Your body mass index, or BMI, is a calculation of your weight relative to your height. It helps you and your healthcare provider evaluate whether you're at a healthy weight or at risk of disease due to being overweight or obese. If you fall within a specific "ideal" weight range for your height, your BMI comes out as normal, or healthy. BMI and height-weight charts aren't infallible, however, and can mistakenly categorize some healthy people as overly fat and some overly fat people as healthy. Height and weight should be among many factors considered when determining your health.
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Calculating BMI from Weight and Height
Your BMI is your weight in kilograms divided by your height in meters squared. In American measurements, the equation includes a conversion factor and reads: BMI = weight in pounds / (height in inches x height in inches) x 703.
If the result is 18.5, you are below an ideal weight for your height. If the result is 18.5 to 24.9, you are at an ideal weight for your height. A BMI of 25 to 29.9 indicates that you're overweight, while a BMI of 30 or more signals obesity.
For example, a 5-foot, 10-inch person who weighs between 132 and 173 pounds would be of an ideal weight; but if he weighs 172 to 208 pounds, he's overweight. Or, a 5-foot, 5-inch person would be at an ideal size if she weighed between 114 and 149 pounds, but overweight from 150 to 179 pounds. Your doctor's office may have a chart that lists the ideal weights for the most common adult heights; these are also available online. Or you can use an online calculator to plug in your weight and height and figure your BMI.
Where you fall in the ideal weight range depends on your body composition and body shape. Lithe, thin people will fall into the lower end of their ideal weight range while stocky, muscular people will be at the higher end. One frame size isn't more ideal than the other -- frame is determined by genetics.
The idea weight as defined by height-weight charts and BMI means you likely have a healthy amount of fat. Too much body fat puts you at greater risk for disease, including type 2 diabetes, cardiovascular disease and some cancers. If you fall below an ideal weight, you may be more susceptible to illness, feel weak and suffer from nutritional deficiencies.
BMI doesn't take gender or age into account. It's a screening tool for risk of disease -- not diagnostic. Height-weight charts do not indicate body fat levels, and BMI isn't a direct measure, so they are only rough estimations of body fat levels. If a man and woman have the same BMI, it's likely that the man has less fat than the woman due to hormonal differences. An older person who has the same BMI as a younger person is also likely to have more fat because of the natural loss of muscle mass as you age.
What Charts and BMI Can Misinterpret
A highly muscular person may register as overweight according to a height-weight chart or BMI. This doesn't mean they're overfat, however. Muscle has a greater density than fat, so a person with a lot of muscle mass may weigh more than the average person. In these cases, additional screenings, such as blood tests and family history questionnaires, help a doctor provider assess health risks.
Relying only on charts and equations doesn't tell you where your fat is located either. You may weigh the "right" amount for your height, but if waist circumference is greater than 40 inches for a man or 35 inches for a woman, you're still at a grave health risk. A large waist indicates you carry an abundance of visceral fat, which surrounds the internal organs and secretes harmful inflammatory substances. People with too much visceral fat, even if they're of normal weight, are at increased risk of developing certain health problems.
Achieving a Healthy BMI
If you're over or under an ideal weight, proper nutrition and exercise can help. For those who are overweight, trimming calorie intake, increasing cardiovascular exercise and performing strength training help achieve weight loss. Aim to consume 250 to 1,000 calories fewer than you burn in a day to lose 1/2 to 2 pounds per week. Reduce your intake of sweets and refined grains like white bread as well as saturated fats from fatty meats and whole dairy. Get at least 150 minutes of moderate-intensity cardio, such as brisk walking, per week; go for more if you want to lose significant weight. Strength train at least twice per week to build lean muscle, which is healthier tissue than fat because it contributes to daily function and stamina.
If you need to gain weight, you'll need to add 250 to 500 calories daily, which will help you put on 1/2 to 1 pound per week. This gradual rate of gain helps ensure you put on muscle rather than just fat. Weight-training regularly encourages the development of muscle, so plan on at least two workouts per week. When you add calories, get them from whole grains, starchy vegetables, fruits, lean protein and unsaturated fats, like nuts and avocados.