Insurance sizing is a replacement problem: if an income stops, what has to be paid anyway? Mortgage and debts, the years of living costs until dependants are independent, education, funeral expenses — less whatever savings and existing cover already handle.
The calculators here give an order of magnitude, not a quote. Underwriting, medical history and occupation move real premiums far more than any formula on this page, and the sums assured they suggest are a starting point for a conversation with a broker rather than a substitute for one.