SUS (System Usability Scale): How to Use It, Score It, and Interpret It

By Philip Burgess | UX Research Leader

Measuring usability is a crucial step in designing products that users find easy and satisfying to use. The System Usability Scale (SUS) offers a quick, reliable way to evaluate usability across a wide range of systems and interfaces. This post explains how to use SUS, how to score it, and how to interpret the results to improve your product’s user experience.

SUS questionnaire printed on paper

What Is the System Usability Scale?

The System Usability Scale is a simple, ten-item questionnaire developed in 1986 by John Brooke. It provides a “usability score” based on users’ responses to statements about a product’s ease of use, complexity, and confidence in using it. SUS is technology-agnostic, meaning it can be applied to websites, software, hardware, mobile apps, and more.

Each question is answered on a five-point scale from Strongly Disagree (1) to Strongly Agree (5). The questions alternate between positive and negative statements to reduce bias.

How to Use SUS in Your Usability Testing

Using SUS is straightforward and fits well into most usability testing sessions. Here’s how to apply it:

• Prepare the questionnaire: Use the standard ten SUS statements. You can find official versions online or create a digital form.

• Administer after task completion: Ask participants to complete the SUS survey immediately after they finish using the product or prototype.

• Ensure clarity: Explain the scale clearly so users understand how to respond.

• Collect responses: Gather all completed questionnaires for scoring.

  
  

SUS works well with small sample sizes. Research shows that even five users can provide meaningful insights, making it efficient for early-stage testing.

Scoring the SUS Questionnaire

Scoring SUS involves a simple calculation that converts user responses into a score from 0 to 100. Follow these steps:

1. For odd-numbered questions (1, 3, 5, 7, 9), subtract 1 from the user’s response.

2. For even-numbered questions (2, 4, 6, 8, 10), subtract the user’s response from 5.

3. Sum all the adjusted scores.

4. Multiply the total by 2.5 to get the final SUS score.

   
   

For example, if a user answers question 1 with a 4, the adjusted score is 4 - 1 = 3. If they answer question 2 with a 2, the adjusted score is 5 - 2 = 3. Add all adjusted scores and multiply by 2.5.

The final score ranges from 0 (worst usability) to 100 (best usability). Keep in mind that SUS scores are not percentages but usability ratings.

Interpreting SUS Scores

Understanding what a SUS score means helps you decide where to focus improvements. Here’s a general guide:

• 85 and above: Excellent usability. Users find the product intuitive and easy to use.

• 70 to 85: Good usability. Some minor issues may exist but overall positive experience.

• 50 to 70: Average usability. Users may struggle with some features or find the interface confusing.

• Below 50: Poor usability. Significant problems likely affect user satisfaction and efficiency.

  
  

SUS scores can also be converted into letter grades or adjective ratings for easier communication:

| SUS Score | Grade | Adjective Rating |

|-----------|-------|--------------------|

| 90-100 | A | Excellent |

| 80-89 | B | Good |

| 70-79 | C | OK |

| 60-69 | D | Poor |

| Below 60 | F | Awful |

Use these categories as a starting point, but always consider the context of your product and users.

Practical Tips for Using SUS Effectively

• Combine SUS with qualitative feedback to understand why users gave certain scores.

• Use SUS repeatedly during development to track usability improvements over time.

• Compare SUS scores across different versions or competitor products to benchmark usability.

• Remember that SUS measures perceived usability, so it complements but does not replace task performance metrics.

  
  

Laptop screen displaying System Usability Scale results and charts

Final Thoughts on Using SUS

The System Usability Scale is a powerful tool for quickly assessing how users perceive your product’s usability. Its simplicity and reliability make it a favorite among UX professionals. By following the steps to use, score, and interpret SUS, you can gather actionable insights that guide design decisions and improve user satisfaction.