Your Free System Usability Scale (SUS) Calculator

The SUS Calculator allows you to quickly determine the usability of a system or product by answering 10 standardized questions.

The System Usability Scale (SUS), developed by John Brooke in 1995, is a widely recognized tool for measuring usability in human-computer interaction.

How It Works

• Answer the 10 usability-related questions.
• Click Submit to calculate your SUS score.
• View your score, which provides a high-level assessment of usability.

While SUS provides a broad usability assessment, it should be used alongside other user experience metrics such as:

• Qualitative feedback (user comments, surveys)
• Task completion rates (success/failure rates)
• Error rates (frequency of user mistakes)

Since SUS is a one-dimensional tool, it is useful for comparing different systems but may not capture detailed usability challenges without further qualitative analysis.

For accurate interpretation, always consider the specific context, user population, and system complexity, rather than relying solely on numerical scores.

Interpreting SUS Scores Accurately

• 0-39: Unacceptable (Not Usable) – Users struggle significantly with the system, indicating severe usability issues that require major redesigns.
• 40-52: Poor Usability – The system has significant usability issues, making it frustrating for users. Improvements are essential.
• 53-73: OK / Marginal Usability – The system is usable but not ideal. Users may experience occasional difficulties, and improvements would enhance the experience.
• 74-85: Good Usability – The system is user-friendly with only minor usability concerns. Most users can complete tasks with ease.
• 86-100: Excellent Usability – The system is highly intuitive and easy to use. Users experience minimal friction when completing tasks.

🔹 Important Notes:

• A SUS score of 68 is considered average based on industry benchmarks.
• Scores above 80 are in the top 10% of usability scores and generally indicate a great user experience.
• The SUS scale isn’t a simple percentage; it’s best understood in relation to industry benchmarks rather than a 0-100 grading system.

Here's how you can apply the SUS score in a project:

Define the purpose: Determine why you want to measure the system's usability using the SUS score. It could be to identify areas for improvement, compare different versions of the system, or evaluate the system against industry benchmarks.

Prepare the questionnaire: Create the SUS questionnaire by including the 10 statements that participants will rate on a scale of 0 to 4, where 0 represents strong disagreement and 4 represents strong agreement. The statements should cover aspects such as ease of use, learnability, efficiency, and satisfaction.

Recruit participants: Identify the target user group for your system and recruit participants who represent this group. Aim for a sample size of at least 20-30 participants to ensure statistical reliability. Participants should be familiar with the system or have enough exposure to provide meaningful feedback.

Conduct the usability test: Administer the SUS questionnaire to the participants after they have interacted with the system. It's best to conduct the test in a controlled environment where you can observe their interaction and provide any necessary clarifications.

Calculate individual SUS scores: For each participant, calculate their individual SUS score using the formula: (Sum of the scores for odd-numbered questions - 5) × 2.5. This step normalizes the scores to achieve a range of 0-100.

Analyze the results: Once you have collected all the SUS scores, calculate the mean score for the entire group. The higher the SUS score, the better the usability of the system. Additionally, you can analyze the individual responses to identify specific areas that need improvement.

Interpret the findings: Interpret the SUS score in the context of your project's goals. If the score is below average, consider conducting further user research or usability testing to pinpoint specific issues. If the score is high, celebrate your success and use the feedback to enhance the system even further.

Repeat the process: If you plan to make iterative improvements to the system, it's advisable to conduct usability tests and measure the SUS score periodically. This will help you track the progress of the system's usability over time.

Remember that the SUS score provides a quantitative measure of usability, but it should be complemented with qualitative feedback from participants to gain a comprehensive understanding of the system's strengths and weaknesses.

Brooke, J. (1996). SUS – A quick and dirty usability scale. Usability Evaluation in Industry, 189(194), 4-7.

Bangor, A., Kortum, P., & Miller, J. (2009). Determining what individual SUS scores mean: Adding an adjective rating scale. Journal of Usability Studies, 4(3), 114-123.

Sauro, J. (2011). A Practical Guide to the System Usability Scale: Background, Benchmarks, & Best Practices. Denver, CO: Measuring Usability LLC.

Lewis, J. R., & Sauro, J. (2009). The factor structure of the System Usability Scale. Lecture Notes in Computer Science, 5619, 94-103.

Tullis, T. S., & Stetson, J. N. (2004). A comparison of questionnaires for assessing website usability. Proceedings of the Usability Professionals Association Conference, 1-12.