content..

Scoring Methodology of the System Usability Scale

The scoring process for the System Usability Scale is straightforward but requires careful attention to detail. Each response is converted into a point value, and the total is transformed into a final score out of 100. Here’s how it works:

For odd-numbered questions (positive), subtract 1 from the user’s response (so a “5” becomes “4”).
For even-numbered questions (negative), subtract the user’s response from 5 (so a “1” becomes “4”, a “2” becomes “3”, etc.).
Sum all the converted values.
Multiply the total by 2.5 to get the final SUS score (since 10 items × 4 max points = 40; 40 × 2.5 = 100).

For example, if a user responds with all 4s across the board, the calculation would be:

Questions 1, 3, 5, 7, 9: (4-1) = 3 → 5 × 3 = 15
Questions 2, 4, 6, 8, 10: (5-4) = 1 → 5 × 1 = 5
Total = 15 + 5 = 20
Final SUS Score = 20 × 2.5 = 50

A score of 50 is considered average. Anything above 68 is generally seen as above average, while scores below 50 indicate significant usability issues.

Why the System Usability Scale Is So Widely Used

The enduring popularity of the System Usability Scale isn’t accidental. Its widespread adoption across academia, industry, and government stems from a combination of practical advantages and empirical validation.

Reliability and Validity of SUS

One of the key reasons the System Usability Scale has stood the test of time is its strong psychometric properties. Numerous studies have confirmed that SUS is both reliable (produces consistent results) and valid (measures what it claims to measure—usability).

A landmark study by James R. Lewis and Jeff Sauro demonstrated that SUS has a high internal consistency, with Cronbach’s alpha typically exceeding 0.9, which is considered excellent in survey research. This means the ten items in the questionnaire are closely related and collectively reflect a single underlying construct: perceived usability.

Moreover, SUS has been shown to correlate well with other usability metrics, including task success rates, time-on-task, and user satisfaction scores. This cross-validation strengthens its credibility as a holistic usability indicator.

Speed and Simplicity in Administration

In fast-paced development environments, time is a critical factor. The System Usability Scale shines here because it can be administered quickly—often immediately after a usability test or product interaction.

Takes less than 5–10 minutes for users to complete.
Requires no specialized training to deploy.
Can be integrated into digital forms, mobile apps, or paper surveys with ease.

This simplicity makes SUS ideal for agile teams, startups, and large organizations alike. Whether you’re running a quick A/B test or conducting a longitudinal study, SUS provides actionable data without adding significant overhead.

“SUS is the Swiss Army knife of usability metrics—simple, versatile, and always useful.” — Dr. Sauro, MeasuringU

Interpreting System Usability Scale Scores

Collecting SUS scores is only half the battle. The real value lies in interpreting what those numbers mean for your product, team, and users.

Understanding the SUS Score Range

The System Usability Scale produces a score between 0 and 100, but not all scores are created equal. While there’s no official pass/fail threshold, researchers have established benchmarks based on aggregated data.

Below 50: Poor usability. Users are likely frustrated and may abandon the system.
50–67: Average. The system is usable but has noticeable friction points.
68–79: Good. Most users find the system intuitive and efficient.
80–100: Excellent. The system is highly usable and likely sets a benchmark in its category.

According to research by Sauro and Lewis, the average SUS score across thousands of studies is around 68. This means a score above 68 places your product in the top 50% of all systems tested.

Benchmarking Against Industry Standards

One of the most powerful uses of the System Usability Scale is benchmarking—comparing your product’s score against competitors or industry averages. For example:

Consumer software averages around 78.
Enterprise software tends to score between 60 and 70.
Medical devices often fall in the 50–60 range due to complexity and regulatory constraints.

By comparing your SUS score to relevant benchmarks, you can contextualize your results. A score of 70 might seem acceptable until you realize that top competitors are scoring 85+. This insight can drive prioritization in UX improvements.

Tools like the SUS Calculator from MeasuringU allow you to input raw responses and instantly get a score with percentile rankings.

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 to improve products, inform design decisions, and validate user experience strategies.

Using SUS in UX Research and Testing

In user experience research, the System Usability Scale is often deployed at the end of usability testing sessions. After participants complete a set of tasks (e.g., signing up, making a purchase, navigating a dashboard), they’re asked to fill out the SUS questionnaire.

This post-task assessment captures the user’s overall impression of the system’s usability. Because SUS is sensitive to design changes, it’s particularly useful for comparing different versions of a product (e.g., before and after a redesign).

For example, a fintech company might run a usability test on their mobile banking app. Version A receives a mean SUS score of 62, indicating room for improvement. After simplifying the navigation and reducing form fields, Version B scores 78—confirming that the changes significantly enhanced usability.

SUS in Product Development and Iteration

Product teams leverage the System Usability Scale throughout the development lifecycle. From early prototypes to post-launch evaluations, SUS provides a consistent metric to track progress.

Early Stage: Use SUS to evaluate low-fidelity prototypes or wireframes (via simulated tasks).
Mid-Development: Test beta versions with real users to identify pain points.
Post-Launch: Monitor SUS scores over time to detect usability regressions or improvements.

Because SUS is quantitative, it integrates well with product analytics dashboards. Teams can set usability KPIs (Key Performance Indicators) and trigger design sprints when scores fall below a threshold.

For instance, a SaaS company might set a goal of maintaining a SUS score above 75. If quarterly surveys show a drop to 69, the UX team investigates and prioritizes fixes.

