SUS Calculator: questions & answers

The SUS is a 10-item questionnaire for measuring the perceived usability of a product, created by John Brooke in 1986. Participants rate ten statements on a five-point scale from strongly disagree to strongly agree. It’s short, technology-agnostic, and one of the most widely used and validated usability measures there is.

It doesn’t tell you what to fix, but it gives you a fast, comparable read on how usable people find an experience.

You collect the ten SUS answers from each participant, then either share a link to the questionnaire, enter responses by hand, or import a file. The tool scores every response and reports the overall SUS (0–100), a letter grade, the percentile against the industry benchmark, the Usability and Learnability sub-scores, and a per-question breakdown.

The odd-numbered statements are positive and the even-numbered ones are negative; the scoring accounts for that automatically, so a high score always means better usability.

The overall SUS captures perceived ease of use of a whole experience. It also splits into two established sub-scales:

• Usability — the eight items other than 4 and 10; how easy and coherent the system feels in general.
• Learnability — items 4 and 10; how easily people feel they can get up to speed without help.

Both are reported on the same 0–100 scale, and together they reconstruct the overall SUS exactly.

• Before or after a redesign, to see whether a change moved perceived usability.
• At the end of a usability test, to attach a number to the session.
• At regular intervals, to track usability over releases.
• To compare two designs, competitors, or segments on a common yardstick.

Best practice is to keep the validated statements as they are: changing them affects reliability and comparability with the benchmark. The one expected customization is swapping in your product referencefor “the system” (e.g. “I found the checkout unnecessarily complex”), which this tool does for you via the product name on the study.

• Not diagnostic: SUS tells you how usable the experience feels, not what to fix. Pair it with usability tests or interviews.
• It’s not a percentage: a SUS of 80 is not “80%”. It’s a point on a 0–100 scale interpreted against the benchmark.
• Context matters: a low score can reflect a hard task or a demanding domain, not just the interface.
• Small samples are noisy: report the number of participants alongside the score.

SUS is unusually robust at small sizes, but precision grows with your sample:

• 5–8 participants: a rough, formative read during a usability test. Interpret cautiously.
• 12–15 participants: a reasonably stable mean for internal decisions.
• 30+ participants: tighter confidence intervals, benchmarking, and comparisons over time.
• Two designs compared: aim for roughly 20+ per group to detect a real difference.

Always report the sample size next to the score — a mean from 6 people and a mean from 60 don’t carry the same weight.

Very. Across many published studies the SUS shows high internal consistency(Cronbach’s alpha around 0.90). This tool computes Cronbach’s alpha for your own responses so you can check whether your participants answered consistently. As a rough guide: ≥ 0.9 excellent, 0.8–0.9 good, 0.7–0.8 acceptable, and below 0.7 worth treating with caution.

It’s the range your true average is likely to sit in given your sample. A wide interval usually means too few participants or inconsistent answers; collecting more responses narrows it. Lean on the interval, not the single mean, when the stakes are high.

The benchmark average, from Sauro & Lewis’s analysis of 500 studies, is 68. So 68 is roughly the 50th percentile — average, not a pass mark. Above 68 is above average; the mid- to high-70s and up is where experiences are generally considered good.

This tool maps the average SUS to a grade:

The percentile places your score against the benchmark distribution of products. A SUS of 68 sits near the 50th percentile; higher scores climb the curve steeply — a small rise near the top can move you up many percentile points. It’s a useful way to say “better than X% of the products in the benchmark” rather than relying on the raw number alone.

Yes — the headline SUS is the mean of each participant’s individual score. Report it with the sample size and, when you can, the confidence interval. Avoid averaging the ten items first and scoring once; score each participant, then average the people.