Ever wondered how companies measure how easy or frustrating their products are to use? Enter the System Usability Scale (SUS) — a simple yet powerful tool that’s become the gold standard in usability evaluation. Let’s dive into what makes it so essential.
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 user experience (UX) research. Its strength lies in its simplicity, reliability, and broad applicability across platforms — from websites and mobile apps to medical devices and software interfaces.
Origins and Development of SUS
The SUS was first introduced in 1986 as a quick and dirty usability scale during usability testing at Digital Equipment Corporation. John Brooke needed a lightweight method to evaluate system usability without requiring extensive resources or time. The original goal wasn’t to create a definitive industry standard but to provide a practical tool for internal assessments.
- Developed by John Brooke in 1986
- Originally intended for internal use in tech companies
- Published formally in 1996, gaining widespread academic and industry attention
Despite its humble beginnings, the SUS quickly gained traction due to its psychometric robustness and ease of administration. It doesn’t require specialized training to deploy, making it accessible even to non-researchers.
Structure of the SUS Questionnaire
The SUS consists of 10 statements, each rated on a 5-point Likert scale ranging from “Strongly Disagree” (1) to “Strongly Agree” (5). The questions alternate between positive and negative phrasing to reduce response bias. For example:
- I think that I would like to use this system frequently. (Positive)
- I found the system unnecessarily complex. (Negative)
- I thought the system was easy to use. (Positive)
After collecting responses, a specific scoring algorithm is applied to calculate a final SUS score between 0 and 100. This score provides a standardized metric for comparing usability across different systems or iterations.
“The beauty of the SUS is that it’s quick to administer, easy to score, and produces a single number that stakeholders can understand.” — Jeff Sauro, MeasuringU
Why the System Usability Scale Matters in UX Research
In today’s competitive digital landscape, usability isn’t just a nice-to-have — it’s a business imperative. Poor usability leads to user frustration, abandonment, and lost revenue. The System Usability Scale offers a quantifiable way to measure user satisfaction and identify pain points early in the design process.
Quantifying Subjective User Experience
One of the biggest challenges in UX research is measuring something inherently subjective — how users *feel* about using a product. While behavioral metrics like task completion time or error rates are useful, they don’t capture the emotional and cognitive aspects of the experience. The SUS bridges this gap by providing a reliable proxy for perceived usability.
- Converts qualitative feedback into a quantitative score
- Enables benchmarking across projects and industries
- Helps communicate UX findings to non-technical stakeholders
Because the SUS score is a single number, it’s easy to track changes over time, compare different versions of a product, or evaluate competitors’ offerings.
Supporting Data-Driven Design Decisions
Design teams often face conflicting opinions about what constitutes “good” usability. The SUS provides objective data to support design decisions, reducing reliance on personal preferences or assumptions. For instance, if two design prototypes receive SUS scores of 68 and 82 respectively, the higher score strongly suggests better perceived usability.
This kind of data is invaluable during sprint reviews, stakeholder presentations, or when justifying design changes to product managers. It transforms UX from a subjective art into a measurable science.
How to Administer the System Usability Scale
Administering the SUS is straightforward, but doing it correctly ensures valid and reliable results. Whether you’re conducting a lab study, remote test, or post-launch survey, following best practices is crucial.
When to Use the SUS in Testing
The SUS can be used at various stages of the product development lifecycle:
- Formative Evaluation: During early prototypes to identify usability issues
- Summative Evaluation: On final versions to benchmark performance
- Comparative Testing: To compare two or more systems (e.g., old vs. new interface)
It’s typically administered immediately after a user completes a set of tasks, while their experience is still fresh. This timing helps ensure accurate recall and reduces bias.
Best Practices for Deployment
To get the most out of the SUS, follow these guidelines:
- Ensure participants have interacted meaningfully with the system before answering
- Use the exact wording of the SUS questions — don’t rephrase
- Collect responses anonymously to encourage honesty
- Pair SUS with qualitative feedback (e.g., interviews or open-ended questions)
Tools like Google Forms, SurveyMonkey, or integrated UX platforms (e.g., MeasuringU) can automate distribution and scoring.
Scoring and Interpreting the System Usability Scale
One of the most appealing features of the SUS is its simple scoring method. Despite having only 10 questions, the scoring algorithm produces a reliable and interpretable result.
