Ever wondered how to measure if a product is truly user-friendly? The System Usability Scale (SUS) is a simple yet powerful tool that gives you the answer—fast, reliable, and with surprising depth.
What Is the System Usability Scale (SUS)?
The System Usability Scale, commonly known as SUS, is a widely used, 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 become one of the most trusted tools in usability testing across industries—from software and websites to medical devices and mobile apps.
Origins and Development of SUS
The System Usability Scale was first introduced in 1986 as a quick, reliable method to assess usability without requiring complex observational studies. At the time, usability professionals needed a lightweight, scalable solution that could be administered easily after user tasks. Brooke’s 10-item questionnaire filled that gap perfectly.
Originally designed for internal use, SUS gained traction in academic and industrial circles due to its simplicity and consistency. Over the decades, it has been validated across thousands of studies and translated into dozens of languages, proving its robustness across cultures and domains.
What makes SUS stand out is its brevity. Despite having only 10 questions, it produces a reliable usability score ranging from 0 to 100. This score allows teams to benchmark performance, compare design iterations, and evaluate user experience (UX) objectively.
Structure of the SUS Questionnaire
The System Usability Scale 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.
Here are the standard 10 items in the SUS:
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.Odd-numbered items are positively worded and scored by subtracting 1 from the user response.Even-numbered items are negatively worded and scored by subtracting the user response from 5.
.The total score is then multiplied by 2.5 to convert it to a 0–100 scale..
“The beauty of the System Usability Scale lies in its simplicity. It doesn’t tell you *why* a system is usable or not, but it tells you *how much*—and that’s often enough to drive decisions.” — Jakob Nielsen, Nielsen Norman Group
Why the System Usability Scale Matters in UX Design
In today’s competitive digital landscape, user experience is a key differentiator. The System Usability Scale provides a quantifiable metric that helps UX teams move beyond subjective opinions and make data-driven decisions. Whether you’re refining a mobile app, launching a new SaaS platform, or redesigning a healthcare portal, SUS offers a consistent way to measure usability.
Objective Measurement of Subjective Experience
Usability is inherently subjective—what feels intuitive to one user might confuse another. However, the System Usability Scale transforms these subjective impressions into an objective score. By aggregating responses from multiple users, teams can identify trends, detect usability issues, and track improvements over time.
For example, a SUS score below 68 is generally considered below average, while scores above 80 are excellent. This benchmarking capability allows teams to set clear goals and evaluate whether a redesign has actually improved the user experience.
Moreover, because SUS is technology-agnostic, it can be applied to websites, mobile apps, kiosks, voice interfaces, and even physical products with digital components. This versatility makes it a staple in both formative and summative usability testing.
Cost-Effective and Time-Efficient Evaluation
Unlike moderated usability testing, which can be time-consuming and expensive, the System Usability Scale can be administered quickly and at scale. It typically takes users less than 5 minutes to complete, making it ideal for remote testing, beta programs, or post-task evaluations in usability labs.
Because it requires no specialized equipment or training, SUS is accessible to startups, small design teams, and large enterprises alike. It integrates seamlessly into existing workflows, whether you’re running A/B tests, conducting user interviews, or analyzing customer feedback.
According to research by Sauro and Lewis (2016), the SUS has a high test-retest reliability (r = 0.83) and strong internal consistency (Cronbach’s alpha > 0.9), making it one of the most psychometrically sound usability questionnaires available. You can learn more about their findings in their comprehensive guide on MeasuringU.
How to Calculate the System Usability Scale Score
One of the most appealing aspects of the System Usability Scale is its straightforward scoring methodology. While the calculation might seem complex at first glance, it’s easy to automate and can be done manually with a simple formula.
Step-by-Step Scoring Process
To calculate a SUS score, follow these steps:
- For each odd-numbered question (1, 3, 5, 7, 9), subtract 1 from the response score (which ranges from 1 to 5).
- For each even-numbered question (2, 4, 6, 8, 10), subtract the response score from 5.
- Sum all the adjusted scores (you’ll have 10 values).
- Multiply the total by 2.5 to get the final SUS score on a 0–100 scale.
For example, if a user responds with all 4s across the 10 questions:
- Odd items: (4-1) = 3 → 3 × 5 = 15
- Even items: (5-4) = 1 → 1 × 5 = 5
- Total = 15 + 5 = 20
- SUS Score = 20 × 2.5 = 50
A score of 50 is slightly below the average SUS benchmark of 68, indicating room for improvement.
Interpreting SUS Scores: Benchmarks and Percentiles
Understanding what a SUS score means in context is crucial. A raw number like “75” doesn’t tell the full story unless you know how it compares to industry standards.
