What is SUS in UX research?
The System Usability Scale is a standardized usability questionnaire developed by John Brooke at Digital Equipment Corporation in 1986. It consists of ten statements that alternate between positive and negative phrasing, each rated on a 5-point agree-disagree scale. After scoring, the responses are combined into a single score from 0 to 100. Example statements include "I thought the system was easy to use" and "I found the system unnecessarily complex." SUS is one of the most widely used usability measurement tools in practice because it is quick to administer, free to use, and has extensive benchmarking data for comparison.
How are SUS scores interpreted?
SUS scores are interpreted relative to an industry benchmark rather than as absolute values. A score above 68 is considered above average. Scores above 80 are considered good. Scores above 90 are considered excellent. The letter grade equivalents popularized by researchers Jeff Sauro and James Lewis provide an intuitive reference: A corresponds to scores above 90.9, B to 80-90.8, C to 68-79.9, D to 51-67.9, and F to below 51. These benchmarks are derived from a large dataset of SUS scores across many products and studies, making it possible to compare a product's score against the distribution of products that have been measured using the same tool.
When should you use SUS in UX research?
SUS is most useful for tracking perceived usability over time, for comparing the usability of different versions or alternatives, and for establishing a baseline before a redesign. It is typically administered after usability testing sessions to capture overall usability perception alongside behavioral observations. SUS measures perception, not performance: a product can score well on SUS while showing poor task completion rates in behavioral testing, or vice versa. Using SUS alongside behavioral metrics and qualitative feedback provides a more complete picture of usability than any single measure alone.