The System Usability Scale, usually shortened to SUS, is a ten-item questionnaire that measures perceived usability. Respondents rate statements on a five-point agreement scale, and their answers are combined into a score from 0 to 100. The number is a scale score, not a percentage of tasks completed or people satisfied.
Developed by John Brooke, SUS is useful when a team needs a consistent overall measure after people have used a system. Its value is different from that of a usability recording: the score summarises a perception, while the recording can help explain the interaction that produced it.
Calculate the score in the correct direction
For the standard questionnaire, subtract 1 from responses to the odd-numbered, positively worded items. For the even-numbered, negatively worded items, subtract the response from 5. Add the ten resulting contributions and multiply by 2.5.
Each contribution therefore runs from 0 to 4. As a simple scoring check, a respondent choosing the middle option, 3, for every item contributes 2 on each, producing a total score of 50. Merely adding the original answers or forgetting to reverse the negative items produces a different and invalid calculation.
MeasuringU’s guide to SUS explains scoring and interpretation. Use the documented instrument and scoring procedure, and establish how missing answers will be handled before analysis. An incomplete response should not quietly be treated as though every item had been answered.
Interpret the reference group as carefully as the score
A score of 68 is often cited as an approximate average in published benchmark datasets. It is not a universal pass mark and does not mean 68% usability. A percentile or adjective label depends on the reference distribution and interpretation scheme used.
For a fictional internal purchasing system, the team may be more interested in whether perceived usability improves after a redesign than in comparison with unrelated consumer products. Keep the participants, tasks and administration sufficiently comparable, and report sample size and uncertainty rather than celebrating a small difference automatically.
A high score does not establish successful task performance, accessibility or demand for the product. A person may rate a familiar system favourably despite difficulties, while another may complete every task and still dislike the experience. Those differences are reasons to examine the evidence together.
Use SUS where an overall judgement is useful
Administer it after relevant use and before a detailed debrief that could reshape the participant’s account. Preserve the standard wording and response scale when relying on evidence or benchmarks for that version. Translations and documented adaptations need their own appropriate support; casual editing can change comparability.
Pair the result with usability testing and follow-up questions to investigate what should change. For perceived ease immediately after an individual task, the Single Ease Question provides a more focused measure. Individual SUS items should not be treated as a ready-made diagnostic map of particular screens or defects.
If questionnaire length is a constraint, UMUX and UMUX-Lite offer related shorter instruments with their own scoring and interpretation. Choose according to the research purpose and comparison needs. More items do not automatically provide a better explanation of the problem, and fewer items do not remove the need for careful measurement.