System Usability Scale

The System Usability Scale (SUS) is a standardized questionnaire that measures the perceived usability of a product or system. As one of the most widely used usability assessment tools, this 10-item questionnaire provides a quick, reliable way to evaluate how users experience your digital product.

What is the System Usability Scale?

The System Usability Scale (commonly abbreviated as SUS) was developed by John Brooke in 1986 while working at Digital Equipment Corporation. Despite its age, it remains a gold standard for measuring perceived usability across various interfaces and products.

As a usability questionnaire, SUS offers several key advantages:

• It's quick and easy to administer (just 10 questions)
• It provides reliable results with small sample sizes (8-12 participants)
• It effectively differentiates between usable and unusable systems
• It generates a single, easy-to-understand score
• It works across different types of interfaces and products

A SUS score ranges from 0 to 100, with higher scores indicating better usability. The industry average is around 68, with scores above 80 generally considered excellent.

How the System Usability Scale Works

The SUS questionnaire consists of 10 statements that participants rate on a 5-point scale from "Strongly Disagree" (1) to "Strongly Agree" (5). The statements alternate between positive and negative phrasing to reduce response biases.

The 10 standard SUS statements are:

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 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.

Calculating the SUS Score

To calculate the overall SUS score:

1. For odd-numbered questions (positive statements), subtract 1 from the user's response
2. For even-numbered questions (negative statements), subtract the user's response from 5
3. This converts all values to a scale from 0 to 4, with 4 being the most positive response
4. Add all converted responses together and multiply by 2.5
5. This gives you a final SUS score between 0 and 100

Interpreting SUS Scores

While SUS produces a numerical score, it's helpful to translate this into more meaningful terms:

• Below 50: Poor usability, significant issues need addressing
• 50-68: Below average, improvements needed
• 68-80: Good usability, above average
• 80-90: Excellent usability
• Above 90: Exceptional, best-in-class experience

Some researchers also convert SUS scores to letter grades:

• 90+ = A (Excellent)
• 80-89 = B (Good)
• 70-79 = C (Acceptable)
• 60-69 = D (Borderline)
• Below 60 = F (Poor)

Best Practices for Using the System Usability Scale

To get the most value from SUS questionnaires:

✅ Administer after task completion: Have participants complete specific tasks with your interface before filling out the SUS questionnaire.

✅ Maintain standard wording: Changing the wording can affect reliability. If you must adapt it (e.g., changing "system" to "website"), be consistent.

✅ Combine with qualitative feedback: SUS tells you if there's a usability problem but not what it is. Pair it with interviews or open-ended questions.

✅ Track scores over time: Use SUS as a benchmark to measure improvement across iterations.

✅ Compare across similar products: SUS scores are most valuable when compared to alternatives or earlier versions.

✅ Use consistent testing environments: Maintain similar conditions for all participants to ensure valid comparisons.

Common Mistakes When Using SUS

❌ Changing the question order or wording significantly: This undermines the validity of the results.

❌ Focusing only on the numerical score: The score alone doesn't tell you what to fix.

❌ Testing with too few participants: While SUS works with smaller samples, aim for at least 8-12 participants for reliable results.

❌ Administering too early in the design process: SUS works best with functional prototypes or products that users can meaningfully interact with.

❌ Misinterpreting the scale: Remember that 68 is considered average, not 50.

❌ Cherry-picking results: Report all findings honestly, even if scores are lower than expected.

How Card Sorting Complements SUS Testing

While the System Usability Scale measures overall usability perception, card sorting helps you understand the specific information architecture issues that might be affecting those perceptions.

For example, if your SUS scores indicate poor usability, a card sorting exercise can help identify if the problem lies in how information is organized. By having users sort content into categories that make sense to them, you can:

1. Discover organizing principles that align with user expectations
2. Identify confusing terminology or categorization
3. Validate or improve navigation structures that may be hurting usability scores

Consider this workflow:

1. Run a card sort to inform your information architecture
2. Build your prototype based on those insights
3. Test with SUS to measure overall usability
4. Iterate based on findings

This combination helps ensure you're not only measuring usability problems but actively identifying and fixing their root causes.

Take Action with System Usability Scale

Ready to measure how usable your interface is? The System Usability Scale offers a scientifically validated way to quantify user perceptions with minimal effort. Start by integrating SUS into your next usability test to establish a baseline score, then use those insights alongside card sorting exercises to create interfaces that truly meet user needs and expectations.