Jeff Sauro, a leading researcher in usability metrics, developed a widely used grading scale for SUS scores:
- 90–100: Excellent
- 80–89: Good
- 70–79: Acceptable
- 60–69: Poor
- 0–59: Awful
Additionally, Sauro’s research shows that the average SUS score across thousands of studies is approximately 68. This means a score above 68 is better than average, while below 68 indicates below-average usability.
You can also convert SUS scores into percentile ranks. For instance, a score of 80 places a product in the 76th percentile, meaning it’s more usable than 76% of systems tested. This comparative insight is invaluable for product managers and UX leads aiming to outperform competitors.
For more detailed analysis, visit MeasuringU’s SUS Calculator, which automatically computes scores and provides percentile rankings.
Advantages of Using the System Usability Scale
The System Usability Scale has stood the test of time because it offers numerous advantages over other usability assessment methods. Its widespread adoption is not accidental—it’s rooted in practical benefits that directly support user-centered design.
High Reliability and Validity
One of the most cited strengths of the System Usability Scale is its psychometric robustness. Studies have consistently shown that SUS has high internal consistency (Cronbach’s alpha typically above 0.9), meaning the 10 items measure the same underlying construct: perceived usability.
It also demonstrates strong test-retest reliability, meaning users tend to give similar scores when evaluating the same system at different times. This stability makes SUS ideal for longitudinal studies and iterative design processes.
Furthermore, SUS correlates well with other usability metrics, such as task success rates and time-on-task, reinforcing its validity as a holistic measure of user experience.
Flexibility Across Platforms and Domains
Unlike some usability tools that are tailored to specific technologies, the System Usability Scale is platform-agnostic. Whether you’re testing a desktop application, a mobile banking app, or a smart home device, SUS can be applied with minimal adaptation.
It’s been successfully used in healthcare (e.g., evaluating electronic health records), automotive (e.g., infotainment systems), education (e.g., e-learning platforms), and government services (e.g., tax filing portals). This cross-domain applicability makes it a universal tool in the UX toolkit.
For example, a study published in the Journal of Biomedical Informatics found that SUS effectively differentiated between usable and poorly designed clinical software, helping hospitals select better EHR systems. You can read more about this application in the ScienceDirect article.
Limitations and Criticisms of the System Usability Scale
Despite its many strengths, the System Usability Scale is not without limitations. Understanding these weaknesses is essential for using SUS effectively and avoiding misinterpretation of results.
Lack of Diagnostic Detail
One of the most common criticisms of the System Usability Scale is that it doesn’t explain *why* a system received a particular score. While it tells you *how usable* a system is, it doesn’t pinpoint specific usability problems.
For instance, a low SUS score could result from poor navigation, confusing terminology, slow performance, or lack of feedback—but the questionnaire itself won’t tell you which issue is to blame. This is why SUS is best used in conjunction with qualitative methods like user interviews, think-aloud protocols, or heuristic evaluations.
To overcome this limitation, many UX researchers pair SUS with open-ended follow-up questions such as: “What did you find most difficult about the system?” or “What one change would make this system easier to use?”
Sensitivity to Context and User Expectations
The System Usability Scale measures perceived usability, which can be influenced by factors outside the system’s design. For example, users’ prior experience with similar systems, their technical proficiency, or their mood during testing can all affect their responses.
A user who is frustrated after a long day may rate a system lower than they would otherwise. Similarly, a tech-savvy user might rate a simple app as “too basic,” while a novice finds it intuitive. These variations can introduce noise into the data.
To mitigate this, it’s important to standardize testing conditions, ensure representative user sampling, and interpret SUS scores in context. Running SUS across multiple user groups (e.g., novices vs. experts) can provide deeper insights.
Best Practices for Administering the System Usability Scale
To get the most value from the System Usability Scale, it’s essential to administer it correctly. Poor implementation can lead to misleading results, undermining its reliability and usefulness.
Timing and Context of Administration
The ideal 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, in a usability test, participants might be asked to complete tasks like creating an account, searching for a product, or submitting a form. Right after finishing, they’re presented with the SUS questionnaire. This post-task timing increases the accuracy of their perceptions.
Administering SUS too long after interaction (e.g., via email a week later) risks memory decay and reduced response accuracy. Conversely, giving it before any interaction defeats its purpose.
Ensuring Representative and Sufficient Sample Size
While SUS can be administered with as few as 5 users (in line with Jakob Nielsen’s heuristic that 5 users uncover 85% of usability issues), larger samples provide more reliable and generalizable results.
