The body mass index (BMI) is the most commonly-used way of classifying overweight and obesity in adult populations and individuals.
BMI is defined as a person's weight in kilograms divided by the square of their height in meters (kg/m2).
Each BMI figure is classified within a range, eg 18-25 is ideal and over 30 is reckoned to be obese.
According to a survey of bodyshapes conducted in the UK in 1951, a woman's average waist size was 70cm (27.5in). A 3-D survey carried out by SizeUK in 2004 found the average woman had a waist measurement of 86cm (34in) and a BMI of 24.4, just inside the ideal range.
There was no comparative data for men in 1951, but the SizeUK survey showed the average man in 2004 had a waist of 94cm (37in) and a BMI of 25.2, technically just outside the ideal range.
But obesity is not just a problem for adults - the spread of obesity among children is also alarming experts.
At least 20 million children under the age of 5 years were overweight globally in 2005, according to the WHO.
Measuring children, aged 5 to 14 years, who are overweight or obese is challenging because there is not a standard definition of childhood obesity applied worldwide. Figures for children in England are shown here.
Childhood obesity is a big problem in the United States. The following graph shows the trend in a number of countries around the world.
Experts are worried that the increase in obesity will lead to more health problems as people who are overweight have a higher risk of heart disease, Type II diabetes and other diseases including some cancers.
As most data sources do not distinguish between Type I and II diabetes in adults, it is not possible to present the data separately. The map below shows the prevalence of diabetes throughout the world in 2007.
Even if the prevalence of obesity remains stable until 2030, the American Diabetes Association, says that the number of people with diabetes will more than double.
It says the increase may be "considerably higher" than this if, as expected, the prevalence of obesity continues to rise around the world.