The SUS Scoring Formula Explained
Here’s how to calculate the SUS score manually:
- For odd-numbered items (1, 3, 5, 7, 9), subtract 1 from the user’s response (so a “1” becomes 0, “2” becomes 1, etc.).
- For even-numbered items (2, 4, 6, 8, 10), subtract the user’s response from 5 (so a “5” becomes 0, “4” becomes 1, etc.).
- Sum the converted values.
- Multiply the total by 2.5 to get the final SUS score (ranging from 0 to 100).
For example, if a user’s adjusted sum is 36, multiplying by 2.5 gives a SUS score of 90 — which is excellent.
Understanding SUS Score Benchmarks
While a score of 68 is often cited as the industry average (based on extensive research by Sauro & Lewis, 2016), here’s a general interpretation guide:
- 0–60: Poor usability — significant improvements needed
- 61–70: Acceptable but below average
- 71–80: Good usability
- 81–100: Excellent usability
It’s important to note that context matters. A score of 75 might be great for a complex enterprise tool but disappointing for a consumer app.
“A SUS score of 68 doesn’t mean your product is ‘okay’ — it means it’s average compared to thousands of other systems tested.” — James Lewis, UX Researcher at IBM
Advantages of the System Usability Scale
The enduring popularity of the SUS isn’t accidental. It offers several key advantages that make it a go-to tool for UX professionals worldwide.
Simplicity and Speed
One of the biggest strengths of the system usability scale is its brevity. Users can complete it in under 2 minutes, minimizing survey fatigue and increasing response rates. Unlike longer questionnaires like the SUMI or QUIS, the SUS strikes a perfect balance between depth and efficiency.
- Only 10 questions
- Takes less than 2 minutes to complete
- Easy to integrate into existing testing protocols
This makes it ideal for agile environments where speed and iteration are critical.
Reliability and Validity
Despite its simplicity, the SUS has strong psychometric properties. Studies have shown it to be both reliable (consistent results across repeated tests) and valid (accurately measuring what it claims to measure — usability).
- High internal consistency (Cronbach’s alpha typically > 0.9)
- Proven validity across cultures and languages
- Effective even with small sample sizes (as few as 5–10 users)
Its robustness has been confirmed through decades of academic research and real-world application.
Limitations and Criticisms of the System Usability Scale
No tool is perfect, and the system usability scale is no exception. While widely praised, it has several limitations that practitioners should be aware of.
Lack of Diagnostic Detail
The SUS provides a global usability score but doesn’t tell you *why* users found a system difficult. A low score indicates a problem, but not where it lies. Was it navigation? Terminology? Performance?
- Does not identify specific usability issues
- Requires supplementary methods (e.g., think-aloud, interviews) for root cause analysis
- Can mask underlying problems if used in isolation
For this reason, the SUS should be used alongside qualitative research, not as a standalone diagnostic tool.
Cultural and Linguistic Sensitivity
While the SUS has been translated into over 40 languages, some items may not translate well across cultures. For example, the phrase “I thought the system was easy to use” might carry different connotations in high-context vs. low-context cultures.
- Translation must preserve original meaning and tone
- Back-translation and pilot testing are recommended
- Response styles (e.g., tendency to agree) vary across cultures
Researchers must be cautious when comparing SUS scores across international user groups.
Alternatives and Complementary Tools to the System Usability Scale
While the system usability scale remains the most popular usability questionnaire, several alternatives and complementary tools exist for more specialized needs.
Usability Questionnaires: SUS vs. Others
Several other usability scales have been developed, each with its own focus:
- SUMI (Software Usability Measurement Inventory): A 50-item questionnaire with subscales for efficiency, affect, and control. More detailed but time-consuming.
- QUIS (Questionnaire for User Interaction Satisfaction): Highly customizable but complex to administer.
- UMUX (Usability Metric for User Experience): A 4-item simplified version of SUS, based on ISO 9241 standards.
- Nielsen’s Usability Goals: Task-based metrics like success rate, time-on-task, and error rate.
While these tools offer deeper insights, the SUS remains unmatched in its balance of simplicity and effectiveness.
Combining SUS with Behavioral Metrics
The most powerful usability evaluations combine the system usability scale with behavioral data. For example:
- Pair SUS scores with task success rates to see if users *feel* the system is usable and actually *can* use it.
