How is the 5×5 risk matrix used?

The calculation

Risk score = Likelihood (1–5) × Severity (1–5) — a result between 1 and 25. The method is widespread because it is simple: two questions, one number.

What has to be said: the bands are not normative

The most frequently skipped fact about this method is that there is no international normative source for the band limits. Sentences like “above 15 is high risk” come not from a standard but from commercial guides, and those guides disagree. The scale definitions — what does likelihood 3 mean? — are left to the organisation as well.

So what should be done

1. The scale definitions are written down: each point is defined in concrete terms (“expected more than once a year”, “permanent incapacity”).
2. The bands are set according to the organisation's size and the complexity of its activities.
3. The bands are approved by top management — because “acceptable risk” is a management decision, not an engineering one.
4. The decision is documented and reviewed periodically.

The known weakness of a multiplicative matrix

Different cells give the same score: 5×1 and 1×5 are the same number, but one is “rare and fatal” and the other “frequent and trivial”. A list that ranks them together loses sight of the fatal one. High-severity lines are therefore assessed separately regardless of score.

Choosing the method

The matrix is quick for hazard screening; process safety, ergonomics and chemical exposure have methods of their own, and the matrix does not replace them.