Calculating excess deaths — deaths above and beyond the expected number of deaths if not for the new variable — is pretty straightforward, at least in countries that have good public statistics. For example, in the US, death totals tend to rise by an average of a little less than one percent per year, as the population as a whole, and the elderly population in particular, increases (This is somewhat offset by age adjusted mortality rates, which continue to decline as they largely have for the 120 years this statistic has been calculated, with striking exceptions for the two big pandemics during that time frame).
Thus the conservative estimate used by the WHO concludes that the US saw about 930,000 excess deaths during 2020 and 2021, while an estimate that just extrapolates from previous age adjusted mortality rate declines would put the number for these two years at around 1.05 million. But in either case, the number isn’t terribly far from the official number of COVID deaths over these two years (about 850,000), keeping in mind that not all COVID deaths are excess deaths statistically, although about 95% are.
But the situation is quite different in developing countries, in which COVID death totals in particular and public health stats in general aren’t as reliable. Thus the WHO estimates that the total number of excess deaths in India alone over the previous two years was about ten times the official COVID death total of around 470,000.
All in all, the WHO estimates nearly 15 million excess deaths globally over the previous two years, compared to six million official COVID deaths. It estimates that the large majority of the excess deaths are COVID deaths that weren’t recorded as such, but some are from spillover effects of the pandemic, such as strained health systems, people not seeking out treatment for other conditions because of fear of COVID infection etc.
A particularly tragic aspect of all this is the hundreds of thousands of excess deaths in the USA that are 100% a product of the right wing campaign to sow doubt about the COVID vaccines, in order to prolong the epidemic for political purposes. Morally speaking that campaign was and is a form of mass murder, although a mass murder for which no one will ever be held responsible.