How To Calculate Sus

Calculate the System Usability Scale (SUS) score for your product or service with this interactive tool. Follow the standard 10-question SUS methodology.

The System Usability Scale (SUS) is a standardized questionnaire developed by John Brooke in 1986 to measure the perceived usability of systems, products, or services. With its 10-item scale and proven reliability, SUS has become the industry standard for usability evaluation across various domains including software, websites, hardware, and consumer products.

Understanding the SUS Methodology

The SUS questionnaire consists of 10 statements that participants respond to using a 5-point Likert scale ranging from “Strongly disagree” (1) to “Strongly agree” (5). The questionnaire alternates between positive and negative statements to prevent response bias and ensure more accurate results.

The 10 Standard SUS Questions

1. I think that I would like to use this system frequently
2. I found the system unnecessarily complex
3. I thought the system was easy to use
4. I think that I would need the support of a technical person to be able to use this system
5. I found the various functions in this system were well integrated
6. I thought there was too much inconsistency in this system
7. I would imagine that most people would learn to use this system very quickly
8. I found the system very cumbersome to use
9. I felt very confident using the system
10. I needed to learn a lot of things before I could get going with this system

Scoring Mechanism

The SUS scoring process involves several steps:

1. Odd-numbered items (1, 3, 5, 7, 9): Subtract 1 from the scale position
2. Even-numbered items (2, 4, 6, 8, 10): Subtract the scale position from 5
3. Sum the scores from all 10 items
4. Multiply the total by 2.5 to convert to a 0-100 scale

Interpreting SUS Scores

SUS scores range from 0 to 100, with higher scores indicating better perceived usability. Here’s how to interpret the results:

Score Range	Adjective Rating	Acceptability	Grade
80.3+	Best Imaginable	Excellent	A+
71.4-80.3	Excellent	Good	A
62.7-71.3	Good	OK	B
51.7-62.6	OK	Poor	C
36.0-51.6	Poor	Awful	D
0-35.9	Worst Imaginable	Worst	F

Percentile Rankings

Based on research by Bangor, Kortum, and Miller (2008), here are the percentile rankings for SUS scores:

Percentile	SUS Score	Adjective Rating
99th	90.9	Best Imaginable
90th	80.8	Excellent
75th	73.3	Good
50th (Median)	68	OK
25th	52.8	Poor
10th	37.1	Awful
1st	12.5	Worst Imaginable

Advantages of Using SUS

• Standardized: Allows for benchmarking across different systems and studies
• Reliable: Proven to be consistent with repeated measurements
• Valid: Correlates well with other usability measures
• Sensitive: Can detect differences between systems
• Technology-agnostic: Works for any type of system or interface
• Quick to administer: Takes participants only about 2-3 minutes to complete
• Cost-effective: Free to use with no licensing requirements

Best Practices for Administering SUS

Before the Test

• Ensure participants have actually used the system being evaluated
• Use the exact wording of the 10 standard questions
• Randomize the order of questions if testing multiple systems to avoid order effects
• Decide whether to use paper or digital format based on your test setup
• Prepare a neutral introduction that doesn’t bias responses

During the Test

• Allow participants to complete the questionnaire without assistance
• Ensure participants understand the 5-point scale
• Encourage honest responses by emphasizing there are no right/wrong answers
• Collect demographic information if you plan to analyze results by user groups
• Consider including open-ended questions for qualitative insights

After the Test

• Calculate scores immediately to identify any data collection issues
• Analyze results both quantitatively (scores) and qualitatively (comments)
• Compare against industry benchmarks when available
• Look for patterns in responses to specific questions
• Present findings with both the numerical score and qualitative insights

Common Mistakes to Avoid

1. Modifying the questions: Changing the wording invalidates the standardized scoring
2. Using incomplete data: All 10 questions must be answered for valid results
3. Small sample sizes: Aim for at least 12-14 participants for reliable results
4. Ignoring context: Consider when and how the questionnaire was administered
5. Overinterpreting single scores: Look at trends and patterns rather than individual responses
6. Neglecting qualitative data: Combine SUS scores with user comments for deeper insights
7. Comparing dissimilar systems: Be cautious when comparing scores across very different products

Advanced SUS Applications

Longitudinal Studies

SUS is particularly valuable for tracking usability improvements over time. By administering the questionnaire at different stages of product development or after major updates, teams can quantitatively measure progress. For example, a software company might track SUS scores across three versions:

