How to Use BMI and BMR to Lose Weight

The success or failure of any weight loss program ultimately comes down to one simple factor: Do you burn more calories per day than you take in? Consume less than your body uses in a given day, and you will lose weight. Consume more, and you'll gain. So, if you're at the beginning of your weight loss journey, it makes sense to start by calculating just how many calories per day your body needs to maintain its current weight. The Body Mass Index (BMI) and Basal Metabolic Rate (BMR) are excellent tools that will allow you to do just that!

What Is the BMI?

BMI stands for Body Mass Index. It gives you a statistical approximation of your ideal weight, and whether you are under or overweight. There are a number of free online BMI calculators you can easily find by using the search term "BMI Calculator." Once you've located one of these calculators, enter your height and weight, and presto! It produces your BMI. Keep in mind that the BMI is only a very rough guideline that does not actually measure your body fat ratio, or take into account your frame. But the BMI does provide a decent starting point for most people to gauge whether they need to lose weight, and what their ideal weight might be.

One important caveat--muscle weighs more than fat. The more heavily muscled you are, the more likely it is that your ideal lean weight is going to be greater than the BMI recommendation. The same holds true for people with large frames. A person with a naturally broad, deep build would have to be underweight to achieve the same BMI that would be ideal for someone with a naturally small, slender frame. So adjust your goals accordingly.

What Is the BMR?

BMR stands for Basal Metabolic Rate, which basically measures how many calories your body burns while at rest. All of the involuntary functions that keep you alive--breathing, your heart pumping blood through your body, the maintenance of all your vital organs--require energy. In fact, this essential bodily maintenance burns more calories than anything else you are likely to do on any given day. As with the BMI, you can find a number of free online calculators to determine your BMR. Enter your age, height, weight and sex to get an estimate of the number of calories your body burns each day to maintain itself. Again, this number is approximate; the greater your muscle mass, the more calories you will burn to maintain a given weight.

Keep in mind that your BMR only includes your body's resting survival functions. Every activity you engage in throughout the day, from running three miles to walking to the refrigerator or picking up the remote control, adds to the number of calories you burn. Depending on exactly how active you are, your actual metabolic rate is usually between 400 and 800 calories a day higher than your BMR.

Your Daily Caloric Intake

Fortunately, most free BMR calculators have a separate field that allows you to estimate your total daily caloric requirements. It takes into account all the factors you have already plugged in to the BMR calculator, then adds your approximate daily activity level to the equation. This allows the calculator to give you a reasonably accurate account of exactly how many calories you need to maintain your current weight. If you are exceptionally muscular, the calculator will underestimate this figure, and if you have an extremely high body fat percentage, it will overestimate your caloric maintenance requirements, but the numbers will still be close enough to give you a good idea of how many calories you burn each day.

Calculating Your Weight Loss Goals

Most BMI calculators provide instructions on how to modify your BMI according to sex and frame, or they include fields that will do this for you. From here, it's easy to determine your ideal weight according to the BMI. You don't have to adopt this supposed perfect weight as your goal; many people find the BMI results to be a bit on the lean side, but it does give you a terrific starting point. As you approach your goal weight, you'll have a better idea of exactly what weight is comfortable for you.

The Final Step

Once you've decided on your ideal body weight, enter this weight into the BMR calculator. Use the resulting number of calories as your daily intake goal. This should enable you to gradually lose weight in a healthy fashion that will allow you to keep the pounds off once you've lost them. If the difference between your current BMR rate and the one based on your ideal weight seem too far apart, consuming calories to match your current BMR rate should still allow you to lose weight at a good pace. For example, most dietitians recommend consuming approximately 500 fewer calories a day than you burn in order to achieve sustainable weight loss. As it happens, this is roughly the difference between your BMR and your actual metabolic rate. Consequently consuming the number of calories the BMR calculator spits out will usually put you on an excellent path to weight loss. And, of course, check with your doctor before starting any diet in case of medical conditions that you need to take into account.