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BMI Categories

Body Mass Index (BMI) is a measurement used to calculate a person's percentage of body fat relative to their height. BMI is used in adults 20 years of age and older. It is calculated by dividing a person's weight by the square of their height to determine in which weight category the person falls. The easiest way to determeine your BMI is either using a free BMI Calculator or looking it up using a BMI Chart.

There are six BMI weight categories used:

  • Severely Underweight: BMI under 15
  • Underweight: BMI between 15 and 18.5
  • Normal Weight: BMI between 18.5 and 24.9
  • Overweight: BMI between 25 and 29.9
  • Obese: BMI between 30 and 40
  • Morbid Obese: BMI over 40

Severely Underweight

The most common cause of being severely underweight is lack of proper nutrition, whether by poverty or eating disorder such as anorexia. A serious physical condition, such as tuberculosis or cancer, can also be the cause. If the weight loss is unexplained, the cause should be properly diagnosed by a doctor. Being severely underweight can cause infertility, lack of menstruation, anemia and hair loss. Although many people think it is better to be underweight than overweight, the mortality rates of both are about the same.

Treatment for being severely underweight consists of eating more food. It should be done in a healthy manner, without consuming too much junk food. Instead, focus on starchy food, such as corn and potatoes. Foods with lots of calories, such as nuts, cheese and dried fruits, are also helpful in gaining weight. Consuming juices and milk will also help with weight gain. Lifting weights is a good way to get exercise and obtain muscle mass.


If you are just slightly underweight, then you should not have to take any drastic measures in order to gain weight. Try eating more throughout the day. If lack of hunger is an issue, there are appetite stimulants you can try. There are also weight gain drinks that can help bring your weight up to a healthy level.

Normal Weight

If your BMI is in the normal range, that means that you are at a healthy weight for your body type and height. However, thinness is not always a sign of good health. You may still be at risk for certain diseases if you do not eat right or exercise. You need to continue with healthy eating habits and exercise. Although it is fine to have sweets occasionally, overdoing it can add pounds and cause you to be overweight.


It is estimated that 63 percent of Americans are in the overweight category, and this number continue to grow at an alarming rate. Although some overweight people are healthy, a majority have health issues. the higher your BMI, the higher your risk for certain health conditions, such as high blood pressure, heart disease, diabetes, breathing problems and cancer. For those who are overweight, weight loss is recommended. Even weight loss of 5-10 percent can dramatically improve your health and lower your risk of developing these deadly conditions.

There are many things you can do to lose weight, but it isn't easy for everyone. It takes willpower and motivation to stay away from fatty foods and sweets. Start by counting your calories and seeing how much food you really eat each day. Many people underestimate the amount they eat. Avoid eating out and buying junk food. Stay away from fast-food restaurants. Reduce your fat intake and eat more fruits and vegetables. Start an exercise routine, even if it's just a long walk each day. Try for a weight loss goal of 1-2 pounds a week. Rapid weight loss can cause you to lose muscle instead of fat and can cause health problems.


There are about 26 percent of Americans in the obese category. Obesity increases one's risk of a variety of long-term health problems, including heart disease, high blood pressure, diabetes, sleep apnea and stroke. To help you lose weight, your doctor may recommend a weight loss plan at first. If the results are not as good as expected, you may undergo behavior and nutrition counseling to find the root cause of your overeating. Your doctor may also prescribe weight loss medication.

Morbid Obese

For those who are morbidly obese, your doctor may recommend weight loss surgery, such as gastric bypass surgery. None of these methods is a quick fix, plus there can be serious side effects, especially with surgery. Even if you lose a lot of weight, you still need to learn healthy eating habits. Otherwise, the weight you lost will quickly come back.

Don't just use your BMI

Once you know your BMI, it is a good idea to measure your waist size as well. Those with a lot of fat stored around their belly are at a higher risk of diseases than one with a lot of fat around their hips. The goal is less than 35 inches for women and under 40 inches for men.

BMI is calculated the same way for children, but used differently. BMI is compared to other children of the same age. For children, a healthy weight is anywhere from the 5th to 85th percentile, with anything under the 5th percentile as underweight. Above the 85th percentile is overweight and above the 95th percentile is obese.

Although the BMI calculator is generally a good gauge to determine if you are at a healthy weight, it does have some limitations. For those with muscular builds, such as athletes, the BMI calculator may determine that you are overweight, even obese. For those who have lost muscle mass, such as the elderly, it may underestimate body fat. The BMI calculator also does not take into account that weight changes with age, and that men and women are built differently but use the same BMI calculator. Also, race and nationality can affect body structure. So as you can see, the BMI calculator is not perfect and does not take into account many factors that can affect body composition.

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