What is BMI?

It is a simple index commonly used to classify overweight and/or obesity in adults.

BMI Calculator
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Definition and Classification of Obesity

According to the World Health Organization (WHO), obesity is defined as abnormal or excessive fat accumulation that may impair health

There are different ways to assess and measure obesity, and the most common measure is the body mass index (BMI) which is an inexpensive and easy assessable tool. BMI is calculated by dividing the person’s weight in kilograms by their height in metres squared

BMI can then be used to classify the person within groups ranging from underweight to obese.

Pre-obesity is defined as a BMI of between ≥25 kg/m2 and <30 kg/m2. Obesity is classified as a BMI of 30 kg/m2 or more, and is split into three classes of increasing severity:

  • Obesity class I with a BMI of ≥30 and <35 kg/m2
  • Obesity class II with a BMI of between ≥35 and <40 kg/m2
  • Obesity class III with a BMI of over kg/m2

Additional Information

Of note, the BMI cut-offs for Asian ethnicities are lower than those presented here, with overweight classed as 23–27.5 kg/m2 and obesity as ≥27.5 kg/m2

  • Obesity is defined as abnormal or excessive fat accumulation that may impair health
  • Body mass index (BMI) provides the most convenient population-level measure of overweight and obesity currently available
weight (kg)height (m2)
Classification BMI (kg/m2)
International 1 Asian population 2,3,4
Underweight <18.5
Normal range ≥18.5 and <25 ≥18 and <23
Pre-obesity* ≥25 and <30 ≥23 and <25
Obesity ≥30 ≥25
Obesity class I ≥30 and <35
Obesity class II ≥35 and <40
Obesity class III ≥40

*Previously described as overweight BMI, body mass index

  1. WHO. Factsheet . Accessed on June 2019;
  2. Misra, A., et al. J Assoc Physicians India 2009; 57:163–70;
  3. Kubota, Y., et al., J Epidemiol, 2015. 25(8): 553-8;
  4. Ota, T., et al., Diabetes Care, 2002. 25(7): 1252-3.

Waist Circumference

Measured midway between the lower rib margin and iliac crest

Gender Increased
disease risk
disease risk
Males ≥94 cm ≥102 cm
Females ≥80 cm ≥88 cm


What is wrong with BMI alone?

BMI should be considered a rough guide because it may not correspond to the same degree of illness in different individuals, especially those who exercise a lot or in the elderly. Furthermore, it provides no information on how the excess weight is affecting the individual’s health, function and wellbeing. This is where something like the Edmonton Obesity Staging System may be of assistance. Other useful measures include, but are not limited to:

  • Waist circumference measurement
  • Body composition via bioelectrical impedance study
  • Body composition via DEXA scan
Anthropometric Predictors of Bio-Impedance Analysis (BIA) Phase Angle in Healthy Adults
bmi and waist circumference thresholds

Guidance and Resources

Learn more about the science behind obesity and access tools to support patient weight management:


Recent NICE Guidance on Obesity

Obesity: NICE Pathway


Obesity: Identification, assessment and management


Obesity in children and young people: prevention and lifestyle weight management programmes


Obesity in adults: prevention and lifestyle weight management programmes


Obesity: working with local communities


Other NICE guidance can be searched at


Obesity Image Bank

The organisations listed below have each developed unique galleries containing images and videos showcasing realistic portrayals of people with obesity. All galleries are free resources which members of the media are invited to view and make use of in accordance with specified guidelines.

  • IFB Adiposity Diseases Centre Germany – Photo Gallery
    The IFB Adiposity Diseases centre is a joint centre of the Medical Faculty of the University of Leipzig and the University Hospital of Leipzig, bringing together both obesity research and treatment experts under one roof. Its photo gallery features a diverse range of images of people with obesity which are available to news publications upon request.
  • Canadian Obesity Network – Perfect at Any Size Gallery
    The Canadian Obesity Networks mission is to act as a catalyst for addressing obesity in Canada and as a platform to foster knowledge translation, capacity building and partnerships among stakeholders. Their Perfect at Any Size Gallery was developed to help promote accurate coverage of obesity-related topics in news reporting and challenge harmful weight-based stereotypes.
  • The UConn Rudd Center for Food Policy & Obesity – Media Gallery
    The Rudd Center for Food Policy & Obesity at the University of Connecticut is a distinguished multi-disciplinary policy research centre dedicated to improving the world’s diet, preventing obesity and reducing weight stigma. Their media gallery now contains several hundred still images and over 80 b-roll videos depicting non-stigmatizing portrayals of individuals with obesity.
  • The World Obesity Federation Image Portal
    The most recent addition to our range of high-quality external resources promoting positive, non-exploitative images of people with obesity in the media. This image bank resource is designed to improve awareness and understanding of practical, effective actions that can be taken to combat the obesity crisis.
  • The Obesity Action Coalition (OAC)
    A nearly 54,000 member-strong non-profit organisation based in the USA and dedicated to giving a voice to the individual affected by the disease of obesity and helping individuals along their journey toward better health through education, advocacy and support. OAC works to raise awareness and improve access to the prevention and treatment of obesity, provide evidence-based education on obesity and its treatments, fight to eliminate weight bias and discrimination, elevate the conversation of weight and its impact on health and offer a community of support for the individual affected.

We will endeavour to include additional imagery from a variety of third party groups and resources as this online portal grows.

  • The University of New South Wales
  • Obesity Australia
  • ANZMOSS – Australian & New Zealand Metabolic and Obesity Surgery Society
  • Australian and New Zealand Obesity Society
  • Royal Australian College of General Practitioners
  • Care Specialist
  • Strategic  Centre for Obesity Professional Education
  • World Obesity