What is the body mass index?
The BMI is an attempt to quantify the amount of tissue mass (muscle, fat, and bone) in an individual and the categorize that person as underweight, normal weight, overweight or obese based on that value. BMI is a screening tool that can indicate whether a person is underweight or if they have a healthy weight, excess weight, or obesity. If a person's BMI is outside of the healthy range, their health risks may increase significantly. The body mass index is universally expressed in kg/m2, resulting from mass in kilograms and height in meters. The formula is stated as weight of the individual in kilograms divided by the height2 of the individual in meters. When using imperial units, the formula is: BMI = lbs x 703/in2
Categories of BMI values
A BMI of less than 18.5 indicates that you are underweight, so you may need to put on some weight. You are recommended to ask your doctor or a dietitian for advice. A BMI of 18.5-24.9 indicates that you are at a healthy weight for your height. By maintaining a healthy weight, you lower your risk of developing serious health problems. A BMI of 25-29.9 indicates that you are slightly overweight. You may be advised to lose some weight for health reasons. You are recommended to talk to your doctor or a dietitian for advice. A BMI of over 30 indicates that you are heavily overweight. Your health may be at risk if you do not lose weight. You are recommended to talk to your doctor or a dietitian for advice.
| CATEGORY | BMI (Kg/m2) | BMI PRIME | ||
|---|---|---|---|---|
| from | to | from | to | |
| Very severely underweight | - | 15 | - | 0.6 |
| Severely underweight | 15 | 16 | 0.6 | 0.64 |
| Underweight | 16 | 18.5 | 0.6 | 0.74 |
| Normal (Healthy Weight) | 18.5 | 25 | 0.74 | 1 |
| Overweight | 25 | 30 | 1 | 1.2 |
| Obese Class I (Moderately obese) | 30 | 35 | 1.2 | 1.4 |
| Obese Class II (Severely obese) | 35 | 40 | 1.4 | 1.6 |
| Obese Class III (Very severely obese) | 40 | 45 | 1.6 | 1.8 |
| Obese Class IV (Morbidly Obese) | 45 | 50 | 1.8 | 2 |
| Obese Class V (Super Obese) | 50 | 60 | 2 | 2.4 |
| Obese Class VI (Hyper Obese) | 60 | - | 2.4 | - |
The BMI values for children
However, measuring BMI in children and teens is slightly different. Girls and boys develop at different rates and have different amounts of body fat at different ages. For this reason, BMI measurements during childhood and adolescence take age and sex into consideration.
It is calculated in the same way as for adults, but then compared to typical values for other children of the same age. Instead of comparison against fixed thresholds for underweight and overweight, the BMI is compared against the percentile for children of the same sex and age.
A BMI that is less than the 5th percentile is considered underweight and above the 95th percentile is considered obese. Children with a BMI between the 85th and 95th percentile are considered to be overweight. BMI is used differently for children. It is calculated in the same way as for adults, but then compared to typical values for other children of the same age. Instead of comparison against fixed thresholds for underweight and overweight, the BMI is compared against the percentile for children of the same sex and age.
A BMI that is less than the 5th percentile is considered underweight and above the 95th percentile is considered obese. Children with a BMI between the 85th and 95th percentile are considered to be overweight.
Doctors and other health professionals do not categorize children by healthy weight ranges because:
- they change with each month of age
- male and female body types change at different rates
- they change as the child grows taller
Doctors calculate BMI for children and teens in the same way as they do for adults, by measuring height and weight. Then they locate the BMI number and person's age on a sex-specific BMI-for-age chart. This will indicate whether the child is within a healthy range.
BMI chart for girls
BMI chart for boys
