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SUS calculator: System Usability Scale score

Paste the answers of all respondents from a spreadsheet or fill in a single questionnaire. The calculator scores every response on the 0–100 SUS scale, averages them with a confidence interval, converts the result to a percentile and a curved letter grade, adds the adjective rating, splits usability from learnability and shows which of the ten items pull the score down.

● Free, no sign-upUpdated: September 16, 2026

Answers, one respondent per line10 answers from 1 to 5 in questionnaire order, separated by commas, semicolons, tabs or spaces. A header row and a first column with IDs are fine — copy straight from Google Sheets or Excel.

Replace one missing answer per respondent with 3 (neutral)

Confidence level

90 %95 %99 %

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SUS score—

Confidence interval—

Percentile—

Grade—

Usability—

Learnability—

Scores on the curved grading scale

respondentsmean and intervalaverage 68

Item breakdown

Mean answer and score contribution for each SUS item
Item	Statement	Mean 1–5	Contribution 0–4

Formula. Odd items: answer − 1; even items: 5 − answer; SUS = sum × 2.5. Interval: mean ± t_n−1 · s / √n. Grades and percentiles: Sauro–Lewis curved scale, where 68 is the 50th percentile.

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How to use it

Run the standard questionnaireShow all ten SUS statements in their original order with a 1–5 agreement scale, right after participants complete tasks with your product.
Paste the answersCopy the answer columns from your survey tool or spreadsheet, one respondent per row. Headers and an ID column are detected automatically; invalid rows are listed, not silently dropped.
Read score, interval and gradeThe mean SUS score comes with a confidence interval, a percentile rank against hundreds of studies, a curved letter grade and an adjective rating.
Find the weak itemsThe item table shows the mean answer and contribution of every statement, so you can see whether the problem is complexity, consistency, learnability or confidence.

What the System Usability Scale is and how it is scored

The System Usability Scale (SUS) is a ten-item questionnaire created by John Brooke at Digital Equipment Corporation in 1986 and published in 1996 as a "quick and dirty" usability scale. It became the most widely used standardized usability questionnaire: it is free, technology-agnostic, takes a minute to complete and works with small samples.

Participants rate each statement from 1 (strongly disagree) to 5 (strongly agree). Odd statements are positive ("I thought the system was easy to use"), even statements are negative ("I found the system unnecessarily complex"), which is why they are scored in opposite directions:

odd items 1, 3, 5, 7, 9: contribution = answer − 1 even items 2, 4, 6, 8, 10: contribution = 5 − answer SUS = (sum of 10 contributions) × 2.5 range 0–100 Example: answers 4 2 3 1 4 2 4 1 3 2 odd: 3 + 2 + 3 + 3 + 2 = 13 even: 3 + 4 + 3 + 4 + 3 = 17 SUS = 30 × 2.5 = 75

SUS is not a percentage. A score of 75 does not mean "75 % usable". To interpret it you need to compare it with other products, which is what percentiles and grades are for.

Several respondents

Each respondent gets a score, then scores are averaged. The confidence interval uses the Student t distribution because SUS samples are usually small:

mean = Σ SUS_i / n s = √( Σ (SUS_i − mean)² / (n − 1) ) interval = mean ± t(1 − α/2, n − 1) × s / √n

Research by Tullis and Stetson (2004) found that SUS reaches stable conclusions with 12–14 participants, faster than other questionnaires they compared. Even with 5 participants the score is informative, but the interval is wide — the calculator shows it so you do not over-read small differences.

Interpreting a SUS score: percentile, grade and adjectives

Jeff Sauro and James Lewis collected SUS data from hundreds of studies. The average score is 68, which corresponds to the 50th percentile. Their curved grading scale, based on 241 industrial studies, maps score ranges to letter grades and percentile ranges:

Grade SUS range Percentile A+ 84.1–100 96–100 A 80.8–84.0 90–95 A− 78.9–80.7 85–89 B+ 77.2–78.8 80–84 B 74.1–77.1 70–79 B− 72.6–74.0 65–69 C+ 71.1–72.5 60–64 C 65.0–71.0 41–59 C− 62.7–64.9 35–40 D 51.7–62.6 15–34 F 0–51.6 0–14

The calculator reports the grade of the mean and a percentile interpolated linearly within these ranges. So a SUS of 72.5 is a C+ at roughly the 65th percentile: slightly above average, not "72.5 % good".

