Using scales for calculating risk

If you are using scale as High, Medium and Low for impact and Likelihood, syou can use a table like this:

                 Low          Medium     High
Low           LOW        LOW          MEDIUM
Medium     LOW        MEDIUM    HIGH
High          MEDIUM  HIGH         HIGH

In the table you just cross the impact with the likelihood (for example, columns are the impact, and rows are the likelihood), and in this way get the result of the risk. For example: I= High and L=High; Risk= HIGH, I=Medium and L=High; Risk=HIGH

Alternative (and simpler) way would be to use the following values: Low = 0, Medium = 1, and High =2; and addition as a way to calculate the risk. So if the I = 2, and L = 1, th en the Risk = 3.

Also this article can help you: How to write ISO 27001 risk assessment methodology https://advisera.com/27001academy/knowledgebase/write-iso-27001-risk-assessment-methodology/