UX glossary
System Usability Scale
The System Usability Scale (SUS) is a short, standardized questionnaire with ten statements. It sums up how usable people perceive a product to be as a single score from 0 to 100.
Imagine a team that has just redesigned the customer portal of an energy provider. The tests went well, but management wants a number: is the new portal really easier to use than the old one? That’s exactly the kind of question the System Usability Scale, or SUS, is built for.
Explained simply
The SUS is a questionnaire with ten statements that participants fill out right after using a product. For each statement, they rate how much they agree on a scale from 1 (“strongly disagree”) to 5 (“strongly agree”). One of the statements reads: “I found the system unnecessarily complex.” The result is a single score between 0 and 100.
Think of it like the star rating after a taxi ride. It quickly tells you how satisfied someone was overall, but not whether it was the driving, the route or the music. John Brooke created the questionnaire in 1986 at the computer company Digital Equipment Corporation (DEC). He himself calls it “quick and dirty,” a fast and deliberately simple tool.
How to calculate the score
The odd-numbered statements are worded positively, the even-numbered ones negatively. For odd items, subtract 1 from the response. For even items, subtract the response from 5. Add up the ten results and multiply the total by 2.5. That’s your SUS score. A simple spreadsheet is all you need.
One important point: the score is not a percentage. Jeff Sauro of MeasuringU analyzed data from around 500 studies and found an average of 68. So a score of 68 is middle of the road, not “68 percent good.” Scores above roughly 80 put you in the top ten percent.
An example
Back to the energy provider. Before: the old portal gets an average SUS score of 58 from twelve participants, which is below average. After: once the redesign is done, twelve different participants complete the same tasks and the same questionnaire, and the new portal scores 77. The team can now show that perceived usability has clearly improved. Which spots still caused trouble, though, only becomes visible through the observations from usability testing.
Common misconceptions
The SUS is not a diagnostic tool. Brooke himself stresses that the individual statements aren’t meant to pinpoint specific problems. For that, you need observation or a heuristic evaluation. To learn more about what usability actually means, read What Is Usability?.
In practice
- Ask right after the test. Have people fill out the questionnaire before you discuss the tasks. Otherwise, the conversation colors their answers.
- Keep changes to the wording minimal. The benchmarks come from studies using the established wording. The more you rephrase, the less you can compare your scores against them.
- Compare instead of celebrating single scores. The SUS becomes meaningful in comparison: old version against new, or your product against the average.
Related terms
Usability Testing, Heuristic Evaluation, User Research