Ever wondered how users truly feel about a product’s ease of use? The System Usability Scale (SUS) is a simple yet powerful tool that cuts through the noise, delivering reliable insights into user experience with just 10 questions.
What Is the System Usability Scale (SUS)?
The System Usability Scale, commonly known as SUS, is a widely adopted, standardized 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 a gold standard in usability testing across industries—from software and websites to medical devices and consumer electronics.
Origins and Development of SUS
Brooke created the SUS to address the need for a quick, reliable, and cost-effective method to measure usability without requiring extensive resources or technical expertise. Unlike more complex usability assessment tools, SUS was designed to be technology-agnostic and easy to administer. Its development was rooted in empirical research, with initial validation involving real-world system evaluations.
- The SUS was first introduced in a 1986 technical report and later published in 1996.
- It was designed to be used after a user completes a specific set of tasks with a system.
- The scale is not tied to any particular type of technology, making it versatile.
Structure of the SUS Questionnaire
The SUS consists of 10 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. After scoring, the final SUS score ranges from 0 to 100, with higher scores indicating better perceived usability.
Example statements include:
- I think that I would like to use this system frequently.
- I found the system unnecessarily complex. (reverse-scored)
- I thought the system was easy to use.
“The beauty of the SUS lies in its simplicity. With just 10 questions, it provides a robust snapshot of usability that correlates strongly with user satisfaction and performance.” — Dr. James R. Lewis, Human Factors Researcher
Why the System Usability Scale Matters in UX Design
In today’s competitive digital landscape, user experience (UX) is a key differentiator. The System Usability Scale provides UX professionals with a quantifiable metric to assess how intuitive and efficient a product feels to users. This data is critical for iterative design, stakeholder reporting, and benchmarking against competitors.
Quantifying Subjective User Experience
Usability is inherently subjective—what feels intuitive to one user might confuse another. The System Usability Scale transforms these subjective impressions into an objective, numerical score. This allows teams to track improvements over time, compare design iterations, and justify design decisions with data.
For example, a mobile banking app might score a 68 on SUS after its first usability test. After redesigning the navigation, the same app scores 82—providing clear evidence of improved usability.
Supporting Agile and Iterative Design Processes
In agile development environments, rapid feedback is essential. The SUS fits seamlessly into sprint reviews and usability testing sessions. Because it takes less than 10 minutes to complete, it can be administered frequently without burdening users or slowing down development.
- Teams can run SUS tests after each prototype iteration.
- Product managers use SUS scores to prioritize usability bugs.
- Designers correlate SUS results with qualitative feedback for deeper insights.
How to Administer the System Usability Scale
Administering the System Usability Scale correctly is crucial to obtaining valid and reliable results. While the process is straightforward, attention to timing, context, and participant selection can significantly impact the quality of the data.
When to Use the SUS in Testing
The optimal time to administer the SUS is immediately after a user completes a set of representative tasks with the system. This ensures that their experience is fresh and contextually grounded. For example, if testing an e-commerce site, users should finish tasks like searching for a product, adding it to the cart, and completing a mock checkout before answering the SUS questions.
Using SUS too early or without task completion can lead to inaccurate perceptions. A user might rate a system poorly simply because they haven’t yet discovered its full functionality.
Best Practices for Participant Selection
To ensure meaningful results, participants should represent the actual or target user base. A common misconception is that large sample sizes are required. In reality, research shows that even 5–10 users can provide reliable SUS scores due to the scale’s high reliability and low variability.
- Recruit users with varying levels of technical proficiency.
- Avoid testing with internal team members unless they mimic real user behavior.
- Ensure participants have completed the intended tasks before answering.
“You don’t need to test with 100 people to get a good idea of usability. With SUS, five well-chosen users can reveal 80% of the major issues.” — Jakob Nielsen, Nielsen Norman Group
Scoring and Interpreting the System Usability Scale
One of the most appealing aspects of the System Usability Scale is its straightforward scoring method. Despite its simplicity, the scoring process must be followed precisely to ensure accuracy.
Step-by-Step Scoring Methodology
To calculate a SUS score:
- For odd-numbered questions (1, 3, 5, 7, 9), subtract 1 from the user’s response.
- For even-numbered questions (2, 4, 6, 8, 10), subtract the user’s response from 5.
