SUS calculator: the System Usability Scale

Calculate your SUS score

How to interpret the SUS score

The SUS score runs from 0 to 100, but it isn’t a percentage. 70 points don’t mean the product is 70% usable. The score only gains meaning when compared with the scores of other products.

In a 2011 MeasuringU article, Jeff Sauro gives the average from 500 studies: 68. A score above 68 counts as above average.

For a finer reading, I use the grade scale by Sauro and Lewis. The grade band in the calculator comes from this table. The percentile shows what share of the compared products your score beats.

An example: a product with 72.5 gets a C+ and sits in the 60th to 64th percentile. On paper that doesn’t look bad. Even so, more than a third of products beat this score.

Method and formula

John Brooke developed the SUS in a usability programme at Digital Equipment Corporation. The copyright notice on the scale dates from 1986. Brooke wanted a measure that could be applied quickly. It still had to be reliable enough to compare versions.

Brooke chose the 10 statements from a pool of 50. Half usually drew strong agreement, the other half strong disagreement. That way, the person has to read each statement and think about it.

That is why the odd statements are worded positively and the even ones negatively.

Brooke describes the calculation in four steps:

1. For statements 1, 3, 5, 7 and 9, the contribution is the marked value minus 1.
2. For statements 2, 4, 6, 8 and 10, the contribution is 5 minus the marked value.
3. Each contribution lies between 0 and 4. Add up the ten contributions.
4. Multiply the sum by 2.5. The result is the SUS score from 0 to 100.

In Brooke’s own example, the sum of the contributions is 22. 22 × 2.5 = 55, so the score is 55. In the calculator, each statement’s contribution appears right below it.

Brooke adds a warning. The values of individual statements mean nothing on their own. Don’t interpret averages of single statements in your report; give the one SUS score.

The original wording and free use

The statements on this page are Brooke’s original English wording. They speak of “the system” because the scale was built for office software. According to MeasuringU, the SUS has since been tested on hardware, consumer software, websites, cell phones, IVRs and even the yellow pages.

I keep the wording as it is. If you change the statements, your score is no longer directly comparable with the average of 68 and the grade table.

The scale is free to use. Brooke writes that the SUS has been made freely available, and that the only prerequisite is that any published report should acknowledge the source of the measure.

How to use the SUS questionnaire

Give the questionnaire after the person has had a chance to use the system. According to Brooke, it should be filled in before any debriefing or discussion begins. I usually put it at the end of a usability test.

Ask the person to mark their immediate response to each statement. All statements must be filled in. If they can’t answer a statement, Brooke says they should mark the centre point, that is 3.

The “Mark empty statements as 3” button in the calculator does exactly that.

According to MeasuringU, the SUS can be used with as few as two people. But the estimate from a small sample is imprecise, so report the score with a confidence interval.

Calculate each person’s score separately, then take the average. If you repeat the same questionnaire for each version, you can compare versions. That is what Brooke built the scale for.

My note

The SUS measures how users perceive usability, but it doesn’t say why. A score of 55 shows there is a problem. Which one, you find out with other methods.

That is why I never use the SUS on its own. In the test I watch the person and take notes, and I put the questionnaire at the end. If the score is low, I look for the reasons in those notes and in a heuristic evaluation.

The biggest mistake, to me, is sending the SUS to people who have never used the product. The questionnaire measures the reaction of someone who has just used the product. Anyone who has never opened it is only guessing.

If your score is below 68 and you can’t find the reason, I go through every step of up to 10 flows in your application with the UX Audit Pro.