What Is Body Mass Index?
BMI (Body Mass Index) is a rough estimate of an individual’s dry body mass or obesity level based on height and weight. However, due to limitations, it should only be used as part of a patient’s overall health assessment. This means that a high BMI does not necessarily mean poor health, and a normal BMI does not necessarily mean poor health.
For adults aged 20 years and older, BMI is based on a standard weight that is the same for both men and women of all ages and body types.
The World Health Organization (WHO) recommends the following standard weight and BMI ranges for adults:
| BMI | Weight Status |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal/Healthy weight |
| 25 – 29.9 | Overweight |
| 30 – 39.9 | Obesity |
| Above 40 | Morbid |
For children and adolescents, BMI is based on age and gender, known as BMI-for-age. A high percentage of body fat in children can lead to weight-related problems and other illnesses. Being underweight is also not a good thing, as it can lead to nutrient deficiencies in the body, which can pose health risks.
According to the U.S. Centers for Disease Control and Prevention (CDC), the BMI ranges for children and adolescents ages 2 to 20 are as follows:
| BMI (percentile range) | Weight Status |
|---|---|
| Less than 5 | Underweight |
| 5 – 85 | Normal/Healthy weight |
| 85 – 95 | Overweight |
| Equal to/greater than 95 | Obesity |
How Is BMI Calculated Manually?
BMI is calculated the same way for both adults and children. Belgian mathematician Adolphe Quetelet devised the formula in 1830. This formula is usually expressed in kg/m². However, if you use English units (pounds and inches), an additional conversion factor is required.
Let’s look at two calculation methods: the metric formula and the English formula.
Calculate BMI in metric units:
Metric BMI formula
BMI = Weight (kg) ÷ Height 2 (m 2 )
Step 1 – Multiply by the key (meters). Convert measurements in centimeters to meters by dividing by 100.
Step 2 – Divide your weight (kilograms) by the value calculated in Step 1.
The following example will help you understand this better.
Consider your height as 180cm and weight as 75kg
Convert centimeters into meters – 180÷100 = 1.8m
BMI = 75 ÷ (1.8 × 1.8) = 75 × 3.24
Therefore, BMI = 23.15.
Calculating BMI in imperial units:
Imperial BMI formula
BMI = Weight (lb) ÷ height 2 (in 2 ) × 703
Step 1 – Multiply your key (in inches).
Step 2 – Divide the value from Step 1 by your weight (in pounds).
Step 3 – Multiply the value from Step 2 by 703.
The following example will help you understand this better.
Consider your height as 5 feet 11 inches and weight as 155 pounds.
Convert height into inches. One foot = 12 inches, therefore multiply 5 feet by 12 and add 11 inches to it. Therefore, your height in inches is ((5 x 12) + 11 =)) 71.
BMI = (155 ÷ (71 × 71)) × 703 = (155 ÷ 5041) × 703
BMI = 0.030747867 × 703
Therefore, BMI = 21.62.