You can punch your stats into online calculators, tweak your Fitbit settings, and meticulously track every calorie you burn, but no amount of math can tell you how many calories your body really burns in a day. To get a real answer, you need to do a little self-experimentation.
Calculators can give you a starting point
If you want a ballpark number for how many calories a person your size “should” burn, there are formulas that can get close. The calculator we mention here uses the Mifflin-St. Jeor equation (or your choice of two other options) to estimate your BMR, or basal metabolic rate. A related statistic is your RMR, or resting metabolic rate. Both of these are meant to account for the calories your body burns just to stay alive.
We burn more than that, though—you also need calories to get up and walk around, to exercise, and to digest your food. The total number of calories a person burns in a day is their TDEE, or total daily energy expenditure. Calculators can give you a guess at this number. For example, the linked calculator says that my BMR is likely to be 1,309 calories per day, and for my activity level, it estimates a TDEE of 2,029. Experience tells me that’s pretty close—good job, calculator!
To get an accurate estimate, you need to track your weight
Everybody is different. Maybe I fidget more than you do; maybe your gut bacteria are better at digesting food than mine are. If you want to track your calories with a goal of gaining, losing, or tightly maintaining your weight, you’ll need to track the calories you eat and your weight.
One way to do this is with the reddit-famous TDEE spreadsheet. Every day you input your weight from that morning, and the number of calories you ate that day. (You will need to track your food meticulously, so this method is not for everybody.) The magic is simple: the spreadsheet averages your weight for the week, and averages your calories for the week. If your weight goes up, you know you were eating more than your TDEE. If your weight goes down, you were eating less.
You can do a similar calculation on the back of any envelope. Basically, if your weight stays steady, then whatever number of calories you’re eating is your TDEE. This is the case for most of us, most of the time!
Even this method isn’t totally accurate
Unfortunately people aren’t machines. We have lives, and we change our activities and foods all the time. If you’re losing weight, your body will start conserving calories a bit. On the other hand, the calculator doesn’t account for how your activities are changing. If you switch up your exercise routine, or you get sick, or you end up walking to work more often than you used to, your energy expenditure will go up.
A TDEE tracker can help you figure out how your expenditure has changed over the past few weeks, but it can’t provide day-by-day guidance on how many calories you should eat or exactly what to expect the scale to say tomorrow. Our weight fluctuates from day to day, anyway. So track your TDEE if you’d like to get the big picture view, but nothing can give you perfect accuracy.