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Short answer
It is a standardized, quick questionnaire of 10 alternating positive and negative questions that measures perceived usability on a total score from 0 to 100.
Explanation
For odd-numbered (positive) questions, subtract 1 from the user's rating. For even-numbered (negative) questions, subtract the user's rating from 5. Add the results and multiply by 2.5 to get the final score. The global SUS average is 68. A score above 68 indicates acceptable, good usability (grade C or higher); a score of 80 or higher indicates an exceptional, very easy-to-use product (grade A).
Common mistakes
- Treating the SUS score as a direct percentage; 68 is not 68%, but average performance at the 50th percentile.
- Changing the wording of the 10 standardized questions, making results less statistically reliable and globally comparable.