Body mass index, or BMI, is a calculation that compares your weight relative to your height. The index isn’t perfect. A bodybuilder’s BMI may indicate that she’s overweight, for example, but her extra weight is muscle rather than fat. For pretty much everyone else, however, BMI is a reasonable measure of your fitness level, demonstrating whether you’re overweight, underweight or just right. If your BMI isn’t where you want it, exercising can help you lose weight, gain muscle, or both.
You can find online calculators to figure your BMI, or you can convert your height and weight into kilograms and meters and tally the results yourself. Just remember that 1 pound equals 0.454 kilograms and 1 inch equals 0.0254 meters. To determine your BMI, divide your weight in kilograms by your height in meters squared. For example, if you’re 5 feet 5 and weigh 150 pounds, you’re 1.65 meters tall and weigh 68.1 kilograms. Divide 68.1 by 2.72 (that’s 1.65 squared) for a BMI of 25. The normal BMI range is 18.5 to 24.9. Anything less than 18.5 is underweight. A range of 25 to 29.9 is overweight, while anything more than 30 is considered obese.
If you’re only a touch overweight, adjusting your diet may be all you need to edge your BMI into the normal range. But the most efficient way to lose weight and keep it off is to combine a healthy diet with aerobic exercise. To lose 5 percent or more of your body weight, for example, you must do approximately five hours of moderate-intensity exercise per week. Try to burn at least 500 extra calories each day, which translates into a loss of 1 pound per week. Aerobic activities can include brisk walking, which burns 314 calories per hour for a 160-pound woman, plus lap swimming (423 calories), running at 5 mph (606 calories), low-impact aerobics (365 calories) or jumping rope (861 calories).
Weight training can offer a double benefit. You’ll work off some calories while you’re exercising, and you’ll raise your metabolic rate if you build more muscle mass, because muscles use more energy than fat, even when you’re at rest. Resistance training itself burns about 365 calories per hour, if you weigh 160 pounds. You’ll burn more calories by circuit training, which involves performing a series of consecutive exercises with no rest between activities. Circuit training burns roughly 600 calories per hour for a 160-pound woman. Try compound exercises -- activities that move multiple joints and target more than one muscle group at a time -- such as deadlifts, rows, standing shoulder presses, barbell squats, bench presses and dumbbell lunges.
Improving your BMI typically means trimming some fat. But if you’re underweight you may need to add a few pounds. To gain weight in a healthy manner, perform compound weight-training exercises two or three times each week, but never on consecutive days. Perform three to five sets of eight to 12 repetitions of each exercise, and make each rep intense by lifting about 75 to 85 percent of your one-repetition maximum. Eat a healthy high-protein meal about 30 to 45 minutes after exercising with a 3- or 4-to-1 ratio of carbohydrates to protein.
M.L. Rose has worked as a print and online journalist for more than 20 years. He has contributed to a variety of national and local publications, specializing in sports writing. Rose holds a B.A. in communications.