Strengths and Limitations of the System Usability Scale

No tool is perfect, and the System Usability Scale is no exception. While it offers numerous advantages, understanding its limitations is crucial for proper application.

Advantages of Using SUS

The strengths of the System Usability Scale are well-documented and widely recognized:

Universality: Applicable to virtually any interactive system, regardless of domain or platform.
Cost-Effectiveness: Requires no special software or equipment—just a questionnaire.
Quick Insights: Delivers a clear, numerical score that’s easy to communicate to stakeholders.
Proven Track Record: Validated across decades of research and thousands of studies.
Language Flexibility: Has been translated into over 30 languages with validated versions available.

These benefits make SUS a favorite among UX professionals, researchers, and product managers who need fast, reliable feedback.

Criticisms and Common Pitfalls

Despite its strengths, the System Usability Scale has several limitations that users should be aware of:

Lack of Diagnostic Detail: SUS tells you how usable a system is, but not why. A low score doesn’t pinpoint specific issues like confusing labels or broken workflows.
Subjective Nature: It measures perceived usability, not objective performance. A user might complete tasks successfully but still rate the system poorly due to aesthetics or expectations.
Context Dependency: Scores can be influenced by factors outside the interface, such as user fatigue, task difficulty, or prior experience.
No Emotional Depth: SUS doesn’t capture emotional responses like delight, frustration, or trust—elements increasingly important in modern UX.

To mitigate these issues, experts recommend pairing SUS with qualitative methods like think-aloud protocols, interviews, or open-ended feedback questions.

“SUS is a thermometer for usability—it tells you if the system has a fever, but not what’s causing it.” — UX Collective

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, data collection, and interpretation.

When and How to Deploy SUS

The timing and method of SUS administration can significantly impact data quality. Here are key guidelines:

Administer After Task Completion: SUS should be given immediately after users interact with the system, while their experience is fresh.
Use Real Tasks: Ensure users have completed meaningful tasks (e.g., booking a flight, configuring settings) rather than just browsing.
Randomize Question Order (Cautiously): While some suggest randomizing to reduce bias, the original SUS structure is optimized for counterbalancing. Altering it may affect score validity.
Collect Demographic Data: Record user experience level, age, and role to enable deeper analysis (e.g., comparing novice vs. expert scores).

For digital deployment, tools like Google Forms, Typeform, or integrated UX platforms (e.g., UserTesting, Lookback) can automate distribution and scoring.

Ensuring Data Quality and Avoiding Bias

High-quality SUS data depends on minimizing bias and ensuring representative sampling.

Sample Size: While SUS can be used with as few as 5 users (common in usability testing), larger samples (n > 30) provide more reliable averages and enable statistical analysis.
Avoid Leading Questions: Don’t influence responses by praising the system beforehand.
Use Neutral Instructions: Tell users: “Please rate your agreement with the following statements based on your experience.”
Ensure Anonymity: Allow anonymous responses to encourage honesty, especially in enterprise or internal tool evaluations.

Additionally, avoid mixing SUS with overly long surveys, as fatigue can distort responses. Keep the overall experience concise and focused.

Alternatives and Complements to the System Usability Scale

While the System Usability Scale is a powerful tool, it’s not the only option. Depending on your goals, you might consider alternatives or complementary metrics.

Combining SUS with Qualitative Methods

To get a complete picture of usability, combine the System Usability Scale with qualitative techniques:

Think-Aloud Protocols: Have users verbalize their thoughts while using the system to uncover cognitive hurdles.
User Interviews: Follow up SUS with open-ended questions like “What did you find most confusing?”
Heatmaps and Session Recordings: Tools like Hotjar or Crazy Egg show where users click, scroll, or hesitate.
Task Success Rate: Track whether users complete key actions successfully—this complements SUS’s subjective data.

For example, a SUS score of 60 might be paired with session recordings showing users repeatedly failing to find the checkout button. This combination guides precise design fixes.

What is a good System Usability Scale score?

A score of 68 or higher is considered above average. Scores above 80 are good, and anything over 90 is excellent. However, what’s “good” depends on your industry and competitors. Always compare against relevant benchmarks.

Can SUS be used for mobile apps?

Yes, absolutely. The System Usability Scale is platform-agnostic and is frequently used for mobile apps, websites, kiosks, and software. Its simplicity makes it ideal for mobile usability testing.

Is the System Usability Scale free to use?

Yes, the System Usability Scale is in the public domain and free for both commercial and academic use. No permission is required, though proper citation is encouraged.

How many users do I need for a reliable SUS score?

For early-stage testing, 5–10 users can reveal major usability issues. For reliable averages and statistical confidence, aim for 30+ users, especially when comparing products or tracking changes over time.

Where can I find a SUS template or calculator?

Reputable resources include the MeasuringU SUS Guide and the Usability.gov official page, which provide templates, scoring tools, and best practices.

The System Usability Scale remains a cornerstone of usability evaluation for good reason. Its blend of simplicity, reliability, and broad applicability makes it an indispensable tool for anyone serious about user experience. While it has limitations—particularly in diagnostic depth—it excels at providing a quick, standardized measure of perceived usability. When used correctly and combined with qualitative insights, SUS empowers teams to build products that are not just functional, but truly user-friendly. Whether you’re a UX researcher, product manager, or developer, mastering the System Usability Scale is a powerful step toward creating better digital experiences.