Ever wondered how companies measure how easy or frustrating their apps and websites are to use? Enter the System Usability Scale (SUS)—a simple yet powerful tool that’s become the gold standard in usability testing.
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 interface. Developed in the late 1980s by John Brooke at Digital Equipment Corporation, SUS has since become one of the most widely used tools in user experience (UX) research. Its popularity stems from its simplicity, reliability, and versatility across different platforms and industries.
Origins and Development of SUS
The System Usability Scale was first introduced in 1986 as a quick and practical way to evaluate usability without requiring complex metrics or extensive user testing. At the time, usability professionals were struggling to find a standardized method that could be applied across various systems—from software interfaces to medical devices. Brooke’s solution was a ten-question survey that could be administered quickly and analyzed easily.
Despite being developed over three decades ago, the SUS has stood the test of time. It’s been validated across numerous studies and remains relevant in today’s fast-evolving digital landscape. Its enduring success lies in its ability to provide a reliable usability score with minimal effort from both researchers and participants.
Structure of the SUS Questionnaire
The SUS consists of 10 statements, each rated on a five-point Likert scale ranging from “Strongly Disagree” to “Strongly Agree.” 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.
- I found the system unnecessarily complex.
- I thought the system was easy to use.
After users complete the survey, their responses are scored using a specific formula that converts raw answers into a single usability score between 0 and 100. This score provides a quick snapshot of how usable a system is perceived to be.
“The beauty of the System Usability Scale is its simplicity. You don’t need a PhD in statistics to understand it, but it still delivers robust, actionable data.” — Dr. James Lewis, UX Researcher at IBM
Why the System Usability Scale Matters in UX Design
In the world of user experience design, understanding how users interact with a product is critical. While analytics can tell you what users are doing, SUS helps explain how they feel about it. This emotional and cognitive feedback is essential for creating products that are not only functional but also enjoyable to use.
Measuring Perceived Usability
One of the key strengths of the System Usability Scale is its focus on perceived usability rather than objective performance. While task success rates and time-on-task are important, they don’t always capture the user’s subjective experience. A user might complete a task successfully but still find the process frustrating or confusing. SUS fills this gap by measuring the user’s overall satisfaction and ease of use.
For instance, imagine a mobile banking app that allows users to transfer money in under 30 seconds. Objectively, this seems efficient. However, if users report high cognitive load or confusion during the process, the SUS score will reflect that dissatisfaction. This insight allows designers to refine the interface for better clarity and confidence.
Supporting Iterative Design Processes
Modern UX design follows an iterative approach—build, test, learn, improve. The System Usability Scale fits perfectly into this cycle. After each design iteration, teams can administer the SUS to track improvements (or declines) in usability over time.
For example, a design team might launch a prototype with a SUS score of 58—below average. After making changes based on user feedback, they retest and see the score rise to 76—well above average. This quantifiable improvement provides clear evidence that the changes had a positive impact, helping justify design decisions to stakeholders.
How to Administer the System Usability Scale
One of the reasons SUS is so popular is its ease of administration. It doesn’t require specialized tools or extensive training. However, to get accurate and meaningful results, there are best practices you should follow.
When to Use SUS in Testing
The System Usability Scale can be used at various stages of product development:
- Early Prototypes: Even low-fidelity wireframes can be tested with SUS after users complete key tasks.
- Mid-Development: Use SUS to compare different design alternatives (A/B testing).
- Post-Launch: Monitor long-term user satisfaction and detect usability regressions after updates.
It’s important to administer the SUS immediately after a user completes a set of tasks. This ensures that their experience is fresh in their mind, leading to more accurate responses.
Best Practices for Accurate Results
To ensure reliable SUS data, consider the following guidelines:
- Use Real Tasks: Don’t ask users to fill out the SUS after passively viewing a screen. Have them complete realistic tasks first.
- Randomize Question Order (Carefully): While some researchers suggest randomizing to reduce bias, SUS relies on a specific scoring algorithm that assumes the original order. Stick to the standard format.
- Sample Size: While SUS can be used with as few as 5 users, larger samples (20+) provide more stable and generalizable results.
