Ever wonder how users truly feel about a product’s ease of use? The System Usability Scale (SUS) is a proven, lightweight tool that delivers powerful insights into user experience—quickly, reliably, and cost-effectively.
What Is the System Usability Scale (SUS)?
The System Usability Scale (SUS) is a globally recognized, standardized questionnaire used 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 adopted usability measurement tools across industries—from software and websites to medical devices and consumer electronics.
Origins and Development of SUS
Brooke created the SUS as a quick, reliable method to assess usability without requiring extensive resources or technical expertise. Unlike more complex usability testing methods, SUS was designed to be simple, quick to administer, and easy to score. It consists of just 10 statements, each rated on a 5-point Likert scale, making it accessible even for non-specialists.
The development process involved iterative testing and validation across various systems. Brooke’s original research demonstrated that SUS produced consistent and reliable results across different user groups and technologies, laying the foundation for its widespread adoption.
Structure of the SUS Questionnaire
The SUS comprises 10 statements that users respond to after interacting with a system. Each item is rated from 1 (Strongly Disagree) to 5 (Strongly Agree). The statements alternate between positive and negative phrasing to reduce response bias.
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.After responses are collected, a specific scoring algorithm is applied to generate a final SUS score between 0 and 100..
Why SUS Stands Out Among Usability Metrics
What makes the System Usability Scale truly powerful is its balance of simplicity and effectiveness. While other usability tools require extensive user observation, task analysis, or eye-tracking equipment, SUS relies solely on user self-reporting—yet still delivers highly predictive and actionable data.
“SUS is not just a usability score; it’s a diagnostic tool that reflects both learnability and user satisfaction.” — Jakob Nielsen, Nielsen Norman Group
Its brevity ensures high completion rates, and its standardized nature allows for benchmarking across products, industries, and time. According to research by Sauro and Lewis (2009), SUS scores correlate strongly with user performance and satisfaction, making it a trusted metric in both academic and commercial settings.
How to Calculate the System Usability Scale Score
One of the most appealing aspects of the System Usability Scale is its straightforward scoring methodology. Despite its simplicity, the calculation process ensures reliable and meaningful results. The final SUS score ranges from 0 to 100, with higher scores indicating better perceived usability.
Step-by-Step Scoring Process
Calculating the SUS score involves a specific formula applied to the 10 responses:
- 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 across all 10 items.
- Multiply the total by 2.5 to convert the range from 0–40 to 0–100.
For example, if a user responds with all 3s (neutral), the adjusted sum is 20, and multiplying by 2.5 gives a SUS score of 50—right at the average benchmark.
Understanding the Final SUS Score
The resulting score is interpreted on a scale where:
- Below 50: Poor usability
- 50–69: Below average
- 70–79: Acceptable (average)
- 80–89: Good
- 90–100: Excellent
According to James Lewis and Jeff Sauro’s extensive research, the average SUS score across thousands of studies is approximately 68. A score above 70 is generally considered above average and indicates a system that users find reasonably easy to use.
Common Scoring Mistakes to Avoid
Despite its simplicity, errors in scoring can occur. Common pitfalls include:
- Forgetting to reverse the scoring for even-numbered items.
- Misapplying the multiplier (e.g., using 2 instead of 2.5).
- Using arithmetic mean instead of the correct formula.
To avoid these, many practitioners use automated SUS calculators. The MeasuringU SUS Calculator is a trusted, free tool that instantly computes scores from user responses.
Advantages of Using the System Usability Scale
The System Usability Scale has endured for decades because it offers tangible benefits that align with real-world usability evaluation needs. Its strengths make it ideal for both small startups and large enterprises.
Speed and Efficiency in Usability Testing
One of the biggest advantages of the system usability scale is its speed. Administering the SUS takes less than 5 minutes per user, making it easy to integrate into usability tests, beta programs, or post-release feedback loops.