- Sum the adjusted values and multiply by 2.5 to get the final score (0–100).
For example, if a user responds with all “4s” across the 10 questions:
- Odd questions: (4-1) = 3 → 3 × 5 = 15
- Even questions: (5-4) = 1 → 1 × 5 = 5
- Total: (15 + 5) × 2.5 = 50
This results in a SUS score of 50, which is below average.
Understanding SUS Score Benchmarks
While the SUS score ranges from 0 to 100, a score of 68 is considered the average based on decades of aggregated data. Here’s a general interpretation guide:
- Below 50: Poor usability — significant redesign needed.
- 50–67: Below average — usability issues present.
- 68–76: Average — acceptable but room for improvement.
- 77–85: Good — above average, competitive.
- 86–100: Excellent — exceptional usability.
It’s important to note that context matters. A medical device may require a higher benchmark than an internal enterprise tool.
Advantages of the System Usability Scale
The enduring popularity of the System Usability Scale is no accident. Its widespread adoption is rooted in a combination of practical benefits that make it ideal for both academic research and industry applications.
Reliability and Validity Across Domains
Extensive research has confirmed the SUS’s high internal consistency (Cronbach’s alpha typically > 0.9) and strong correlation with other usability metrics like task success rate and user satisfaction. It has been validated across cultures, languages, and technologies, making it one of the most robust usability instruments available.
A study published in the Journal of Usability Studies found that SUS scores correlated strongly with user performance and perceived ease of use across 15 different software applications.
Speed and Simplicity of Administration
Unlike lengthy usability questionnaires, the SUS takes less than 5–10 minutes to complete. This low user burden increases response rates and reduces fatigue, especially in longitudinal studies or multi-phase testing.
- No training is required to administer the SUS.
- It can be delivered via paper, email, or integrated into digital testing platforms.
- Results are easy to aggregate and visualize.
Limitations and Criticisms of the System Usability Scale
Despite its strengths, the System Usability Scale is not without limitations. Understanding these weaknesses helps practitioners use SUS more effectively and complement it with other methods when necessary.
Lack of Diagnostic Detail
One of the most common criticisms of the SUS is that it provides a global usability score without pinpointing specific usability problems. A low score tells you that something is wrong, but not what exactly needs fixing.
For instance, a SUS score of 55 could result from poor navigation, unclear labels, or slow performance—each requiring different solutions. To address this, UX researchers often pair SUS with qualitative methods like think-aloud protocols or interviews.
Cultural and Linguistic Sensitivity
While the SUS has been translated into over 40 languages, nuances in language and cultural expectations can affect how users interpret the statements. For example, in some cultures, users may avoid extreme responses (e.g., “Strongly Agree”), leading to artificially lower scores.
Researchers recommend using culturally adapted translations and pilot testing the questionnaire in the target population.
“SUS is a great thermometer for usability, but it doesn’t tell you which organ is infected.” — Bill Albert, Founder, Beyond Curious
Comparing SUS with Other Usability Metrics
While the System Usability Scale is one of the most popular tools, it’s not the only one. Understanding how SUS compares to alternatives helps teams choose the right tool for their needs.
SUS vs. SUPR-Q
The SUPR-Q (Standardized User Experience Percentile Rank Questionnaire) measures not only usability but also credibility, loyalty, and appearance. Unlike SUS, which focuses solely on usability, SUPR-Q is designed specifically for websites and provides percentile rankings compared to industry benchmarks.
While SUPR-Q offers broader insights, it requires a license and is less flexible than the public-domain SUS.
SUS vs. UMUX and UMUX-Lite
The UMUX (Usability Metric for User Experience) is a four-item scale based on ISO definitions of usability. UMUX-Lite, a two-item version, correlates highly with SUS but is even shorter. However, due to its brevity, it may lack the reliability of the full SUS in some contexts.
UMUX is ideal for situations where survey fatigue is a concern, but SUS remains the preferred choice for comprehensive assessments.
Real-World Applications of the System Usability Scale
The System Usability Scale is not just a theoretical tool—it’s actively used across industries to improve products and services. From tech startups to government agencies, SUS provides actionable insights that drive design decisions.
