System Usability Scale (SUS): questionnaire, calculation and interpretation

What is the System Usability Scale?

The System Usability Scale is a standardized questionnaire of ten statements that expresses the perceived usability of a system as a score between 0 and 100. John Brooke developed it in 1996 at Digital Equipment and called it “quick and dirty” himself.

That is not a put-down but the strength of the method: the SUS takes participants about two minutes, works equally well for software, websites, devices and machine controls, and has been used so often over decades that there is a reliable benchmark.

When does the SUS fit — and when not?

The SUS fits when you need a fast, comparable overall score for usability. Typical occasions:

• a baseline before a redesign that you measure against later
• comparing two versions or two products on the same scale
• a regular measurement point, for example after onboarding
• ruling something out: checking whether usability is the cause of a problem at all

The SUS does not fit when you need to know what the problem is. It does not distinguish between clarity, speed and dependability. UEQ+ is built for that. And it does not answer questions about motives or needs — that is what the Jobs-to-be-Done variants are for.

How is the SUS questionnaire structured?

Ten statements on a five-point agreement scale, alternating between positive and negative wording. The options range from “strongly disagree” (1) to “strongly agree” (5).

1. I think that I would like to use this system frequently.
2. I found the system unnecessarily complex.
3. I thought the system was easy to use.
4. I think that I would need the support of a technical person to be able to use this system.
5. I found the various functions in this system were well integrated.
6. I thought there was too much inconsistency in this system.
7. I would imagine that most people would learn to use this system very quickly.
8. I found the system very cumbersome to use.
9. I felt very confident using the system.
10. I needed to learn a lot of things before I could get going with this system.

The switch between positive and negative is intentional. It forces people to read every statement instead of ticking down one column. That is also why: Do not change the wording. The benchmark of 68 depends on everyone answering the same ten sentences.

How is the SUS score calculated?

For each participant, the responses are recoded, added up and multiplied by 2.5; only then is the average taken across everyone.

1. Odd statements (1, 3, 5, 7, 9): response minus 1
2. Even statements (2, 4, 6, 8, 10): 5 minus response
3. add up the ten values — result 0 to 40
4. multiply by 2.5 — result 0 to 100
5. average across all participants

SUS = ( Σ(odd − 1) + Σ(5 − even) ) × 2.5

An example for a single person with the responses 4, 2, 4, 1, 5, 2, 4, 2, 4, 3:

	Statements	Responses	Recoded	Sum
odd	1, 3, 5, 7, 9	4, 4, 5, 4, 4	3, 3, 4, 3, 3	16
even	2, 4, 6, 8, 10	2, 1, 2, 2, 3	3, 4, 3, 3, 2	15
total				31 × 2.5 = 77.5

calculate You do not have to calculate this by hand: the SUS calculator (in German) scores individual responses or an entire survey, with grade and confidence interval.

How do you interpret a SUS score?

68 points is the average, not a “satisfactory” grade. Jeff Sauro combined more than 500 published SUS studies and arrived at this mean. A score of 68 puts you in the middle of all measured systems.

Two more thresholds matter in practice: from 80.8 points you get grade A and with it a place among the top ten percent; below 51.7 points usability is no longer considered acceptable. The full grade scale and the typical misreadings are covered in the article Interpreting a SUS score (in German).

What matters most: The scale is relative. A single score without a before, without a competitor and without a comparison segment is a number without meaning.

Example: the SUS as a rule-out in checkout

In the Online shop case study (in German), seven out of ten customers abandon a full shopping cart. The third redesign of the checkout form is planned. The SUS runs alongside a JTBD Switch survey of 1,140 customers who abandoned their cart and delivers:

Metric	Value
SUS score of the checkout	78
corresponds to grade	B+
average	68
difference	+10

This rules out usability as the cause. The SUS found nothing, and that was exactly its job: the planned form redesign is canceled, and attention goes where JTBD Switch finds the problem — the unclear delivery time.

What are the limits of the SUS?

• No cause. The SUS does not show which screen, step or function is the problem.
• One score for everything. Clarity, speed, dependability and design all flow into a single number.
• Perception, not behavior. It measures how participants experience usability, not whether they actually complete their task.
• Noisy with small samples. A jump from 71 to 76 with 15 participants is noise, not progress.

How does the SUS work in usabiq?

The SUS is included in every plan, including Free Discovery, the free plan. The questionnaire is set up as a template, and participants take part via a link without an account.

• Scored without a language model. The SUS is calculated from 0 to 100 using the standard scoring, with correct recoding per participant before averaging.
• Combinable. As a rule-out or a complement to a method that asks about causes — as in the Online shop case study.
• Compare segments from Decision Pro: rating differences between groups are reported with statistical confidence. If a group is too small to support a conclusion, usabiq says so.
• One limitation: the CSV export does not cover the SUS yet.