Ever wondered how to measure if your users actually enjoy using your product? Enter the System Usability Scale (SUS)—a simple, reliable tool that turns user frustration into actionable data. It’s fast, free, and shockingly accurate.
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 usability evaluation across industries—from software and websites to medical devices and consumer electronics.
Origins and Development of SUS
The System Usability Scale was first introduced in 1986 as a quick and dirty usability assessment tool. Its goal was not to replace in-depth usability testing but to provide a standardized, lightweight method for comparing the usability of different systems. Despite its informal beginnings, SUS quickly gained traction due to its simplicity and reliability.
Brooke’s original research was focused on evaluating interactive systems within a corporate environment, but the flexibility of the SUS allowed it to be applied far beyond its initial scope. Over time, researchers and practitioners found that the scale could be used across platforms and languages with minimal adaptation, making it a global standard.
- Created in 1986 by John Brooke
- Originally intended for internal use at Digital Equipment Corporation
- Designed to be platform- and domain-independent
“The SUS is not tied to any specific type of technology or user group, which is one of the reasons for its enduring popularity.” — Jim Lewis, Human Factors Researcher, IBM
Structure of the SUS Questionnaire
The System Usability Scale consists of 10 statements, each followed by a five-point Likert scale ranging from “Strongly Disagree” (1) to “Strongly Agree” (5). The odd-numbered items are positively worded, while the even-numbered ones are negatively worded, helping to reduce response bias.
Here are the 10 standard SUS questions:
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.Notice how questions 2, 4, 6, 8, and 10 are negatively phrased..
This counterbalancing helps prevent users from mindlessly agreeing with all statements, increasing the validity of responses..
Why SUS Stands Out Among Usability Metrics
There are many ways to measure usability—task success rates, time-on-task, error counts, Net Promoter Score (NPS), and more. But what makes the System Usability Scale unique is its balance of simplicity and depth. Unlike observational metrics, SUS captures the user’s subjective perception, which is often just as important as objective performance.
It’s also remarkably efficient. Administering the SUS takes less than 10 minutes, requires no special training, and can be deployed at any stage of development. Whether you’re testing a prototype or a live product, SUS provides a consistent benchmark.
- Quick to administer and score
- Validated across cultures and languages
- Produces a single, easy-to-interpret score
According to research published in the Journal of Usability Studies, the SUS has demonstrated high internal consistency (Cronbach’s alpha typically above 0.9), making it one of the most reliable subjective usability scales available.
How to Calculate the System Usability Scale Score
One of the biggest advantages of the System Usability Scale is its straightforward scoring method. Despite its academic roots, anyone can calculate a SUS score with basic arithmetic. The result is a single number between 0 and 100, where higher scores indicate better perceived usability.
Step-by-Step Scoring Process
Scoring the SUS involves a few simple steps:
- For odd-numbered questions (1, 3, 5, 7, 9): Subtract 1 from the user’s response (which ranges from 1 to 5).
- For even-numbered questions (2, 4, 6, 8, 10): Subtract the user’s response from 5.
- Sum the adjusted values across all 10 questions.
- Multiply the total by 2.5 to convert it to a 0–100 scale.
Let’s walk through an example. Suppose a user responds to the 10 SUS items with the following answers: [4, 1, 5, 1, 5, 1, 5, 1, 5, 1]
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Applying the formula:
- Q1 (odd): 4 – 1 = 3
- Q2 (even): 5 – 1 = 4
- Q3 (odd): 5 – 1 = 4
- Q4 (even): 5 – 1 = 4
- Q5 (odd): 5 – 1 = 4
- Q6 (even): 5 – 1 = 4
- Q7 (odd): 5 – 1 = 4
- Q8 (even): 5 – 1 = 4
- Q9 (odd): 5 – 1 = 4
- Q10 (even): 5 – 1 = 4
Total sum = 3 + 4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 = 41
Multiply by 2.5: 41 × 2.5 = 102.5 → capped at 100
Final SUS score: 100 (excellent usability)
“The beauty of the SUS scoring system is that it’s transparent and reproducible. There’s no black box—just clear math.” — Sauro, J., & Lewis, J.R. (2016), Measuring Usability
Common Scoring Mistakes to Avoid
While the SUS scoring method is simple, mistakes are common—especially when automating surveys or using third-party tools. Here are the most frequent errors:
- Forgetting to reverse-score even-numbered items: This is the most common mistake. If you don’t reverse the even items, your scores will be artificially low.
