System Usability Scale

The System Usability Scale (SUS) is a structured attitudinal survey for quantifying how users feel about the usability of a product, using 10 questions and resulting in a single numerical score. It is quick, robust and reliable. Because it provides a numerical score, it can be used for benchmarking, relative comparisons of different systems, to track trends and changes over time and to detect unexpected impacts.

The SUS scale method was developed by John Brooke, while working at the Digital Equipment Corporation in 1986. The survey structure was published in the 1996 compilation book Usability Evaluation In Industry.

Benefits of SUS

According to Andrew James Holyer, the great advantage of SUS is its “astonishing ease of use”.

Fitting easily on a single side of paper, it is quick and easy to administer, and since it is less intimidating than longer questionnaires a far higher percentage of users are likely to actually complete it - a serious issue when you consider that many more “serious” questionnaires constitute a substantial document in their own right. It is also extremely quick and easy to score… The SUS can be scored by hand in around 30 seconds.

Andrew James Holyer, Methods for evaluating user interfaces

Hoyler’s paper claims that University College Cork evaluated SUS and compared it to their much more comprehensive 50-question survey Software Usability Measurement Inventory, concluding that the results correlated with a reliability of 0.86, “which considering that it is far simpler and cheaper to administer is very impressive”.

SUS Survey Template

The SUS survey covers various aspects of system usability in 10 statements, including complexity, need for support and training. The survey participants rate each statements based on how strongly they agree or disagree with it, on a 5 point Likert Scale. The statements are somewhat redundant, and alternate between positive and negative, to intentionally reduce biases and variability.

The SUS survey template alternates between redundant positive and negative statements to reduce bias
# Question Strongly Disagree Strongly Agree
1 I think that I would like to use this system frequently 1 2 3 4 5
2 I found the system unnecessarily complex 1 2 3 4 5
3 I thought the system was easy to use 1 2 3 4 5
4 I think that I would need the support of a technical person to be able to use this system 1 2 3 4 5
5 I found the various functions in this system were well integrated 1 2 3 4 5
6 I thought there was too much inconsistency in this system 1 2 3 4 5
7 I would imagine that most people would learn to use this system very quickly 1 2 3 4 5
8 I found the system very cumbersome to use 1 2 3 4 5
9 I felt very confident using the system 1 2 3 4 5
10 I needed to learn a lot of things before I could get going with this system 1 2 3 4 5

To calculate the total usability score, subtract 1 from answers in positive questions (odd rows 1, 3, 4, 5, 9), and for the negative questions (even rows 2, 4, 6, 8, 10), subtract the answer from 5. This normalizes each row to the scale of 0 to 4. The final score is the sum of normalized row scores, multiplied by 2.5, giving it a range of 0 to 100.

As SUS survey format has been around for several decades, there was plenty of academic research evaluating that survey format, testing different combinations of questions and wording. In the book Quantifying the User Experience (2016), Jeff Sauro and James R. Lewis suggest that question 8 can be rephrased to use the word “awkward” instead of “cumbersome”, and that the word “system” can be replaced with “product” or “web site” to improve relatability, particularly for non-native English speakers.

Running SUS Surveys

Brooke suggests filling in the SUS survey after a full evaluation session (so SUS is used for post-test, not post-task evaluations), but before any discussion or debriefing. In 10 Things to Know About the System Usability Scale (SUS), Jeff Sauro suggest that even samples as small as 5 respondents can produce relevant results.

SUS benchmarks

Jeff Sauro and James R. Lewis provide some interesting benchmarks from a set of almost 500 usability studies in the book Quantifying the User Experience:

Category SUS Score (99% confidence)
B2B software 63-72
B2C software 69-78
Large public web sites 64-69
Internal software 71-82
Interactive voice systems 75-84

In the article 10 Things to Know About the System Usability Scale (SUS) Jeff Sauro suggests that “SUS scores predicts around 40% of why customers recommend software and websites” and compares SUS scores to Net Promoter Score categories:

Detractors have an average SUS score of 67 (slightly below average usability) and Promoters have an average score of 82 (well above average usability). In independent, large datasets, we’ve seen that you can estimate the Likelihood to Recommend question used in the Net Promoter Score (a 0 to 10 scale) by simply dividing the SUS by 10. For example, an SUS score of 72 would predict a LTR response of 7.2.

Jeff Sauro, 10 Things to Know About the System Usability Scale (SUS)

Learn more about the System Usability Scale

Alternatives to the System Usability Scale