Likelihood and impact

The National Risk Register 2025 says it "assesses the likelihood and impact for each risk". Likelihood is "the percentage chance of the reasonable worst-case scenario occurring at least once in the assessment timescale", scored from 1 to 5, where 5 means a greater than 25 per cent chance and 1 less than 0.2 per cent. Impact is scored across dimensions including fatalities and casualties, essential services, economic damage, the environment, security and international standing, each from 0 to 5 on the scope, scale and duration of harm, then combined into one score. The two scores are plotted on a matrix with logarithmic scales so that very different risks can be compared.

Regulation 14 of the Contingency Planning Regulations 2005 lets a Minister of the Crown issue guidance or an assessment to a Category 1 responder on "the likelihood of a particular emergency or an emergency of a particular kind occurring" and the extent of the damage it would cause. A responder must take the guidance into account and adopt the assessment.

The Met Office uses the same pair. Its warnings are coloured yellow, amber or red "depending on a combination of both the impact the weather may have and the likelihood of those impacts occurring".