System Usability Scale (SUS)

What is the System Usability Scale?

The System Usability Scale, or SUS, is a standardized questionnaire for measuring perceived usability. It has ten statements that participants rate on a five-point scale from "strongly disagree" to "strongly agree". Positive and negative statements alternate, so people can't just tick the same box all the way down.

John Brooke created the questionnaire in 1986. The answers are converted into a score between 0 and 100. That score is an index, not a percentage. As a rough guide, a result around 68 is considered average.

Why the SUS matters

Watching people use your product shows you where they get stuck. The SUS adds a number you can compare: between two design variants, across releases or against a competitor. It takes about two minutes to fill in, costs nothing and works just as well for SaaS tools, apps and online stores. That makes it an easy way to track usability over time and show stakeholders that changes actually made a difference.

How to use the SUS in practice

• Ask right after the session: Hand out the questionnaire once all tasks are done, while the experience is still fresh.
• Keep the wording: Use the established statements and only swap "system" for "website" or "app".
• Score it correctly: For odd-numbered items, subtract 1 from the answer. For even-numbered items, subtract the answer from 5. Multiply the total by 2.5.
• Measure before and after: Collect a baseline before a redesign and repeat the measurement afterwards under the same conditions.

Common mistakes

A frequent mistake is reading individual statements on their own. The SUS is designed as a single overall score. It also won't tell you why something is hard to use. For that, you need the observations from your usability test. And a score based on three responses is not a reliable benchmark yet.