There are a number of competing equations to calculate your Resting Metabolic Rate (RMR) - the number of calories your body burns at rest in a single day. While these formulas offer a ballpark number of calories you can eat based on your age and weight, the most commonly used equations come from decades-old research. In fact, one study found the formulas to be off by as much as 15 percent, especially in obese individuals. All equations, even those based on body composition, can over- or underestimate the number of calories you should eat.
When scientists who study metabolism need to get it right, they rely on a "metabolic cart" - an elaborate tool that calculates RMR based on the amount of oxygen you breathe in and carbon dioxide you exhale. In the past, this kind of technology was expensive and inaccessible, but for $40- $100, you can get results via a simple, hand-held breath test that is used for metabolic assessment at gyms and spas nationwide.
To calculate your RMR on your own, follow this equation:
* Multiply your healthy weight by 10 calories for your RMR. Example: You weigh 150 but 140 is a healthier weight for you. Multiply 10 calories x 140 pounds (appropriate weight) = 1,400 calories for your RMR.
* To your RMR, add half that number (assuming you are moderately active throughout the day). Example: 50% x 1,400 calories for RMR = 700 calories for daily activity.
* Add 200 calories per 30 minutes of aerobic exercise. Example: 30 minutes = 200 calories. Total calorie needs 1,400 (RMR) + 700 (daily activity) + 200 = 2,300.
* Subtract 20% to determine your calorie budget for losing weight. Example: 20% x 2,300 calories = about 500; 2,300 calories - 500 = 1,800 calories -- the number you'd need to lose weight.
You can increase your activity to burn 200 more calories; then eat 300 fewer calories to cut the 500 calories. Use this plan and you'll lose 1/2-1 pound per week.