- Not capping the score at 100: While rare, some calculations can exceed 100 due to rounding. Always cap the final score at 100.
- Using incorrect Likert scale anchors: The SUS assumes a 1–5 scale with specific labels. Changing the labels (e.g., “Very Poor” to “Excellent”) can affect interpretation.
- Averaging individual item scores instead of calculating the total: The SUS is not an average of 10 items. It’s a transformed sum.
Always double-check your scoring logic, especially if using a custom-built survey tool or spreadsheet.
Interpreting SUS Scores: What Do the Numbers Mean?
A SUS score alone doesn’t tell the full story—it needs context. However, researchers have established useful benchmarks over the years. According to data compiled by Jeff Sauro and James R. Lewis, here’s how to interpret SUS scores:
- Below 50: Poor usability. Users are likely frustrated and may abandon the system.
- 50–69: Marginal or acceptable. There are noticeable usability issues, but the system is usable with effort.
- 70–79: Good. The system is reasonably easy to use, with minor pain points.
- 80–89: Excellent. Users find the system intuitive and efficient.
- 90–100: Outstanding. Rare in practice, but indicates near-perfect usability.
The average SUS score across thousands of studies is around 68, which falls into the “acceptable” range. Anything above 70 is considered above average.
For example, a mobile banking app scoring 85 would be seen as highly usable, while a legacy enterprise software scoring 45 would likely require significant redesign.
Advantages of Using the System Usability Scale
The System Usability Scale has stood the test of time for good reason. Its widespread adoption is not just due to historical inertia—it offers tangible benefits that make it indispensable in UX research and product development.
Speed and Simplicity in Data Collection
One of the biggest advantages of the System Usability Scale is how fast it is to administer. Unlike longitudinal studies or complex cognitive walkthroughs, SUS can be completed in under 10 minutes. This makes it ideal for:
- Guerrilla usability testing
- Remote user testing
- Iterative design sprints
- Post-task feedback in usability labs
Because it’s so lightweight, teams can collect SUS data from dozens of users in a single day, enabling rapid iteration. It’s also easy to integrate into existing workflows—simply add it as the final step after a usability task.
For startups and agile teams, this speed is a game-changer. Instead of waiting weeks for insights, they can get immediate feedback on whether a new feature improves or harms usability.
Reliability and Validity Across Domains
Despite its simplicity, the System Usability Scale is backed by decades of empirical research. Studies have confirmed its high internal consistency, test-retest reliability, and construct validity.
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For instance, a meta-analysis by Bangor, Kortum, and Miller (2008) reviewed over 100 studies and concluded that the SUS is a “robust, reliable, and valid tool” for measuring perceived usability. It performs consistently across:
- Different languages and cultures
- Age groups and technical expertise levels
- Device types (desktop, mobile, kiosk, wearable)
- Industry sectors (healthcare, finance, education, e-commerce)
This cross-context reliability makes SUS a trusted benchmark. If you’re comparing a telehealth app in Germany to a fintech platform in Japan, SUS provides a common language for usability.
“The SUS has been translated into over 30 languages and maintains its psychometric properties across cultures.” — Kortum, P., & Sorber, M. (2015), Universal Access in the Information Society
Cost-Effectiveness for Teams of All Sizes
Unlike proprietary usability tools that require expensive licenses, the System Usability Scale is completely free to use. There are no fees, no restrictions, and no need for permission. This open-access nature has fueled its global adoption.
Small design teams, freelancers, and nonprofits can leverage SUS without budget constraints. Even large enterprises use it as a standardized metric across departments.