Because it doesn’t require specialized software or training, teams can collect usability data rapidly during iterative design cycles. This agility supports agile and lean UX methodologies, where quick feedback is essential.
Cost-Effectiveness for Teams of All Sizes
Unlike eye-tracking labs or in-depth cognitive walkthroughs, the system usability scale is virtually free to implement. The questionnaire is in the public domain, and no licensing fees are required.
This makes SUS especially valuable for startups, educational institutions, or small design teams with limited budgets. Even large organizations use SUS as a first-line screening tool before investing in more resource-intensive evaluations.
Reliability and Validity Across Contexts
Extensive research has confirmed the reliability (consistency) and validity (accuracy) of the system usability scale. Studies show high internal consistency, with Cronbach’s alpha typically above 0.9, indicating strong reliability.
Moreover, SUS has been validated across diverse domains, including healthcare, finance, e-commerce, and mobile apps. A 2013 meta-analysis by Bangor, Kortum, and Miller confirmed that SUS produces consistent results regardless of the technology being evaluated.
Limitations and Criticisms of the System Usability Scale
While the system usability scale is widely praised, it is not without limitations. Understanding these weaknesses helps practitioners use SUS more effectively and know when to complement it with other methods.
Lack of Diagnostic Depth
One of the most common criticisms of the system usability scale is that it provides a score but not specific insights into *why* a system is usable or not. A low SUS score signals a problem, but it doesn’t pinpoint whether the issue lies in navigation, terminology, layout, or functionality.
For example, a SUS score of 45 tells you users are struggling, but not whether they’re confused by the menu structure or can’t find the search bar. To address this, SUS should be paired with qualitative methods like think-aloud protocols or interviews.
Sensitivity to Context and User Expectations
The system usability scale can be influenced by factors outside the system itself, such as user expectations, prior experience, or emotional state. A user familiar with advanced software may rate a simple app as “too basic,” dragging down the score, even if it’s well-designed for its target audience.
Similarly, users under time pressure or in a negative mood may give lower ratings unrelated to actual usability. This context sensitivity means SUS results should always be interpreted alongside behavioral data and user demographics.
Language and Cultural Biases
Although SUS has been translated into over 30 languages, direct translations may not capture the intended meaning. Cultural differences in how people interpret Likert scales (e.g., tendency to avoid extreme responses) can affect scores.
For global products, it’s essential to validate translated versions and consider using culturally adapted versions. The UXmatters article on international adaptation offers guidance on localization best practices.
Best Practices for Administering the System Usability Scale
To get the most value from the system usability scale, it’s crucial to follow best practices in administration, timing, and interpretation. Proper use ensures data accuracy and actionable outcomes.
When to Administer SUS in a Usability Test
The optimal time to administer the system usability scale is immediately after the user completes a set of representative tasks. This ensures their experience is fresh and contextually relevant.
For example, in a website usability test, users might complete tasks like finding a product, adding it to a cart, and checking out. Right after, they’re presented with the SUS questionnaire. Delaying the survey risks memory decay and less accurate responses.
Ensuring Representative User Sampling
While SUS can be administered to as few as 5 users (as per Jakob Nielsen’s usability testing theory), larger samples improve statistical confidence. For benchmarking or comparative studies, aim for at least 20–30 users.
Ensure your sample reflects your actual user base in terms of age, tech proficiency, and domain knowledge. Testing a medical app only with tech-savvy millennials may skew SUS scores upward, masking usability issues for older or less experienced users.
Combining SUS with Other UX Metrics
The system usability scale is most powerful when used alongside other metrics. Common combinations include:
- Task Success Rate: Percentage of users who complete a task successfully.
- Time on Task: How long users take to complete key actions.
- Net Promoter Score (NPS): Measures user loyalty and willingness to recommend.
- SEQ (Single Ease Question): A single post-task question asking, “How easy was this task?”
Together, these metrics provide a 360-degree view of usability—quantitative performance data from task metrics and subjective perception from SUS.
