What Is the System Usability Scale (SUS) and How Do You Score It?

If you have ever needed a quick, reliable way to measure how usable a product or website feels to real users, the System Usability Scale (SUS) is one of the most widely used tools in the industry. Developed by John Brooke in 1986, SUS gives you a single score from 0 to 100 that reflects overall perceived usability. It is short, easy to administer, and does not require a large sample to be informative.

What It Is

The SUS is a standardized questionnaire consisting of 10 statements about a product. Users respond to each statement on a 5-point Likert scale, from "Strongly disagree" (1) to "Strongly agree" (5). The statements alternate between positive and negative phrasing to reduce response bias. The result is a single numeric score that helps you compare usability across versions, products, or competitors.

Because Brooke published the scale openly, you can use it freely in both academic and commercial settings. The scoring rules are fixed, so results are comparable across studies.

How It Works: The Scoring Formula

The SUS score is not a simple average. You must apply the standard transformation:

1. For each of the 10 items, note the user's response (1 to 5).
2. For odd-numbered items (1, 3, 5, 7, 9): subtract 1 from the response.
3. For even-numbered items (2, 4, 6, 8, 10): subtract the response from 5.
4. Sum all 10 adjusted values.
5. Multiply the sum by 2.5.

The result is a SUS score between 0 and 100. A score above 70 is generally considered acceptable usability, while scores below 50 indicate serious problems.

A Worked Illustrative Example

Example data (illustrative only): Imagine one user gives the following responses to the 10 SUS items:

Item | Response (1–5)

• 1 | 4
• 2 | 2
• 3 | 5
• 4 | 1
• 5 | 3
• 6 | 4
• 7 | 4
• 8 | 2
• 9 | 5
• 10 | 3

Now apply the adjustments:

• Item 1 (odd): 4 − 1 = 3
• Item 2 (even): 5 − 2 = 3
• Item 3 (odd): 5 − 1 = 4
• Item 4 (even): 5 − 1 = 4
• Item 5 (odd): 3 − 1 = 2
• Item 6 (even): 5 − 4 = 1
• Item 7 (odd): 4 − 1 = 3
• Item 8 (even): 5 − 2 = 3
• Item 9 (odd): 5 − 1 = 4
• Item 10 (even): 5 − 3 = 2

Sum of adjusted values = 3 + 3 + 4 + 4 + 2 + 1 + 3 + 3 + 4 + 2 = 29.
Multiply by 2.5 → SUS score = 72.5.

In this illustrative case, the product would be considered usable (above 70). To get a robust result, you should average SUS scores across multiple users.

Common Pitfalls

• Do not average raw responses. You must apply the odd/even adjustment before summing, or your score will be meaningless.
• Do not treat SUS as a percentage. A score of 70 is not "70% correct"; it is a normalized index.
• Use consistent wording. The original scale uses specific phrasing; changing it can affect validity.
• Report sample size. SUS scores from a single user are not statistically reliable.

Get Started with a Free Tool

Manually calculating SUS for many respondents is tedious and error-prone. To save time and avoid mistakes, use the free SUS usability scoring calculator at https://www.6sq.com/tools/sus/. Enter the responses, and the tool applies the standard Brooke formula instantly, giving you a reliable score every time.