Body mass index (BMI) is a screening tool healthcare professionals use to assess a person’s weight relative to their height. Although it can help identify people who may be at risk of weight-related health problems, it does not provide a complete picture of an individual’s health.
BMI is calculated by dividing a person’s weight in kilograms by the square of their height in metres. The result is expressed in kilograms per square metre (kg/m²).
For adults aged 20 and older, a BMI below 18.5 is classified as underweight, 18.5 to below 25 as healthy weight, 25 to below 30 as overweight, and 30 or higher as obesity.
These categories are screening guidelines rather than definitive diagnoses. Healthcare professionals consider other factors, including medical history, blood pressure, cholesterol levels and physical examination findings, when assessing a person’s health.
How to calculate BMI for men and women
The BMI calculation is the same for adult men and women. It uses height and weight rather than separate formulas based on sex.
The formula is:
BMI = Weight (kg) ÷ Height (m)²
For example, a person weighing 70 kilograms and measuring 1.75 metres tall would have a BMI of approximately 22.9.
Online calculators can perform the calculation automatically. The US Centers for Disease Control and Prevention (CDC) provides an adult BMI calculator for people aged 20 and older.
The MealPro website also provides a BMI calculator that readers can consult.
BMI does not directly measure body fat. It also cannot distinguish fat from muscle and bone, or show where fat is distributed in the body. As a result, two people with the same BMI may have different body compositions and health risks.
Understanding BMI categories
For adults aged 20 and older, standard BMI categories are applied regardless of sex or age.
- Underweight: BMI below 18.5.
- Healthy weight: BMI from 18.5 to below 25.
- Overweight: BMI from 25 to below 30.
- Obesity: BMI of 30 or higher.
Obesity is further divided into three classes: class 1, with a BMI from 30 to below 35; class 2, from 35 to below 40; and class 3, with a BMI of 40 or higher.
These classifications do not mean that everyone in a particular category has the same health risks. A clinician may also consider waist circumference, physical activity, family history and existing medical conditions.
BMI is interpreted differently in children and teenagers because their bodies are still growing. Healthcare professionals use age- and sex-specific BMI percentiles rather than applying adult cut-offs directly.
Health risks associated with obesity
Excess body fat is associated with an increased risk of several chronic health conditions. These include high blood pressure, abnormal cholesterol levels, type 2 diabetes, cardiovascular disease, fatty liver disease and sleep-related breathing disorders.
Obesity can also be associated with osteoarthritis and certain cancers. During pregnancy, obesity is linked to a higher risk of complications, including gestational diabetes and pre-eclampsia, a condition involving high blood pressure that can threaten the health of the mother and baby.
These are increased risks, not inevitable outcomes. A person’s overall health depends on multiple factors, and BMI alone cannot establish whether someone has any of these conditions.
Family history can also influence the likelihood of developing obesity and related illnesses. Genetics, medication, sleep, dietary habits, physical activity and social and environmental conditions may contribute to weight changes.
How healthcare professionals assess obesity
BMI is commonly used as an initial screening measure because it is inexpensive, straightforward and easy to calculate. However, a healthcare professional should not rely on it alone when assessing an individual’s health.
An assessment may include waist circumference, blood pressure, medical history, physical examination and laboratory tests to check blood glucose and cholesterol levels.
Waist circumference can provide additional information about abdominal fat, which is associated with cardiometabolic risks, including type 2 diabetes and cardiovascular disease.
A BMI in the overweight or obesity range does not automatically establish the presence or severity of a medical condition. Similarly, a BMI within the healthy-weight range does not rule out health problems.
Anyone concerned about weight changes or a possible weight-related condition can consult a healthcare professional for an assessment based on their individual circumstances.
Treatment and management
Treatment depends on a person’s health, medical history, risk factors and individual needs. Where appropriate, healthcare professionals may recommend changes to eating habits, regular physical activity, nutritional counselling or a structured programme to support long-term health.
For some adults, prescription medicines may be considered alongside lifestyle changes. Weight-management surgery may also be an option for eligible patients, depending on clinical criteria and associated health conditions.
Treatment should be tailored to the individual rather than determined by a BMI figure alone. Medicines should be used under appropriate medical supervision, and patients should discuss potential benefits, side effects and other treatment options with their healthcare providers.
People with conditions such as diabetes, high blood pressure or high cholesterol may need additional treatment to manage those conditions alongside any weight-related concerns.
Using a BMI calculator
A BMI calculator can provide an initial estimate of weight status, but its result should be interpreted in the appropriate age group and clinical context.
For adults, the CDC’s online calculator provides a BMI value and its corresponding category. Children and teenagers should use an age-appropriate calculator and have the results interpreted using growth charts.
A calculator cannot diagnose obesity-related diseases or determine which treatment is appropriate. A healthcare professional can help interpret the result alongside other health indicators and recommend further assessment or treatment where necessary.