Adjective ratings. Bangor, Kortum and Miller (2009) added a seventh question asking people to describe the product with an adjective, then looked at the mean SUS for each: worst imaginable 12.5, awful 20.3, poor 35.7, OK 50.9, good 71.4, excellent 85.5, best imaginable 90.9. The calculator shows the adjective whose mean is nearest. Bangor and colleagues also proposed acceptability ranges: below 50 not acceptable, 50–70 marginal, above 70 acceptable.

Usability and learnability

Lewis and Sauro (2009) found that items 4 and 10 — needing technical support and needing to learn a lot first — form a separate learnability factor. The calculator scales them to 0–100 (sum of the two contributions × 12.5) and the remaining eight items to a usability score (sum × 3.125). A product with good usability but low learnability usually needs better onboarding, not a redesign.

Running SUS well

Ask after tasks, not after a demo. SUS measures perceived usability of an experience; participants should have actually used the product first.
Keep the original wording and order. Changing statements, removing items or using a different scale breaks comparability with the benchmark. Replacing "system" with "website" or your product name is accepted practice.
Do not skip negative items. The alternating tone is part of the method. An all-positive version exists (Sauro and Lewis, 2011) and scores similarly, but do not mix versions within one study.
Handle missing answers explicitly. A common convention is to replace a single skipped item with the neutral 3; rows with more gaps should be excluded. The calculator lists skipped lines so you can decide.
Compare, don't just report. SUS is most useful for comparing versions, competitors or releases. If confidence intervals overlap heavily, the difference is not proven — check the sample size with the survey sample size calculator.
Pair the score with behavior. Low learnability and weak items 3, 7 and 10 often point to a first-run experience problem. Watching activation metrics after improving onboarding tells you whether the change worked.

SUS measures how usable people find the product; NPS measures whether they would recommend it. They correlate, but a product can be easy to use and still not worth recommending.

Sources

Brooke J. SUS: A "quick and dirty" usability scale. In: Jordan P. W. et al. (eds.) Usability Evaluation in Industry. Taylor & Francis, 1996.
Sauro J., Lewis J. R. Quantifying the User Experience: Practical Statistics for User Research, 2nd ed. Morgan Kaufmann, 2016 — curved grading scale, benchmark mean 68.
Bangor A., Kortum P., Miller J. Determining What Individual SUS Scores Mean: Adding an Adjective Rating Scale. Journal of Usability Studies, 4(3), 2009.
Bangor A., Kortum P. T., Miller J. T. An Empirical Evaluation of the System Usability Scale. International Journal of Human–Computer Interaction, 24(6), 2008 — acceptability ranges.
Lewis J. R., Sauro J. The Factor Structure of the System Usability Scale. HCD 2009, LNCS 5619 — usability and learnability subscales.
Tullis T. S., Stetson J. N. A Comparison of Questionnaires for Assessing Website Usability. UPA Conference, 2004 — sample size.

FAQ

How do you calculate a SUS score?

For odd items (1, 3, 5, 7, 9) subtract 1 from the answer; for even items (2, 4, 6, 8, 10) subtract the answer from 5. Add the ten contributions and multiply by 2.5. The result is a score from 0 to 100 for one respondent; average the scores for a group.

What is a good SUS score?

The benchmark average is 68, the 50th percentile. Scores above 80.8 get an A on the Sauro–Lewis curved scale and are in the top 10 %. Scores below 51.7 are an F, in the bottom 15 %. Above 70 is usually considered acceptable.

Is a SUS score a percentage?

No. SUS ranges from 0 to 100, but 68 is average, not 68 % usable. Convert the score to a percentile or a grade to interpret it: 68 is the 50th percentile, 80.3 is about the 89th.

How many participants do I need for SUS?

SUS gives usable results with small samples: Tullis and Stetson found stable conclusions from about 12–14 participants. With 5 you still get a score, but the confidence interval will be wide, so avoid comparing designs that differ by a few points.

Can I change the wording of the SUS questions?

Replacing "system" with "website", "app" or your product name is common and does not affect results noticeably. Rewording statements, dropping items or changing the 5-point scale breaks comparability with published benchmarks.

What do usability and learnability subscores mean?

Items 4 and 10 (needing technical support, needing to learn a lot first) measure learnability; the other eight measure usability. Both are rescaled to 0–100. A low learnability score with decent usability suggests people struggle to get started — an onboarding problem.

How should I handle missing answers?

A common convention is to replace one skipped item with the neutral answer 3 and to exclude respondents with more missing answers. Tick the option in the calculator to impute one missing answer per row; all excluded rows are listed.