What Is BMI and What Is the Correct BMI for My Age?

BMI (body mass index) is a simple mathematical calculation based on only your height and weight. The correct BMI for your age is 5th percentile to less than the 85th percentile.

BMI (body mass index) is a simple mathematical calculation based on only your height and weight. The result of this calculation will tell you if you are at a healthy weight, underweight, or overweight. This will also warn you of the health risks you face in the future, enabling you to take corrective action. People with high BMI are overweight or obese and at high risk for stroke, heart disease, and some cancers. A high BMI also puts you at risk for death.

Think of your BMI as an early warning system. It doesn't tell you that you're sick but warns you are at increased risk for chronic disorders and death. A BMI in the unhealthy range should prompt you to consult your healthcare provider and take appropriate steps to reduce your risk. BMI does not measure the fat content of the body, but its predictive ability for long-term diseases is as good as many more sophisticated measurements.

BMI is an indicator of body mass. It uses your height and weight to determine if you are at a healthy weight. It can also warn you if you are underweight or overweight. BMI doesn't measure the fat content of your body but is a good substitute for the more difficult and expensive tools that do analyze body composition.

BMI takes various body shapes into account. It gives a range for healthy weight based on your height. It tells you if you are at risk for chronic diseases and premature death. But BMI isn't the only thing to consider. Heredity, lifestyle, habits, and other factors also influence health outcomes.

BMI Categories: 

Less than 18.5: underweight.

18.5 to 24.9: healthy weight

25 to 29.9: overweight

More than 30: obesity

You can calculate your body mass index if you know your height and weight. When weighing yourself, try to be in light clothing and bare feet, fasting, and with an empty bladder. If you're monitoring weight gain or loss, it's best to measure yourself at the same time of day.

To calculate your BMI, use one of these methods:

Divide your weight in pounds by the square of your height in inches, then multiply this by 703. If you're 5 feet 7 inches tall and weigh 155 pounds, your calculation would look like this: 

155 ÷ (67)2 × 703 = 24.27

Using the metric system, you can calculate your BMI by dividing your weight in kilograms by the square of your height in meters. If you're 170 centimeters tall and weigh 70 kilograms, your calculation would look like this: 

70 ÷ (1.70)2 = 24.22 

You can also use the CDC's Adult BMI calculator, which allows both metric and imperial measures. 

The BMI calculation is meant for adults over the age of 20 years. The categorization into healthy, underweight, and overweight is the same for adults of all genders and ages.

The BMI for children and adolescents uses the same calculation as for adults. But the numbers are interpreted differently since boys and girls differ in body fat composition. Body fat content also changes with age. 

For children and adolescents, the Centers for Disease Control and Prevention CDC) provides charts based on studies of thousands of American children. The categories are based on these percentiles rather than on the numbers used for adults:

Underweight: Less than the 5th percentile

Healthy weight: 5th percentile to less than the 85th percentile

Overweight: 85th to less than the 95th percentile

Obesity: Equal to or greater than the 95th percentile

The World Health Organization also provides age-appropriate information for children and adolescents based on a broader population.

What should you do if your BMI is over 25? Take it as a warning. You are at risk for several disorders that accompany overweight and obesity:

• Type 2 diabetes mellitus (T2DM)
• Hypertension (high blood pressure)
• Dyslipidemia (high blood levels of low-density lipoproteins and triglycerides and low levels of high-density lipoprotein)
• Heart disease
• Osteoarthritis (joint damage)
• Cancers of the breast, uterus, gallbladder, colon, and liver
• Sleep apnea
• Mental health issues such as anxiety and depression
• Gallbladder disease
• Reduced quality of life

A high BMI increases the risk of all causes of death. Women who never smoked but are overweight have a 13% higher risk of death than women with a BMI between 22.5 and 24.9. A BMI of 30 to 34.9 is associated with a 44% increased risk for death. A BMI of 35 to 39.9 has an 88% higher risk for death. The mortality risk for men is similar. Each five-unit increase in BMI increases the risk of death by 31%.

