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.