Even the best calorie calculators (like you’ll find here) come with a crucial caveat: 

They produce informed estimates that should be viewed as starting points, not pin-sharp and incontrovertible computations. (And in case you’re wondering—activity trackers are even worse.)

There are a few reasons for this:

  1. The majority of your total daily energy expenditure (TDEE) comes from your resting metabolic rate (RMR)—the number of calories your body burns at rest in 24 hours—and this number is different even among people with identical body compositions. Thus, calorie calculators assume an average RMR, but you may be above or below this amount.
  2. The second-biggest component of TDEE comes from your physical activity level, and while most of this is captured by inputting time and general intensity, actual numbers will vary based on types of activity, types of exercises, amounts of effort, and other factors that can’t be practically represented mathematically. And so again, calorie calculators work with averages that may or may not be accurate in your case.
  3. Your physical activity level also includes non-exercise activity thermogenesis (NEAT), which refers to the many miscellaneous movements we engage in like standing, walking, fidgeting, and so forth. Research shows the amount of calories burned through NEAT can differ greatly among people, ranging from 100 calories to 800 calories per day or more, and there’s no pat way to determine where you are on that spectrum. Like most of us, you’re probably close to the average (probabilities working as they do), but if you’re not, a calorie calculator won’t account for this.
  4. The final element of your total daily energy expenditure worth discussing here is the thermic effect of food (TEF), which is the amount of calories you burn digesting food. Studies show this accounts for 5-to-15% of total daily energy expenditure in most people, and the two primary factors in TEF are macronutrients (protein costs more energy to digest than carbohydrate—about 25-to-30% and 10-to-15%, respectively—and carbohydrate costs more than fat, which requires almost no energy to digest and absorb) and food processing (unprocessed foods cost more energy to digest than highly processed ones). Calorie calculators don’t customize your results according to exactly how much protein and unprocessed food you’re eating—they just assign a middle-ground number that hopefully is right for you—and this can lead to an estimate of calorie expenditure that’s too high or low.

All that isn’t to say you shouldn’t use calorie calculators, however—they’re quite handy—only that you may need to adjust your calories up or down based on how your body actually responds, usually in the range of 5-to-10% (if you’re losing or gaining weight too slowly, you may need to eat less or more and vice versa).

(And if you’d like even more specific advice about how many calories, how much of each macronutrient, and which foods you should eat to reach your health and fitness goals, take the Legion Diet Quiz, and in less than a minute, you’ll know exactly what diet is right for you. Click here to check it out.)

+ Scientific References