- Context Matters: Always collect qualitative feedback alongside SUS. A low score is more actionable when you know why users felt the system was hard to use.
For more details on proper administration, refer to the official Usability.gov guide on SUS.
Scoring and Interpreting the System Usability Scale
One of the most powerful aspects of the System Usability Scale is its scoring model. Despite its simplicity, the scoring method is mathematically sound and produces a standardized score that can be compared across studies and industries.
The SUS Scoring Formula Explained
Each of the 10 SUS questions is scored individually, with adjustments made for negatively worded items. Here’s how it works:
- For odd-numbered questions (positively worded): Subtract 1 from the user’s response (which ranges from 1 to 5), then multiply by 2.5.
- For even-numbered questions (negatively worded): Subtract the user’s response from 5, then multiply by 2.5.
After converting all responses, sum the scores to get the final SUS score, which ranges from 0 to 100. For example, a perfect score of 100 means every response aligned perfectly with high usability.
While manual calculation is possible, many teams use online SUS calculators to automate the process and reduce errors.
What Does Your SUS Score Mean?
Interpreting SUS scores is straightforward thanks to established benchmarks:
- Below 50: Poor usability. Significant redesign is likely needed.
- 50–69: Average. Room for improvement.
- 70–79: Good. Users find the system reasonably easy to use.
- 80–100: Excellent. Top-tier usability.
According to research by Sauro and Lewis (2006), the average SUS score across thousands of studies is around 68. So, a score above 70 is already better than average.
“A SUS score of 70 is the tipping point. Above it, users are generally willing to use your product without resistance. Below it, they’re likely to complain, make errors, or abandon the system.” — Jeff Sauro, MeasuringU
Advantages of Using the System Usability Scale
The widespread adoption of the System Usability Scale isn’t accidental. It offers several compelling advantages that make it a favorite among UX professionals, product managers, and researchers.
Simplicity and Speed
One of the biggest strengths of the System Usability Scale is its brevity. The entire survey takes less than 5 minutes to complete, making it easy to integrate into usability tests without burdening participants. This low overhead allows teams to collect usability data frequently and consistently.
Unlike more complex usability frameworks that require extensive training or software, SUS can be deployed by anyone—from startup founders to academic researchers. Its accessibility has democratized usability measurement across industries.
Reliability and Validity
Despite its simplicity, the System Usability Scale is statistically robust. 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) found that SUS has high internal consistency (Cronbach’s alpha typically above 0.9), which is exceptional for a 10-item scale. This means the questions are cohesive and measure a single underlying construct: perceived usability.
Moreover, SUS scores correlate well with other usability metrics, such as task success and user satisfaction, further validating its effectiveness.
Limitations and Criticisms of the System Usability Scale
While the System Usability Scale is a powerful tool, it’s not without limitations. Understanding these weaknesses is crucial 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 tell you why a system is usable or not. A low score indicates a problem, but it doesn’t pinpoint where the issue lies.
For example, if a user gives a SUS score of 45, you know usability is poor, but you don’t know whether the problem is with navigation, terminology, layout, or something else. This is why SUS should always be paired with qualitative methods like think-aloud protocols, interviews, or observational studies.
As Dr. Elizabeth Rosenzweig notes, “SUS is a thermometer for usability. It tells you if the system has a fever, but you still need a doctor to diagnose the illness.”
Cultural and Linguistic Sensitivity
The System Usability Scale was developed in English and reflects Western cognitive patterns. When translated into other languages or used in different cultural contexts, subtle nuances in meaning can affect responses.
For instance, the phrase “I would like to use this system frequently” might be interpreted differently in cultures where modesty or indirectness is valued. Some researchers recommend adapting the wording slightly to maintain clarity and cultural relevance, though this can affect score comparability.
To address this, teams should pilot-test translated versions of SUS with local users and consider using culturally validated adaptations when available.
Comparing SUS with Other Usability Metrics
While the System Usability Scale is one of the most popular usability tools, it’s not the only one. Understanding how SUS compares to other metrics helps you choose the right tool for your needs.
