That’s the simplest way. You stand on the scale and tell how many pounds you gain. But weight may be misleading because it related to body height and body composition.

Body Mass Index (BMI) is a simple index of weight-for-height that is commonly used to classify underweight, overweight and obesity in adults. It is defined as the weight in kilograms divided by the square of the height in metres (kg/m2). For example, an adult who weighs 70kg and whose height is 1.75m will have a BMI of 22.9. BMI = 70 kg / (1.75 m)2 = 70 / 3,0625 = 22.9 .

Table 1: The International Classification of adult underweight, overweight and obesity according to BMI

Source: Adapted from WHO, 1995, WHO, 2000 and WHO 2004.
BMI values are age-independent and the same for both sexes. However, BMI may not correspond to the same degree of fatness in different populations due, in part, to different body proportions. The health risks associated with increasing BMI are continuous and the interpretation of BMI gradings in relation to risk may differ for different populations.

In recent years, there was a growing debate on whether there are possible needs for developing different BMI cut-off points for different ethnic groups due to the increasing evidence that the associations between BMI, percentage of body fat, and body fat distribution differ across populations and therefore, the health risks increase below the cut-off point of 25 kg/m2 that defines overweight in the current WHO classification.

There had been two previous attempts to interpret the BMI cut-offs in Asian and Pacific populations3,4, which contributed to the growing debates. Therefore, to shed the light on this debates, WHO convened the Expert Consultation on BMI in Asian populations (Singapore, 8-11 July, 2002)5.

The WHO Expert Consultation5 concluded that the proportion of Asian people with a high risk of type 2 diabetes and cardiovascular disease is substantial at BMI's lower than the existing WHO cut-off point for overweight (= 25 kg/m2). However, the cut-off point for observed risk varies from 22 kg/m2 to 25 kg/m2 in different Asian populations and for high risk, it varies from 26 kg/m2 to 31 kg/m2 . The Consultation, therefore, recommended that the current WHO BMI cut-off points (Table 1) should be retained as the international classification.

PBF Cutoff Point
BMI methods has its own problem. you can not tell a muscular body builder is “fatty” than a moderate person, although he has higher BMI.
The most accurate way to determine obesity is on the basis of body composition. BMI cannot tell the difference between muscle, water or fat, the PBF(percent body fat) can. So we recommend PBF, the reflection of fat tissue, as the standard for identifying obesity. In Fact, there is no internatinal standard for BF cutoff point,in general, PBF >20 for man or PBF >30 for women is regarded with higher health risk.This is based on the result of various research, such as:

Abel Romero-Corral1, Virend K. Somers1, Justo Sierra-Johnson2, Yoel Korenfeld1, Simona Boarin3, Josef Korinek1, Michael D. Jensen1, Gianfranco Parati3 and Francisco Lopez-Jimenez1
Normal weight obesity: a risk factor for cardiometabolic dysregulation and cardiovascular mortality
European Heart Journal Volume31, Issue6 Pp. 737-746.

National Health and Nutrition Examination Survey
www.cdc.gov/nchs/nhanes.htm

Soren Snitker
Use of Body Fatness Cutoff Points
http://www.mayoclinicproceedings.com/content/85/11/1057.1.full