Moreover, because it doesn’t require specialized software, SUS can be deployed using free tools like Google Forms, Typeform, or even paper surveys. This accessibility ensures that usability measurement isn’t limited to well-funded organizations.
Limitations and Criticisms of the System Usability Scale
While the System Usability Scale is powerful, it’s not without limitations. Understanding its weaknesses is crucial for using it effectively and avoiding misinterpretation.
Lack of Diagnostic Detail
One of the most common criticisms of the SUS is that it tells you how usable a system is, but not why. A low score indicates problems, but it doesn’t pinpoint which features or interactions are causing frustration.
For example, if a user gives a SUS score of 40, you know usability is poor—but is it due to confusing navigation, slow load times, or unclear error messages? The SUS alone can’t answer that.
To overcome this, experts recommend combining SUS with qualitative methods like:
- Think-aloud protocols
- Post-test interviews
- Observational notes
- Heatmaps and session recordings
By pairing SUS with qualitative insights, you get both the “what” and the “why” of usability issues.
Sensitivity to Context and User Expectations
The SUS measures perceived usability, which can be influenced by factors outside the system itself. For instance:
- A user’s prior experience with similar systems
- Their mood or fatigue level during testing
- Their expectations based on brand reputation
- The difficulty of the tasks they were asked to perform
This means that two identical systems could receive different SUS scores simply because one was tested with more experienced users or more challenging tasks.
To mitigate this, it’s important to standardize testing conditions as much as possible. Use the same task scenarios, instructions, and environment across participants.
Not Designed for Formative Feedback
The System Usability Scale is best suited for summative evaluation—assessing a system after it’s been used. It’s less effective during early design phases when the goal is to identify specific interaction problems.
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For formative testing, methods like cognitive walkthroughs, heuristic evaluation, or iterative prototyping are more appropriate. SUS can still be used, but its value increases when the system is relatively stable.
As Sauro and Lewis note, “SUS is great for tracking progress over time, but it shouldn’t be the only tool in your usability toolkit.”
Best Practices for Administering the System Usability Scale
To get the most accurate and actionable results from the System Usability Scale, it’s essential to follow best practices in administration, timing, and data interpretation.
When and How to Deploy SUS
The timing of SUS administration significantly impacts its validity. It should be given immediately after the user completes a set of representative tasks with the system. Delaying the survey risks memory decay and reduced accuracy.
Ideal scenarios for deploying SUS include:
- After a usability test session
- At the end of a product onboarding flow
- Following a key user journey (e.g., checkout, registration)
- Post-update or post-redesign to measure impact
It’s also important to ensure that users have had enough interaction with the system to form a meaningful opinion. Administering SUS after only 30 seconds of use will yield unreliable data.
Ensuring Consistent Question Wording
While the SUS is flexible, altering the wording of its questions can compromise its validity. Even small changes—like replacing “system” with “app” or “website”—can affect how users interpret the items.
To maintain consistency, always use the original 10-item questionnaire. If you must adapt it for clarity (e.g., in a mobile context), document the changes and be cautious when comparing scores to benchmarks.
For example, instead of changing the word “system,” you can add a clarifying instruction: “In this survey, ‘system’ refers to the mobile app you just used.”
“Stick to the standard wording. Deviations may seem minor, but they can introduce bias.” — Brooke, J. (1996), Usability.gov
Sample Size and Statistical Confidence
How many users do you need to get a reliable SUS score? While there’s no fixed rule, research suggests that even small samples (5–10 users) can provide useful directional insights.
However, for higher confidence—especially when comparing versions or making go/no-go decisions—larger samples are better. With 15–20 users, you can achieve a margin of error of around ±10 points at a 90% confidence level.
Keep in mind that SUS scores are typically not normally distributed, so non-parametric statistics (like medians and interquartile ranges) are often more appropriate than means and standard deviations.
Comparing SUS to Other Usability Metrics
The System Usability Scale is just one of many tools available for measuring usability. Understanding how it compares to alternatives helps you choose the right method for your goals.