- Correlate SUS with time-on-task to identify efficiency issues.
- Use eye-tracking or clickstream analysis to understand where users struggle, then validate with SUS.
This mixed-methods approach provides a holistic view of usability — both perceived and actual.
“The SUS tells you *how* users feel, but behavioral data tells you *what* they do. You need both.” — Jakob Nielsen, Nielsen Norman Group
Real-World Applications of the System Usability Scale
The system usability scale isn’t just a theoretical tool — it’s used daily by companies, governments, and researchers around the world to improve real products.
Case Study: Improving a Healthcare App
A U.S.-based telehealth company used the SUS to evaluate a new patient portal. Initial testing with 15 users yielded an average SUS score of 54 — well below the benchmark. Follow-up interviews revealed confusion around appointment scheduling and medication tracking.
After redesigning the navigation and simplifying the interface, a second round of testing showed a SUS score of 81. This 27-point improvement demonstrated a dramatic increase in perceived usability, leading to higher patient engagement and satisfaction.
Enterprise Software Evaluation
A global financial institution used the SUS to compare two CRM systems during a vendor selection process. System A scored 69, while System B scored 83. Despite System B being more expensive, the significantly higher usability score justified the investment, as it promised lower training costs and faster adoption.
The SUS provided a clear, data-driven rationale for the decision, reducing internal debate and aligning stakeholders.
Future of the System Usability Scale in UX Evolution
As technology evolves, so do the ways we measure usability. Yet, the system usability scale continues to adapt and remain relevant in modern UX practice.
Integration with AI and Automated Testing
Emerging tools are beginning to integrate SUS-like metrics into automated usability testing platforms. AI-driven systems can now predict SUS scores based on user behavior patterns, facial expressions, or voice tone during interactions.
- Machine learning models trained on historical SUS data can estimate scores in real-time
- Chatbots can administer SUS post-interaction without human intervention
- Automated dashboards track SUS trends across product updates
While these innovations don’t replace human judgment, they enhance scalability and speed.
SUS in Agile and Continuous UX
In agile development, where releases happen weekly or even daily, traditional usability testing can feel too slow. The SUS fits perfectly into this environment because it’s fast, lightweight, and repeatable.
- Teams run micro-SUS tests after each sprint
- Product owners monitor SUS trends like any other KPI
- Continuous feedback loops enable rapid iteration
The system usability scale is evolving from a one-off assessment tool into a continuous monitoring metric — a usability heartbeat for digital products.
What is the ideal sample size for the System Usability Scale?
A minimum of 5–10 users can provide reliable insights for formative testing. For summative or benchmarking studies, 15–20 users are recommended to achieve statistical confidence. Larger samples (30+) allow for more robust comparisons and subgroup analysis.
Can the SUS be used for non-digital products?
Yes, the system usability scale has been successfully applied to physical products like medical devices, ATMs, and even household appliances. As long as users interact with a system in a goal-oriented way, SUS can assess perceived usability.
Is the SUS 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 authors (Brooke, 1996) is encouraged. You can find the official questionnaire and scoring guide on Usability.gov.
How does SUS compare to Net Promoter Score (NPS)?
While both are survey-based metrics, SUS measures perceived usability, whereas NPS measures customer loyalty and likelihood to recommend. They serve different purposes but can be used together — a high SUS score often correlates with a high NPS, indicating both usability and satisfaction.
Can SUS scores be compared across different types of systems?
Generally, yes — the SUS is designed to be a universal metric. However, comparisons should be made cautiously, especially between systems with vastly different complexity (e.g., a banking app vs. a smartwatch). Context and user expectations play a significant role in interpretation.
The System Usability Scale remains one of the most powerful, practical, and enduring tools in the UX researcher’s toolkit. Its ability to deliver a reliable, standardized measure of usability in just a few minutes explains its global adoption. While it has limitations — particularly in diagnostic depth — its strengths far outweigh them when used appropriately. By combining SUS with qualitative insights and behavioral data, teams can make smarter, more user-centered decisions. As technology continues to evolve, the SUS will likely remain a cornerstone of usability evaluation, adapting to new contexts and methodologies. Whether you’re a designer, product manager, or researcher, understanding and applying the system usability scale is a critical step toward building better, more usable products.
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