Version	Release Date	Avg. SUS Score	Change
1.0	Q1 2022	62	–
2.0 (UI Redesign)	Q3 2022	75	+13
3.0 (New Features)	Q1 2023	72	-3

Competitive Benchmarking

Organizations can use SUS to compare their products against competitors. For instance, an e-commerce platform might evaluate:

• Their own website: SUS = 78
• Competitor A: SUS = 72
• Competitor B: SUS = 65
• Industry average: SUS = 68

Segment Analysis

Breaking down SUS scores by user segments can reveal important insights:

User Segment	Avg. SUS Score	Sample Size
First-time users	65	42
Returning users	81	78
Power users	87	15
Mobile users	68	53
Desktop users	79	82

SUS in Academic Research

The System Usability Scale has been extensively studied and validated in academic research. Key findings include:

• Bangor, Kortum, and Miller (2008) analyzed 2,324 SUS responses across 206 studies, establishing reliable percentile rankings
• Lewis and Sauro (2009) demonstrated that SUS is sensitive enough to detect differences between systems with sample sizes as small as 12-14 participants
• Tullis and Stetson (2004) found that SUS correlates well with other usability metrics while being more sensitive to changes
• Research shows SUS works equally well for both expert and novice users across various domains

For those interested in the academic foundations of SUS, these studies provide valuable insights:

• US Government’s guide on SUS methodology
• Human Factors and Ergonomics Society resources on usability measurement
• ACM Digital Library for SUS research papers

Alternatives and Complements to SUS

While SUS is the most widely used usability questionnaire, other instruments can provide complementary insights:

UMUX (Usability Metric for User Experience)

A 4-item questionnaire that measures perceived usability and user experience. UMUX scores correlate highly with SUS (r = 0.92) but can be administered more quickly.

UMUX-LITE

An even shorter 2-item version of UMUX that maintains strong correlation with SUS (r = 0.88). Ideal for situations where brevity is critical.

UEQ (User Experience Questionnaire)

Measures both pragmatic (task-oriented) and hedonic (non-task-oriented) aspects of user experience across 26 items. Provides a more comprehensive view than SUS but takes longer to complete.

NASA-TLX (Task Load Index)

Assesses workload across six dimensions: mental demand, physical demand, temporal demand, performance, effort, and frustration. Useful when cognitive load is a primary concern.

Single Ease Question (SEQ)

A simple 7-point scale asking “Overall, how difficult or easy was the task to complete?” Best for evaluating specific tasks rather than overall system usability.

Implementing SUS in Your Organization

To successfully implement SUS in your usability testing program:

1. Start small: Begin with one product or feature to establish your process
2. Train your team: Ensure everyone understands how to administer and interpret SUS
3. Integrate with other metrics: Combine SUS with task success rates, time-on-task, and qualitative feedback
4. Create benchmarks: Establish internal baselines for comparison over time
5. Automate where possible: Use tools to calculate scores and generate reports
6. Share results widely: Make usability metrics visible to stakeholders
7. Act on findings: Use SUS data to prioritize improvements
8. Track over time: Monitor how changes affect usability perceptions

Future of Usability Measurement

As technology evolves, so do methods for measuring usability. Emerging trends include:

• AI-powered analysis: Natural language processing to analyze open-ended responses at scale
• Biometric integration: Combining SUS with eye-tracking, facial expression analysis, and other physiological measures
• Continuous measurement: In-product micro-surveys to collect usability data continuously rather than in lab settings
• Personalized benchmarks: Dynamic comparison against similar products/users rather than static norms
• Predictive modeling: Using SUS data to forecast business outcomes like adoption rates or support costs

Despite these advancements, SUS remains foundational because of its simplicity, reliability, and broad applicability. As Brooke originally intended, it provides a quick, dirty, and reliable way to measure usability that has stood the test of time.

Conclusion

The System Usability Scale is more than just a questionnaire—it’s a powerful tool for driving user-centered design decisions. By regularly measuring and tracking SUS scores, organizations can:

• Quantify the user experience of their products
• Identify specific usability problems
• Track improvements over time
• Benchmark against competitors
• Make data-driven design decisions
• Demonstrate the value of UX investments

Whether you’re a UX professional, product manager, or developer, understanding how to properly calculate and interpret SUS scores is an essential skill. The calculator provided at the top of this page gives you an easy way to compute scores, while this guide offers the context needed to use SUS effectively in your work.

Remember that while SUS provides valuable quantitative data, it should be combined with qualitative research methods for a complete understanding of your users’ experiences. The most successful products are those that continuously measure, learn from, and improve based on user feedback.