How Stroop scores the SUS
The System Usability Scale (SUS) is a ten-item questionnaire about how usable a product felt, answered on a five-point scale and scored from 0 to 100. John Brooke published it in 1996.
Updated . Describes scoring version 2026.10.0, the version stored with every score Stroop computes. All method notes
Half the items are worded positively and half negatively, which is why odd and even items score in opposite directions [1]. The score is not a percentage, and a 50 is well below average.
The procedure
- Collect all ten answers, each from 1 (strongly disagree) to 5 (strongly agree), in the standard item order.
- For each odd-numbered item (1, 3, 5, 7, 9), subtract 1 from the answer.
- For each even-numbered item (2, 4, 6, 8, 10), subtract the answer from 5.
- Add the ten results. The total runs from 0 to 40.
- Multiply by 2.5. The score runs from 0 to 100.
- If any of the ten answers is missing, there is no score for that participant.
A worked example
One participant from the sample checkout study on the usability page answered:
| Item | Answer | Rule | Item score |
|---|---|---|---|
| 1 | 4 | answer minus 1 | 3 |
| 2 | 2 | 5 minus answer | 3 |
| 3 | 4 | answer minus 1 | 3 |
| 4 | 2 | 5 minus answer | 3 |
| 5 | 4 | answer minus 1 | 3 |
| 6 | 2 | 5 minus answer | 3 |
| 7 | 4 | answer minus 1 | 3 |
| 8 | 2 | 5 minus answer | 3 |
| 9 | 4 | answer minus 1 | 3 |
| 10 | 2 | 5 minus answer | 3 |
The item scores add up to 30. Times 2.5, that's a SUS of 75.
Across all 10 participants the scores were 75, 85, 62.5, 77.5, 65, 85, 62.5, 77.5, 47.5, 77.5. Stroop reports their mean, 71.5.
The grade and the percentile
Beside the mean, Stroop shows a letter grade from the curved scale in Sauro and Lewis (2016) [3], and an approximate percentile. The study above gets a C+ and sits at about the 61th percentile.
The percentile treats SUS scores as normally distributed with a mean of 68 and a standard deviation of 12.5, the benchmark MeasuringU publishes from its pooled studies. Bangor, Kortum and Miller (2008) [2] found a similar centre, a mean near 70 across more than 2,300 surveys. Read the percentile as a rough guide: the benchmark mixes every kind of product, and a banking app and an internal admin tool don't compete for the same score.
| Grade | SUS from | SUS to |
|---|---|---|
| A+ | 84.1 | 100.0 |
| A | 80.8 | 84.0 |
| A- | 78.9 | 80.7 |
| B+ | 77.2 | 78.8 |
| B | 74.1 | 77.1 |
| B- | 72.6 | 74.0 |
| C+ | 71.1 | 72.5 |
| C | 65.0 | 71.0 |
| C- | 62.7 | 64.9 |
| D | 51.7 | 62.6 |
| F | 0.0 | 51.6 |
What changes the score
Use all ten items in the standard wording. Dropping items or rewording them makes the score impossible to compare with anything, including your own last round. If ten items is too many for the study, Stroop also scores UMUX-Lite, which has two, and converts it to a SUS-equivalent with the published regression (0.65 times the UMUX-Lite score, plus 22.9).
Used on Usability testing. Stroop is the all-in-one research and testing platform. Run usability tests, prototypes, surveys, tree tests, diaries and IATs in one study, and get results back transcribed, scored and charted, with every finding linked to the session it came from.
Sources
- [1]Brooke, J. (1996). SUS: a “quick and dirty” usability scale. In P. W. Jordan, B. Thomas, B. A. Weerdmeester and I. L. McClelland (eds), Usability Evaluation in Industry, 189 to 194. London: Taylor and Francis.
- [2]Bangor, A., Kortum, P. T. and Miller, J. T. (2008). An empirical evaluation of the System Usability Scale. International Journal of Human-Computer Interaction, 24(6), 574 to 594.
- [3]Sauro, J. and Lewis, J. R. (2016). Quantifying the User Experience: Practical Statistics for User Research, 2nd edition. Cambridge, MA: Morgan Kaufmann. Curved grading scale.