Interpreting and Benchmarking System Usability Scale Scores
Understanding what a SUS score means in context is critical. A score of 75 might be excellent for a complex enterprise tool but disappointing for a consumer mobile app.
Industry Benchmarks for SUS Scores
Jeff Sauro’s research provides valuable benchmark data. Across thousands of studies:
- Consumer software: Average SUS ~78
- Enterprise software: Average SUS ~68
- Mobile apps: Average SUS ~73
- Medical devices: Average SUS ~70
These benchmarks help contextualize your results. A score of 72 for an internal HR system might be above average, while the same score for a social media app could indicate room for improvement.
Using SUS for Competitive Analysis
The system usability scale is ideal for comparing your product against competitors. By testing users on both your system and a rival’s, you can calculate SUS scores for each and identify usability gaps.
For example, if your e-commerce platform scores 82 and a competitor scores 76, you have a clear usability advantage. Conversely, if your score is lower, the difference highlights areas needing redesign.
Tracking SUS Over Time for Product Improvement
One of the most powerful uses of the system usability scale is longitudinal tracking. By measuring SUS before and after a redesign, you can quantify the impact of UX changes.
For instance, a fintech app might have a SUS score of 65 before a navigation overhaul. After implementing a new menu structure, the score rises to 80—demonstrating a significant usability improvement. This data is invaluable for justifying UX investments to stakeholders.
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 real products and services.
SUS in Software and Web Development
Software companies use SUS during beta testing to identify usability bottlenecks before launch. For example, a SaaS platform might administer SUS to 50 beta testers and discover a score of 60, prompting a redesign of the onboarding flow.
Web developers integrate SUS into A/B testing. Two versions of a checkout page can be tested with different user groups, and the SUS scores compared to determine which design feels more intuitive.
Healthcare and Medical Device Usability
In healthcare, usability is a matter of safety. Regulatory bodies like the FDA encourage the use of SUS in human factors testing for medical devices.
A study published in the Journal of Biomedical Informatics found that SUS effectively identified usability issues in electronic health record (EHR) systems, helping reduce clinician errors and improve workflow efficiency.
E-Commerce and Customer Experience Optimization
Online retailers use the system usability scale to optimize the customer journey. After users complete a simulated purchase, they’re asked to complete the SUS.
Low scores on specific items—like “I felt very confident using the system”—can signal anxiety during checkout, prompting changes like clearer error messages or progress indicators.
What is a good System Usability Scale score?
A score of 68 is the average across all systems. Scores above 70 are considered above average, 80+ is good, and 90+ is excellent. However, what’s “good” depends on your industry and product type.
Can I modify the System Usability Scale questions?
It’s not recommended. The validity of SUS relies on the exact wording and structure. Modifying questions can invalidate the scoring model. If you need a customized tool, consider the UMUX (Usability Metric for User Experience) instead.
How many users do I need for a reliable SUS score?
As few as 5 users can reveal major usability issues. For reliable averages and statistical confidence, aim for 15–20 users. Larger samples (30+) are better for benchmarking or comparative studies.
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 or licensing is required.
How does SUS compare to NPS or SEQ?
SUS measures overall system usability, while NPS measures loyalty and SEQ measures ease of a single task. SUS is more comprehensive than SEQ but less focused on advocacy than NPS. They are often used together for a complete picture.
The System Usability Scale remains one of the most powerful, accessible tools in the UX researcher’s toolkit. Its simplicity, reliability, and versatility make it indispensable for evaluating how users perceive a system’s ease of use. While it has limitations—particularly in diagnostic depth—its value is maximized when combined with qualitative insights and other performance metrics. Whether you’re refining a mobile app, validating a medical device, or benchmarking against competitors, SUS provides a fast, cost-effective way to quantify usability. By understanding how to properly administer, score, and interpret SUS, teams can make data-driven decisions that lead to better user experiences and stronger product outcomes.
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