Notes: This calculator is based on research data pulled together by a team of researchers at the London School of Hygiene and Tropical Medicine.
Using UN data on population size in 177 countries, together with estimates of global weight from the WHO and mean height from national health examination surveys the team were able to calculate average BMI figures for each country.
Using the values that you input into the calculator, it works out your BMI as well as where you are in relation to the rest of the population in your country and the world for your gender and age.
As with all large data sets, there is a margin of error in this calculation, something statisticians call "standard deviation". This is indicated on the graph by the faint band either side of your indicated BMI measure.
* Your range is a reference to the "standard deviation from the mean", in other words your result is likely to vary within these boundaries.
http://www.bbc.co.uk/news/health-18770328

Don't know how accurate it is given the problems with BMI calculation but a fun idea.