Healthcare and Medical Devices
In healthcare, usability can be a matter of life and death. Regulatory bodies like the FDA encourage the use of SUS in human factors testing for medical devices. A high SUS score can support a device’s approval by demonstrating that it is safe and easy to use under stress.
For example, a study on an insulin delivery device used SUS to compare two interface designs, ultimately selecting the one with a 20-point higher SUS score.
Digital Banking and Financial Services
Banks and fintech companies use SUS to evaluate mobile apps and online banking platforms. A high SUS score correlates with customer satisfaction and retention. One major European bank reduced customer support calls by 30% after improving its app’s SUS score from 62 to 81 through a redesign.
- SUS helps identify pain points in account setup, fund transfers, and security features.
- It’s used in A/B testing of new features before full rollout.
- Regulatory compliance teams use SUS data to demonstrate usability due diligence.
How to Improve Your System Usability Scale Score
A low SUS score isn’t a dead end—it’s a starting point for improvement. By analyzing both the score and the qualitative feedback, teams can implement targeted changes to boost usability.
Identify Problematic Questions
Break down the SUS responses by question to identify weak areas. For example, if users consistently disagree with “I found the system easy to use,” focus on simplifying workflows. If “I needed to learn a lot of things before I could get going” scores poorly, improve onboarding.
Drilling into individual item scores helps prioritize design sprints and resource allocation.
Combine SUS with Qualitative Feedback
The most effective usability improvements come from combining SUS data with direct user feedback. After administering the SUS, ask follow-up questions like:
- What was the most frustrating part of using the system?
- Where did you feel lost or confused?
- What one change would make this system easier to use?
This mixed-methods approach provides both the “what” (SUS score) and the “why” (user quotes), enabling more informed design decisions.
“Numbers tell you something is wrong. Stories tell you what to fix.” — Tomer Sharon, Senior UX Researcher
Future of the System Usability Scale in UX Research
As technology evolves, so do the methods for evaluating it. Yet, the System Usability Scale continues to remain relevant, adapting to new contexts and research paradigms.
Integration with Automated Testing Tools
Modern UX platforms are beginning to integrate SUS into automated usability testing workflows. Tools like UsabilityHub and Optimal Workshop allow researchers to embed SUS at the end of remote tests, automatically calculate scores, and generate reports.
This integration reduces manual effort and enables real-time usability monitoring.
Adaptation for Emerging Technologies
The SUS is being successfully applied to voice interfaces, augmented reality (AR), and AI-driven systems. While the original scale was designed for graphical user interfaces, researchers are exploring minor adaptations to better capture the nuances of conversational and immersive experiences.
For example, a modified SUS might include questions about voice command accuracy or AR interface clarity, while retaining the core 10-item structure for comparability.
What is the ideal sample size for SUS testing?
A sample size of 5–10 users is often sufficient to obtain a reliable SUS score, especially in formative testing. While larger samples increase statistical confidence, the high reliability of SUS means that even small groups can detect meaningful differences between designs.
Can the System Usability Scale be used for non-digital products?
Yes, absolutely. Although commonly used for software and websites, the SUS has been successfully applied to physical products like medical devices, kiosks, and consumer electronics. Its language is general enough to assess any interactive system.
Is the SUS questionnaire 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 of the original work by John Brooke is recommended.
How does SUS compare to NPS?
While both SUS and Net Promoter Score (NPS) are simple metrics, they measure different things. SUS assesses perceived usability, while NPS measures customer loyalty and likelihood to recommend. A product can have a high NPS but a low SUS if users love the brand but find the interface clunky.
Can SUS scores be averaged across different systems?
Averaging SUS scores across different types of systems (e.g., a mobile app and a desktop software) is not recommended, as usability expectations vary by context. However, averaging is valid when comparing iterations of the same system or similar products within the same category.
The System Usability Scale remains a cornerstone of usability evaluation—a tool that combines scientific rigor with practical simplicity. From its humble beginnings in a corporate lab to its global adoption across industries, SUS continues to empower designers, researchers, and businesses to build better, more user-friendly products. While it has limitations, its strengths in reliability, speed, and versatility make it an indispensable part of the UX toolkit. By understanding how to administer, interpret, and act on SUS data, teams can turn subjective user experiences into actionable insights that drive real improvement.
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