SUS vs. SUPR-Q
The SUPR-Q (Standardized User Experience Percentile Rank Questionnaire) builds on SUS by measuring not just usability, but also trust, appearance, and loyalty. While SUS focuses narrowly on ease of use, SUPR-Q provides a broader picture of the overall user experience.
SUPR-Q is particularly useful for websites and e-commerce platforms where brand perception matters. However, it requires a larger sample size and is not as universally applicable as SUS.
Learn more about SUPR-Q at SurveyMonkey’s UX page.
SUS vs. UMUX and UMUX-Lite
The UMUX (Usability Metric for User Experience) is a four-item scale based on ISO standards. It’s shorter than SUS and focuses on two core dimensions: usability and usefulness. The UMUX-Lite version reduces this to just two questions, making it ideal for quick surveys.
While UMUX-Lite is faster, it lacks the depth and reliability of SUS. Studies show that UMUX-Lite scores correlate highly with SUS, but they don’t replace the diagnostic power of the full 10-item scale.
For teams needing a balance between brevity and accuracy, UMUX-Lite can be a good supplement—but not a substitute—for the System Usability Scale.
Real-World Applications of the System Usability Scale
The System Usability Scale isn’t just a theoretical tool—it’s used every day by companies, governments, and researchers around the world to improve digital products and services.
Healthcare and Medical Devices
In healthcare, usability isn’t just about convenience—it’s a matter of safety. The FDA recommends using SUS in human factors testing for medical devices. A poorly designed insulin pump interface, for example, could lead to dosing errors.
Hospitals and device manufacturers use SUS to evaluate electronic health records (EHRs), patient portals, and diagnostic tools. A high SUS score indicates that clinicians can use the system efficiently under pressure, reducing the risk of errors.
Software and App Development
From enterprise software to consumer apps, SUS is a staple in the development lifecycle. Companies like Google, Microsoft, and Adobe use SUS internally to benchmark usability across product versions.
For example, a fintech startup might use SUS to compare two onboarding flows. If Flow A scores 62 and Flow B scores 81, the team has clear evidence to choose the more user-friendly option.
Academic and Government Research
Universities and government agencies use the System Usability Scale to evaluate public-facing digital services. The U.S. Digital Service and UK’s GOV.UK have both referenced SUS in their design guidelines.
In academic research, SUS is frequently used in human-computer interaction (HCI) studies to compare interface designs, evaluate new technologies, or assess the impact of accessibility features.
“We used the System Usability Scale to evaluate a new voting interface for visually impaired users. The SUS score jumped from 54 to 79 after adding screen reader compatibility—proof that accessibility improvements directly enhance usability.” — Dr. Anita Taylor, HCI Researcher
What is the System Usability Scale used for?
The System Usability Scale is used to measure the perceived usability of a product, system, or interface. It helps teams evaluate how easy or difficult users find a system to use, track usability improvements over time, and compare different design alternatives.
How many questions are in the SUS questionnaire?
The System Usability Scale consists of 10 questions. Each question is rated on a 5-point scale from “Strongly Disagree” to “Strongly Agree.”
What is a good SUS score?
A SUS score above 70 is considered good. Scores between 70–79 indicate above-average usability, while 80–100 are excellent. The average SUS score across industries is around 68.
Is the System Usability Scale free to use?
Yes, the System Usability Scale is free to use. It is not copyrighted, and researchers and practitioners can administer it without permission or licensing fees.
Can SUS be used for mobile apps?
Absolutely. The System Usability Scale is platform-agnostic and can be used to evaluate websites, mobile apps, software, hardware, and even voice interfaces.
From its humble beginnings in a corporate lab to its current status as a global standard, the System Usability Scale has proven its worth time and again. It’s not just a survey—it’s a window into the user’s mind. By providing a quick, reliable, and standardized way to measure usability, SUS empowers teams to build better products, reduce frustration, and create more satisfying user experiences. While it has limitations, especially in diagnostic depth, its strengths far outweigh its weaknesses when used appropriately. Whether you’re a UX designer, product manager, or researcher, understanding and applying the System Usability Scale is a critical step toward user-centered design.
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