To calculate how many calories you need each day, you estimate your resting metabolic rate using a simple formula, then multiply it by a number that reflects how active you are. The result is your total daily energy expenditure, or TDEE, the number of calories your body uses in a full day. From there, you adjust up or down depending on whether you want to maintain, lose, or gain weight.
Step 1: Estimate Your Resting Metabolic Rate
Your resting metabolic rate (RMR) is the energy your body burns just to stay alive: breathing, circulating blood, maintaining body temperature, keeping your brain running. For most people, this accounts for the largest share of daily calorie burn. The most widely recommended formula for estimating it is the Mifflin-St Jeor equation, which the Academy of Nutrition and Dietetics considers the most accurate for the general population.
For men: (9.99 × weight in kg) + (6.25 × height in cm) − (4.92 × age in years) + 5
For women: (9.99 × weight in kg) + (6.25 × height in cm) − (4.92 × age in years) − 161
If you think in pounds and inches, convert first. Divide your weight in pounds by 2.2 to get kilograms. Multiply your height in inches by 2.54 to get centimeters. So a 170-pound, 5’10” man who is 35 years old would plug in 77.3 kg, 177.8 cm, and 35 years, giving him an RMR of roughly 1,735 calories per day. That’s just the baseline, before any movement is factored in.
Step 2: Factor In Your Activity Level
Once you have your RMR, multiply it by an activity factor to get your total daily calorie needs. These multipliers account for everything from walking around the office to structured workouts:
- Sedentary (little or no exercise): RMR × 1.2
- Lightly active (light exercise 1–3 days per week): RMR × 1.375
- Moderately active (moderate exercise 3–5 days per week): RMR × 1.55
- Very active (hard exercise 6–7 days per week): RMR × 1.725
- Super active (intense exercise plus a physical job): RMR × 1.9
Using the example above, the 35-year-old man with an RMR of 1,735 who exercises moderately three to five days a week would multiply by 1.55, giving him a TDEE of about 2,689 calories. That’s his estimated maintenance number, the amount he’d eat to stay at the same weight.
Be honest when picking your activity level. Most people with desk jobs who exercise a few times a week fall into the “lightly active” category, not “moderately active.” Overestimating here is the single most common reason calorie calculations don’t match real-world results.
Why These Numbers Are Estimates
Formulas like Mifflin-St Jeor use your total body weight, but they can’t tell the difference between muscle and fat. Muscle tissue is far more metabolically active than fat tissue, which means two people who weigh the same can have very different calorie needs. A lean, muscular person at 180 pounds burns more at rest than someone at 180 pounds with a higher body fat percentage.
If you know your body fat percentage (from a DEXA scan, for example), you can use a formula designed for this. The Katch-McArdle equation calculates RMR from lean body mass alone: 370 + (21.6 × lean body mass in kg). This tends to be more accurate for people who are notably muscular or notably overweight, since weight-only formulas can overshoot or undershoot for both groups.
Another variable these formulas miss is non-exercise activity, the calories you burn fidgeting, standing, walking to the kitchen, or pacing during phone calls. Research from the Mayo Clinic found this type of everyday movement can vary by up to 2,000 calories per day between two people of similar size. That’s an enormous range and helps explain why some people seem to eat freely without gaining weight while others gain easily.
What Your Body Does With Calories
Your TDEE isn’t just RMR plus exercise. A smaller but real portion of your daily burn comes from digesting food itself. Your body uses energy to break down, absorb, and process what you eat. Protein costs the most to digest, raising your metabolic rate by 15–30% of the calories in that protein. