Ever wondered how to measure if your app, website, or software is truly user-friendly? Enter the System Usability Scale (SUS)—a simple, reliable tool that turns subjective feelings into hard data. 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 evaluate the perceived usability of a system, product, or service. Developed in 1986 by John Brooke at Digital Equipment Corporation, SUS has become one of the most widely used tools in usability testing across industries—from healthcare apps to enterprise software.
Origins and Development of SUS
Brooke created the System Usability Scale as a quick, reliable way to assess usability without requiring complex lab setups or extensive user testing. It was initially developed during usability research for voice recognition systems but quickly proved applicable across a broad range of technologies.
Unlike other early usability metrics that focused on task completion time or error rates, SUS captured the user’s subjective experience—how easy or difficult they felt the system was to use. This shift toward perceived usability was revolutionary at the time.
Why SUS Stands Out Among Usability Metrics
What makes the System Usability Scale so enduring? Its simplicity and versatility. SUS is technology-agnostic, meaning it can be applied to websites, mobile apps, medical devices, kiosks, and even voice assistants. It doesn’t require users to be tech-savvy, and it can be administered quickly after a usability test session.
- Only 10 questions long
- Produces a single score from 0 to 100
- Validated across cultures and languages
- Free to use (no licensing fees)
“SUS is the ‘Swiss Army knife’ of usability questionnaires—simple, reliable, and effective.” — Jakob Nielsen, Nielsen Norman Group
How the System Usability Scale Works
The System Usability Scale operates on a straightforward structure: 10 statements rated on a 5-point Likert scale, ranging from “Strongly Disagree” to “Strongly Agree.” The questions alternate between positive and negative phrasing to reduce response bias.
After users complete a task or interaction with a system, they answer the SUS questionnaire. Their responses are scored using a specific algorithm to generate a final SUS score between 0 and 100. Higher scores indicate better perceived usability.
The 10 SUS Questions Explained
Here are the standard 10 items in the System Usability Scale, along with explanations of what each measures:
I think that I would like to use this system frequently.— Measures user satisfaction and willingness to reuse.I found the system unnecessarily complex.— Assesses perceived complexity (reverse-scored).I thought the system was easy to use.— Direct measure of perceived ease of use.I think that I would need the support of a technical person to be able to use this system.— Evaluates self-sufficiency (reverse-scored).I found the various functions in this system were well integrated.— Looks at coherence and consistency of features.I thought there was too much inconsistency in this system..
— Measures perceived inconsistency (reverse-scored).I would imagine that most people would learn to use this system very quickly.— Assesses learnability.I found the system very cumbersome to use.— Evaluates efficiency and friction (reverse-scored).I felt very confident using the system.— Measures user confidence and trust.I needed to learn a lot of things before I could get going with this system.— Assesses initial learning curve (reverse-scored).Each odd-numbered question is positively worded and scored as (response – 1), while even-numbered (negatively worded) questions are reverse-scored as (5 – response).The total is then multiplied by 2.5 to get the final SUS score..
Scoring and Interpreting SUS Results
Once responses are collected, the scoring process is simple but precise. For each participant:
- Subtract 1 from the response to each odd-numbered item.
- Subtract the response from 5 for each even-numbered item.
- Sum all adjusted scores.
- Multiply the total by 2.5 to get the SUS score (0–100).
Interpretation of the score is guided by established benchmarks. According to research by Sauro and Lewis (2006), a SUS score above 68 is considered above average. Here’s a general breakdown:
- 90–100: Excellent
- 80–89: Good
- 70–79: Acceptable
- 60–69: Poor
- 50–59: Awful
- Below 50: Unacceptable
It’s important to note that SUS doesn’t diagnose specific usability problems—it gives a holistic snapshot of perceived usability. For deeper insights, it should be paired with qualitative feedback.
Advantages of Using the System Usability Scale
The System Usability Scale has earned its reputation as a gold standard in usability assessment for several compelling reasons. Its widespread adoption is not accidental but rooted in practical, methodological strengths.
Reliability and Validity Across Contexts
One of the most cited strengths of the System Usability Scale is its psychometric robustness. Numerous studies have confirmed its reliability (consistency of results) and validity (accuracy in measuring what it claims to measure).
For example, a meta-analysis by Bangor, Kortum, and Miller (2008) validated SUS across hundreds of studies and found it to be a consistent predictor of user satisfaction. It performs well even when translated into different languages, making it ideal for global product testing.
Its validity has been confirmed in diverse domains, including:
- Healthcare interfaces (e.g., electronic health records)
- Automotive infotainment systems
- E-commerce platforms
- Mobile banking apps
Learn more about its validation in real-world settings at Nielsen Norman Group’s comprehensive guide.
Speed and Simplicity of Administration
In fast-paced development cycles, teams need quick feedback. The System Usability Scale delivers exactly that. It takes users less than 5 minutes to complete and can be administered via paper, email, or integrated into digital testing platforms like UserTesting or Lookback.
Because it’s short and non-technical, participants are more likely to complete it honestly and without fatigue. This increases data quality and response rates—critical for remote usability studies.
Moreover, the scoring algorithm is easy to automate using spreadsheets or tools like MeasuringU’s SUS calculator, which instantly computes scores and percentiles.
Limitations and Criticisms of the System Usability Scale
Despite its popularity, the System Usability Scale is not without limitations. Understanding these weaknesses helps researchers and designers use it more effectively and know when to complement it with other methods.
Lack of Diagnostic Detail
One of the most common criticisms of the System Usability Scale is that it doesn’t tell you why a system scored poorly. A low SUS score indicates poor perceived usability, but it doesn’t pinpoint whether the issue lies in navigation, terminology, layout, or performance.
For example, if users rate a mobile app with a SUS score of 45, you know it’s problematic, but you don’t know if the login process is confusing, the buttons are too small, or the loading times are too long. To uncover root causes, SUS must be paired with qualitative methods like think-aloud protocols, interviews, or heatmaps.
Sensitivity to Context and User Expectations
The System Usability Scale measures perceived usability, which can be influenced by factors outside the system itself. For instance, a user’s prior experience with similar tools, their mood during testing, or even brand perception can skew results.
Additionally, SUS scores can vary based on the task difficulty. If users are asked to complete a complex task, their SUS score may be lower—not because the system is unusable, but because the task was inherently hard.
Researchers must carefully control testing conditions and pair SUS with behavioral metrics (e.g., task success rate, time on task) to get a balanced view.
How to Administer the System Usability Scale Effectively
While the System Usability Scale is simple to use, administering it effectively requires attention to timing, context, and participant selection. A poorly timed or misapplied SUS can yield misleading results.
Best Practices for Timing and Context
The ideal time to administer the SUS is immediately after a user completes a set of representative tasks with the system. This ensures their experience is fresh and contextually relevant.
For example, in a usability test for an e-commerce site, users might be asked to:
- Search for a product
- Add it to the cart
- Proceed through checkout
Right after completing these steps, the SUS questionnaire should be presented. Delaying it by hours or days risks memory decay and reduced accuracy.
Avoid using SUS as a general satisfaction survey months after product launch. It’s designed for specific interaction sessions, not long-term usage sentiment.
Ensuring Representative and Diverse Participants
To get meaningful SUS scores, your participant pool must reflect your actual user base. Testing only with tech-savvy employees, for instance, will inflate scores and misrepresent real-world usability.
Best practices include:
- Recruiting users with varying levels of technical proficiency
- Including different age groups and backgrounds
- Testing with both new and experienced users (if applicable)
A sample size of 8–12 users is often sufficient to get a reliable SUS score, thanks to its high sensitivity. However, for comparative studies (e.g., A/B testing two designs), larger samples (20+) improve statistical power.
Comparing SUS with Other Usability Questionnaires
While the System Usability Scale is the most popular usability metric, it’s not the only one. Several alternative and complementary tools exist, each with its own strengths and use cases.
Usability Metric for User Experience (UMUX)
The Usability Metric for User Experience (UMUX) is a 4-item questionnaire derived from SUS, designed to be even shorter. It focuses on two core dimensions: usability and usefulness.
UMUX-Lite, a 2-item version, is often used in mobile apps where screen space and user patience are limited. While faster, it sacrifices some of the depth and reliability of the full SUS.
Despite its brevity, UMUX correlates strongly with SUS (r > 0.90), making it a viable alternative when time is critical.
Post-Study System Usability Questionnaire (PSSUQ)
Developed by IBM, the Post-Study System Usability Questionnaire (PSSUQ) is a 16-item scale that dives deeper into satisfaction, system usefulness, information quality, and interface quality.
Unlike the System Usability Scale, PSSUQ provides more granular insights into specific aspects of the user experience. However, its length makes it less suitable for quick tests or mobile contexts.
PSSUQ is ideal for enterprise software evaluations where detailed feedback is worth the extra time.
Net Promoter Score (NPS) vs. SUS
While NPS measures overall customer loyalty (“How likely are you to recommend this product?”), SUS focuses specifically on usability. A product can have a high NPS due to brand loyalty but a low SUS if it’s hard to use.
Using both metrics together provides a fuller picture: SUS tells you how usable the system is, while NPS tells you how much users like it overall.
Real-World Applications of the System Usability Scale
The true power of the System Usability Scale lies in its real-world impact. From startups to Fortune 500 companies, SUS is used to drive design decisions, validate prototypes, and benchmark performance.
Case Study: Improving a Healthcare App
A telehealth startup used the System Usability Scale to evaluate its patient onboarding flow. Initial testing with 10 users yielded an average SUS score of 52—well below the acceptable threshold.
Qualitative feedback revealed that users were confused by medical jargon and struggled to upload insurance documents. After simplifying language and adding tooltips, a second round of testing raised the SUS to 78.
This 26-point improvement demonstrated the effectiveness of the redesign and gave the team confidence before launch.
Enterprise Software Benchmarking
A global bank used SUS to compare three internal CRM systems across departments. By administering SUS after standardized tasks, they identified that one system had a significantly lower score (58) due to slow load times and inconsistent navigation.
This data supported the business case for upgrading to a modern platform, ultimately improving employee productivity and reducing training costs.
Academic Research and SUS
The System Usability Scale is one of the most cited tools in human-computer interaction (HCI) research. Its simplicity makes it ideal for student projects, thesis work, and peer-reviewed studies.
Researchers at universities worldwide use SUS to compare interface designs, evaluate assistive technologies, and study the impact of AI on user experience. Its open-access nature removes barriers to entry, promoting reproducibility and collaboration.
Future of the System Usability Scale
Despite being over 35 years old, the System Usability Scale remains remarkably relevant. But as technology evolves—into voice interfaces, AR/VR, and AI-driven systems—can SUS keep up?
Adapting SUS for Emerging Technologies
Researchers are exploring ways to adapt the System Usability Scale for non-traditional interfaces. For example, in voice assistants like Alexa or Google Assistant, traditional concepts like “navigation” or “buttons” don’t apply.
Modified versions of SUS are being tested with reworded items, such as:
- “I found it easy to get the information I needed using voice commands.”
- “The voice assistant understood my requests consistently.”
Preliminary studies suggest that with minor tweaks, SUS can still provide valuable insights in these new contexts.
Integration with Automated Usability Testing
As AI-powered usability testing tools emerge, the System Usability Scale is being integrated into automated workflows. Platforms like UserZoom and Maze now allow SUS to be embedded directly into unmoderated tests, with instant scoring and benchmarking.
Future developments may include sentiment analysis of open-ended feedback to enrich SUS scores, or real-time SUS prediction based on user behavior patterns.
What is the System Usability Scale used for?
The System Usability Scale (SUS) is used to measure the perceived usability of a system, product, or service. It helps designers, researchers, and developers evaluate how easy or difficult users find a system to use, providing a quantitative score that can be used for benchmarking, comparing designs, or validating improvements.
Is the System Usability Scale reliable?
Yes, the System Usability Scale is highly reliable and has been validated across numerous studies and industries. Research shows it has strong internal consistency (Cronbach’s alpha typically above 0.9) and correlates well with other usability metrics. It’s widely trusted in both academic and industry settings.
Can I use SUS for free?
Yes, the System Usability Scale is completely free to use. There are no licensing fees or restrictions. It was designed as an open tool for the research and design community. However, proper attribution to John Brooke (1986) is recommended in academic or published work.
How many users do I need for a SUS test?
For a reliable SUS score, 8–12 users are typically sufficient. SUS is sensitive enough to detect usability differences with small sample sizes. For comparative studies (e.g., A/B testing), larger samples (20+) are recommended to increase statistical power.
What is a good SUS score?
A SUS score above 68 is considered above average. Scores above 80 are good, and above 90 are excellent. However, what constitutes a “good” score depends on the context, industry, and user expectations. Always compare against benchmarks or previous versions of your product.
The System Usability Scale remains one of the most powerful, practical tools in the UX researcher’s toolkit. Its blend of simplicity, reliability, and flexibility makes it indispensable for anyone serious about improving user experience. While it doesn’t replace qualitative insights, it provides a crucial quantitative anchor for decision-making. Whether you’re testing a prototype, benchmarking a product, or validating a redesign, SUS delivers actionable data fast. As technology evolves, so too will SUS—adapting to new interfaces while staying true to its core purpose: measuring usability, one question at a time.
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