System Usability Scale (SUS) and the usability of your website

When we design a digital interface, we always ask ourselves a key question that guides all our efforts: Is what we created easy for people to use? We know that an interface can have an attractive visual design, but if it’s not understandable or easy to use, it’s not fulfilling its purpose. That’s why, beyond design, evaluation plays a crucial role in our projects.

One tool we often use during validation phases is the System Usability Scale (SUS) — a simple yet powerful questionnaire that helps us clearly understand how people perceive the usability of a digital product.

Photo by Brett Jordan on Unsplash.

What is SUS?

The System Usability Scale was created by John Brooke in 1986 and has been widely used ever since to evaluate the usability of products and digital systems.

Its value lies in being quick to apply, resource-light, and providing a score that can easily be compared to a benchmark.

Over nearly four decades, it has become a reference tool for its reliability and simplicity, often described as a “quick and dirty scale to measure usability in your web”.

How does the SUS work?

The questionnaire consists of 10 statements that users respond to after interacting with a system. These statements reflect their experience, for example:

• “I would like to use this system frequently”
• “I found the system unnecessarily complex”
• “I think most people would learn to use this system quickly”

Each statement is rated on a scale from 1 to 5, where 1 means “strongly disagree” and 5 means “strongly agree”. Some statements are positive, others are negative, to avoid users responding automatically without thinking.

Once the responses are collected, a calculation transforms the results into a score between 0 and 100. This score does not represent a percentage, but serves as a reference for perceived usability.

How is the SUS score calculated?

The System Usability Scale calculation may seem complex initially but actually follows a very clear procedure.

Step-by-step calculation

• Review the responses: Each user answers 10 questions rated from 1 to 5.
• Separate odd and even questions: Odd questions (1, 3, 5, 7, 9) are positively phrased. Even questions (2, 4, 6, 8, 10) are negatively phrased.
• Transform the scores: For odd questions: subtract 1 from the user’s response. Example: if someone answered 4 in question 1, the transformed value will be 3 (4–1). For even questions: subtract the user’s response from 5. Example: if someone answered a 2 in question 2, the transformed value will be 3 (5–2).
• Sum all transformed scores.
• Multiply the total by 2.5 to adjust the final score to a range of 0–100.

Practical example

Suppose a user responds as follows:

Example of SUS score calculation

Sum of transformed values: 3 + 3 + 4 + 3 + 2 + 2 + 3 + 3 + 3 + 4 = 30

Multiply by 2.5: 30 × 2.5 = 75

Final SUS Score: 75

This normalization ensures that results are standardized and easy to interpret, balancing positive and negative statements and using a broader scale for clearer visual interpretation.

How to interpret the SUS score

• 0 to 50: Indicates serious usability issues. People have significant difficulties interacting with the system.
• 51 to 70: Acceptable but with room for improvement. It is useful for identifying specific areas for review.
• 71 to 100: Good usability, easy to understand and use for most users.

The global SUS average is 68.

• Scores below this indicate aspects to review and improve.
• Scores above this reflect a positive user experience.

Practical application in projects

At Torresburriel Estudio, we frequently apply the SUS during validation phases or when we need a quick usability diagnosis before implementing design improvements.

For example, in a recent project for an educational platform, we applied the questionnaire after a moderated testing session. The result: 74, giving us confidence to move forward with the proposed solution while identifying navigation adjustments that further improved the experience.

Thanks to its ease of application and reliable results, the SUS is an extremely effective tool. Its simplicity allows for quick integration at various stages of development without requiring complex tools.

How to apply the SUS step-by-step

1. Define evaluation objectives: What exactly are we validating? A redesign? A specific functionality? Overall perception?
2. Select representative participants: Ensure that respondents are similar to real users.
3. Administer the test after a real interaction: Users should test the system before responding.
4. Analyze results in context: Beyond the final score, observe where responses vary most — this often reveals critical insights.

In summary

The System Usability Scale is a tool we use frequently because of its simplicity and effectiveness in making informed decisions.

While it does not replace a comprehensive usability study, it is a quick and reliable method to assess how users perceive an interface.

If we aim to design clearer, more intuitive experiences aligned with user expectations, using methods like the SUS gives us a trustworthy compass to guide our work.

This is a translation of the following article from our corporate website:

El System Usability Scale (SUS) y la usabilidad de tu página web | Torresburriel Estudio