Meeting your doctor should be your first step. They will measure your blood pressure and ask for laboratory tests to measure your blood lipids and sugar. You may also need a glucose tolerance test. These measurements and reports will identify your risks of severe heart disease or diabetes.

These risk factors indicate a higher probability of heart disease.

• Hypertension
• High blood level of low-density lipoprotein
• Low blood level of high-density lipoprotein
• High blood level of triglycerides
• High blood sugar level or abnormal results of the glucose tolerance test
• Family history of heart disease
• Sedentary lifestyle
• Smoking

If your BMI is over 30 (obesity), or 25 to 29.9 (overweight) with two or more risk factors, you should make determined efforts to lose weight. Losing 5% to 10% of weight can reduce your risk of obesity-associated diseases. But if you are overweight, don't have a high waist measurement, and have fewer than two risk factors, just preventing further weight gain may be adequate.

The next step is weight management. Use the dual strategy of a reduced calorie diet and increased physical activity. You will lose weight if you eat fewer calories than you use for daily activities. 

The BMI doesn't take age and gender into account. It uses the same formula and tables for all adults. Women naturally have more fat than men, and older people also have more body fat. The BMI doesn't consider any of this.

The BMI also doesn't consider the location of your body's fat. Visceral fat, the fat around the waist and abdominal organs is an important indicator of health. People with more fat in these areas are at greater risk of health problems. Fat on the hips and thighs is less metabolically active than visceral fat.

Athletes and bodybuilders have a large muscle mass. Muscles weigh more than other tissues, and these people have a high BMI as a result. They're in good physical health despite their high BMI.

Ethnicity also matters. People of South Asian origin can have a higher risk of health problems with a BMI of 23, which is within the healthy range for others.

Measuring your waist is an excellent way of learning your health status. Increased waist measurement is associated with hypertension (high blood pressure), dyslipidemia (abnormalities of blood lipids), insulin resistance, inflammation, and coronary heart disease.

The ratio of your waist to hip measurements are also good indicators of disease probability. But a large waist is the most dependable indicator of vascular events like stroke and diabetes. This measurement works independently of hip circumference, BMI, and other risk factors.

You can assess your risk of obesity-related diseases by measuring your waist yourself. Find the bottom of your ribs and the top of your hip bones. Measure yourself at the mid-point of these. It will be at your navel or close to it.

Use a tape measure around your waist at the navel, and measure just after breathing out. Use these measurements to assess your risk status:

Low risk: Less than 37 inches (94 centimeters) in men and 31 inches (80 centimeters) in women

Increased risk: 37 to 40 inches (94 to 101.9 centimeters) in men and 31 to 34½ inches (80 to 87.9 centimeters) in women

High risk: More than 40 inches (102 centimeters) in men and 34½ inches (88 centimeters) in women

Being underweight is bad for health, too. You could have a weakened immune system, fragile bones, and nutritional deficiencies. Poor stamina and tiredness also happen in those who are underweight.

You should consult your doctor if you've lost weight recently or have always found it difficult to gain weight. They will test you for chronic diseases, cancers, eating disorders, malabsorption, and other causes of poor weight gain. Apart from treating any disorders they find, they'll also advise you about safe ways to gain weight:

Have regular meals with plenty of vegetables and fruit. 

Include wholegrain, potatoes, bread, rice, and pasta in your meals. 

Have full-fat dairy for a while till your weight is at a healthy level.

Protein foods are important — meat, fish, eggs, beans, and pulses. 

Don't depend on food with lots of saturated fats and sugar to gain weight. These foods increase fat instead of lean body mass. You could also have increased blood cholesterol levels. 

BMI is not a diagnostic tool. It does not tell you what diseases you have or may have. BMI is a screening tool that tells you about the increased risk for certain conditions. Calculating your BMI from time to time can give you an early warning of impending overweight or obesity. 

BMI isn't perfect. It doesn't take body composition into account and can't tell whether excess weight is healthy muscle or unhealthy fat. It also uses the same criteria for all adults, regardless of age and gender. You shouldn't be alarmed by an increased BMI, but you shouldn't ignore it either. Consult your doctor and take the required steps to keep yourself safe.