SUS vs. Net Promoter Score (NPS)
Both SUS and NPS are short, quantitative surveys, but they measure different things. NPS asks, “How likely are you to recommend this product to a friend?”—focusing on loyalty and satisfaction. SUS, on the other hand, focuses specifically on perceived ease of use.
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While NPS is great for measuring customer advocacy, it’s less sensitive to usability issues. A user might love a brand but still find its app hard to use. That’s where SUS shines.
In practice, many teams use both: SUS to diagnose usability problems and NPS to track overall customer satisfaction.
SUS vs. UMUX (Usability Metric for User Experience)
The UMUX is a shorter alternative to SUS, consisting of only four items. It was designed to correlate highly with SUS while being even quicker to complete.
Studies show that UMUX scores are highly correlated with SUS (r > 0.9), making it a viable option when brevity is critical. However, because it’s newer and less validated, SUS remains the gold standard for most applications.
UMUX-Lite, a two-item version, is even more concise but sacrifices some reliability.
SUS vs. Task Success and Performance Metrics
Objective metrics like task success rate, time-on-task, and error count provide direct evidence of usability. If users can’t complete a task, it doesn’t matter how they feel about the system.
However, these metrics don’t capture the user’s emotional experience. A user might complete a task successfully but still feel frustrated or confused. That’s where SUS complements performance data by adding the subjective layer.
The most comprehensive usability evaluations combine both: objective performance + subjective perception (via SUS).
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 worldwide to improve real products and services.
Healthcare and Medical Devices
In healthcare, usability can be a matter of life and death. The FDA recommends using SUS to evaluate the usability of medical devices, including infusion pumps, glucose monitors, and electronic health record (EHR) systems.
For example, a study published in Applied Ergonomics used SUS to compare two EHR interfaces. The version with better workflow design scored 82 on SUS, while the legacy system scored 54—clear evidence of improved usability.
High SUS scores can also support regulatory submissions by demonstrating that a device is safe and easy to use.
E-Commerce and Digital Platforms
Online retailers use SUS to optimize checkout flows, product search, and account management. A low SUS score on the checkout page might indicate that users are abandoning carts due to complexity.
For instance, an e-commerce company redesigned its mobile app and saw the SUS score jump from 61 to 78. This 17-point increase correlated with a 22% reduction in cart abandonment.
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By tracking SUS over time, digital teams can quantify the impact of design changes and prioritize improvements.
Government and Public Services
Government websites often serve diverse populations, including older adults and people with limited digital literacy. SUS helps ensure these platforms are accessible and easy to use.
The UK’s Government Digital Service (GDS) uses SUS as part of its “digital by default” strategy. After redesigning a tax filing portal, they reported a SUS score of 85—well above the benchmark—indicating high usability across user groups.
Public sector organizations also use SUS to meet accessibility standards and improve citizen satisfaction.
What is a good System Usability Scale score?
A score of 68 is average. Anything above 70 is considered good, 80+ is excellent, and below 50 indicates significant usability problems.
Can I modify the SUS questionnaire?
It’s not recommended. Altering the wording or structure can affect reliability. If you must adapt it, document changes and avoid comparing to standard benchmarks.
Is the System Usability Scale free to use?
Yes, the SUS is completely free and in the public domain. No permission or licensing is required.
How many users do I need for a reliable SUS score?
As few as 5 users can provide useful insights, but 15–20 users offer greater statistical confidence, especially for comparisons.
Can SUS be used for mobile apps?
Absolutely. SUS is platform-agnostic and widely used for mobile apps, websites, software, and hardware interfaces.
system usability scale – System usability scale menjadi aspek penting yang dibahas di sini.
The System Usability Scale remains one of the most powerful, practical tools in the UX researcher’s arsenal. Its simplicity, reliability, and versatility make it indispensable for measuring perceived usability across industries. While it has limitations—particularly in diagnostic depth—it excels at providing a quick, standardized snapshot of user experience. When combined with qualitative insights and objective metrics, SUS enables teams to make data-driven decisions that improve real-world usability. Whether you’re a solo designer or part of a large organization, mastering the SUS can transform how you evaluate and enhance your digital products.
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