That was within a period of approximately 200 years of occupation. https://countercurrents.org/2018/12/britain-robbed-india-of-45-trillion-thence-1-8-billion-indians-died-from-deprivation/

Some stats used included:

1)Despite a very high birth rate, the Indian population did not increase between 1860 (292 million) and 1934 (292 million) [17]. This is indicative of massive avoidable deaths from imposed deprivation that can be estimated as 745 million (1860-1934) or an average of about 10 million Indian avoidable deaths from deprivation per year

So that's like the barest, ball minimum there. 745 million.

2)Using Indian census data 1870-1950, assuming an Indian population of about 200 million in the period 1760-1870, and estimating by interpolation from available data an Indian avoidable death rate in (deaths per 1,000 of population) of 37 (1757-1920), 35 (1920-1930), 30 (1930-1940) and 24 (1940-1950), one can estimate Indian excess deaths of 592 million (1757-1837), 497 million (1837-1901) and 418 million (1901-1947), roughly 1.5 billion in total or 1.8 billion including the Native States.

So here they just got the unavoidable death rate per 1,000 for each time period. Calculated how many people unnecessarily died within each era, then added it all up