For benchmarking or comparative studies, a sample size of 15–20 users is recommended. This allows for meaningful statistical analysis, such as calculating confidence intervals or conducting t-tests between design versions.
It’s also important to ensure diversity in your sample. Testing only with power users or only with novices can skew results. Aim for a mix that reflects your actual user base in terms of age, technical skill, and familiarity with the domain.
Advanced Applications and Variants of the System Usability Scale
While the original System Usability Scale remains the gold standard, researchers and practitioners have developed several adaptations to address specific needs or contexts.
SUS Variants: SUS-S, SUS-M, and SUS-8
Over the years, several shortened or modified versions of SUS have emerged:
- SUS-S (Short SUS): A 5-item version that maintains high correlation with the full SUS, useful when time is extremely limited.
- SUS-M (Mobile SUS): Adapted for mobile contexts, with slightly reworded items to reflect mobile-specific interactions.
- SUS-8: An 8-item version proposed to improve internal consistency by removing two less reliable items.
While these variants offer convenience, the original 10-item SUS is still recommended for most applications due to its extensive validation and benchmarking data.
Integration with Other UX Metrics
The System Usability Scale is rarely used in isolation. It’s most powerful when combined with other UX metrics to create a holistic view of user experience.
Common complementary metrics include:
- Net Promoter Score (NPS): Measures user loyalty and willingness to recommend.
- Task Success Rate: Quantifies how many users complete key tasks.
- Time-on-Task: Tracks efficiency of interaction.
- Customer Satisfaction (CSAT): Assesses overall satisfaction with the experience.
Together, these metrics form a “UX dashboard” that provides both qualitative and quantitative insights. For example, a high SUS score paired with low task success might indicate that users *feel* the system is easy but struggle to complete actions—highlighting a gap between perception and performance.
Real-World Case Studies Using the System Usability Scale
The true power of the System Usability Scale becomes evident when we look at how it’s been applied in real-world scenarios across industries.
Healthcare: Improving EHR Usability
In a 2018 study conducted at a major U.S. hospital, SUS was used to evaluate two competing Electronic Health Record (EHR) systems. Nurses and physicians completed a series of clinical tasks (e.g., prescribing medication, updating patient notes) using both systems.
System A received a mean SUS score of 62, while System B scored 84. Follow-up interviews revealed that System A had inconsistent navigation and required too many clicks, leading to user frustration. Based on these findings, the hospital chose System B, citing its superior usability as a key factor.
This case demonstrates how SUS can support high-stakes procurement decisions in critical environments where usability directly impacts patient safety.
E-Commerce: Optimizing Checkout Flow
An online retailer redesigned its checkout process to reduce cart abandonment. Before and after the redesign, they administered the System Usability Scale to 30 users each time.
Prior to the redesign, the SUS score was 58 (“awful”). After simplifying form fields, adding progress indicators, and enabling guest checkout, the score improved to 79 (“acceptable”).
Alongside a 22% decrease in cart abandonment, this SUS improvement provided clear evidence that the redesign enhanced usability. The company used these results to justify further UX investments.
For more on e-commerce usability, check out the Nielsen Norman Group’s research.
What is the System Usability Scale used for?
The System Usability Scale is used to measure the perceived usability of a system, product, or service. It helps teams evaluate user experience, compare design alternatives, benchmark performance, and identify areas for improvement in a quick and reliable way.
How many questions are in the System Usability Scale?
The System Usability Scale consists of 10 questions, each rated on a 5-point Likert scale. The questions alternate between positive and negative statements to reduce response bias and improve reliability.
What is a good SUS score?
A SUS score above 68 is considered above average. Scores above 80 are classified as “Good” or “Excellent,” while scores below 68 indicate usability issues that may require attention.
Can I modify the System Usability Scale?
While you can adapt the wording slightly for context (e.g., replacing “system” with “app”), it’s not recommended to change the core items or scale, as this may affect the validity of the results. The original SUS has been extensively validated, and modifications can compromise its reliability.
Is the System Usability Scale free to use?
Yes, the System Usability Scale is in the public domain and free to use for both commercial and non-commercial purposes. No permission is required, though proper attribution to John Brooke is appreciated.
The System Usability Scale remains one of the most effective, efficient, and widely adopted tools for measuring usability. Its simplicity, reliability, and versatility make it indispensable for UX professionals, product managers, and researchers. While it doesn’t replace in-depth usability testing, it provides a powerful snapshot of user experience that can guide design decisions, support business cases, and drive continuous improvement. When used correctly—paired with qualitative insights and other metrics—SUS empowers teams to build products that are not just functional, but truly user-friendly.
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