System Usability Scale (SUS): A Quick Usability Score

WHY IT EXISTS Teams needed a fast, reliable, and cheap way to measure the subjective usability of a system. Instead of creating long, custom questionnaires for every study, the SUS provides a standardized "good enough" tool that gives a single number that can be tracked and compared over time.

THE MENTAL MODEL Think of SUS as a thermometer for usability. It gives you a single temperature reading (the score) that tells you if things are hot (unusable), cold (great), or just okay. It doesn't diagnose the specific illness, but it quickly tells you if the user has a fever, indicating that a deeper look is needed.

HOW IT WORKS A user answers 10 standard questions on a 5-point scale from "Strongly Disagree" to "Strongly Agree." The questions have alternating positive and negative phrasing (e.g., "I found the system easy to use" vs. "I found the system unnecessarily complex"). The responses are then converted through a specific formula—summing adjusted scores for each question and multiplying the total by 2.5—to produce a final score between 0 and 100.

WHEN TO USE IT Use SUS when you need a quantitative measure of usability with a small sample size; as few as five users can give a reliable score. It's great for comparing usability across different versions of your product or for benchmarking against competitors. Because it's a well-researched industry standard, your scores are immediately comparable to published benchmarks.

WHEN NOT TO USE IT Don't use SUS if you need to identify specific usability problems. The score tells you that users might be struggling, but not why or where. For that, you need qualitative methods like think-aloud usability testing. Also, never present the raw score to stakeholders without explaining that it's not a percentage, as a score of 70 (which sounds like a C-) is actually a good, above-average result.

ONE CANONICAL EXAMPLE A product team wants to measure the impact of a major UI redesign. They administer the SUS survey to a group of users on the old design and get an average score of 62. After launching the new design, they test with a similar group and the score jumps to 81. The team now has a quantitative metric showing their work resulted in a significant, measurable improvement in perceived usability, moving from below average to excellent.