BMI Formula
Body mass index (BMI) is a measurement of a person's weight in relation to their height. It offers an inexpensive and simple method of categorising people according to their BMI value so that we can screen people’s weight category and indicate their potential risk for health conditions.
BMI formulas
There are two formulas to calculate BMI depending whether you’re using metric or imperial measurements.
Metric BMI formula
BMI = Weight (kg) / Height (m)²
To calculate BMI, the metric formula is your weight in kilograms divided by your height in meters squared.
For example
If you are 175cm (1.75m) in height and 75kg in weight, you can calculate your BMI as follows:
75kg / (1.75m²) = 24.49kg/m²
Imperial BMI formula
BMI = [Weight (lbs) / Height (inches)²] x 703
The imperial BMI formula is your weight in pounds (lbs) divided by your height in inches, squared and then you multiply this figure by a conversion factor of 703.
For example
If you’re 5’5” (65”) in height and 150lbs in weight, you would calculate your BMI as follows:
[150lbs / (65 inches²)] x 703 = 24.96
BMI weight categories
Based on your height and weight, you will be assigned to one of the following weight categories:
Underweight - BMI less than 18.5
Normal healthy weight - BMI between 18.5 and 24.9
Overweight - BMI between 25.0 and 29.9
Obese - BMI between 30.0 and 39.9
Morbidly obese – BMI 40.0 and above
Weight categories can be further classified. The table below is from the WHO and shows the international classification of adult underweight, overweight and obesity according to BMI¹
|
Classification |
BMI(kg/m2) |
|
|
|
Principal cut-off points |
Additional cut-off points |
|
Underweight |
<18.50 |
<18.50 |
|
Severe thinness |
<16.00 |
<16.00 |
|
Moderate thinness |
16.00 - 16.99 |
16.00 - 16.99 |
|
Mild thinness |
17.00 - 18.49 |
17.00 - 18.49 |
|
Normal range |
18.50 - 24.99 |
18.50 - 22.99 |
|
|
|
23.00 - 24.99 |
|
Overweight |
≥25.00 |
≥25.00 |
|
Pre-obese |
25.00 - 29.99 |
25.00 - 27.49 |
|
|
|
27.50 - 29.99 |
|
Obese |
≥30.00 |
≥30.00 |
|
Obese class I |
30.00 - 34.99 |
30.00 - 32.49 |
|
|
|
32.50 - 34.99 |
|
Obese class II |
35.00 - 39.99 |
35.00 - 37.49 |
|
|
|
37.50 - 39.99 |
|
Obese class III |
≥40.00 |
≥40.00 |
Online BMI calculator
For ease, you can use our automated BMI calculator. Simply tap in your weight and height details in either metric or imperial measurements and this tool will calculate your BMI. It also offers a useful reference of the BMI weight categories and their corresponding BMI values.
How to use BMI figures
BMI is a screening tool of your body fatness and your health. The higher your BMI the greater the indication of high body fatness and, excess body fat is known to be linked to both current and future morbidity.
If your BMI figure indicates you are a normal healthy weight that’s great news. You may want to monitor your BMI on a regular basis to keep abreast and check that you remain in this category.
If you have a high BMI and you fall into the overweight or obese categories, you should arrange an appointment with your doctor who may recommend further diagnostic tests to explore any health risks. These diagnostic tests might include: skinfold thickness and waist measurements, diet and physical activity evaluations, family history and, other appropriate health screenings. Your doctor may also offer lifestyle and diet advice to help lower your BMI.
If you have a low BMI and you are classified as underweight, then you may want to review your diet with a view to gaining some weight. Your doctor will be able to give you advice.
Limitations of BMI
Although there is a correlation between BMI and body fatness, there are limitations of BMI that have been greatly discussed in the medical field. These include
- Athletes and body builders who have a high proportion of muscle (muscle weighs more than fat) are categorised with a high BMI
- There are height and weight ratio differences between races.
- Age and sex are not taken into account. Women tend to have more body fat than men of equal BMI and older people tend to have more body fat than younger people with the same BMI
- BMI also does not account for pregnant or lactating women and, children and teenagers who are still growing and are not yet physically mature.
Health consequences of obesity in adults
Research has shown that people whose BMI falls into the overweight or obese BMI categories are more likely to experience health problems associated with excess weight.
These diseases and health conditions include: high blood pressure (hypertension), type 2 diabetes, coronary heart disease (CHD), stroke, musculo-skeletal problems, sleep apnoea and breathing problems and some cancers. Many of these are often preventable by engaging in a healthy and active lifestyle.
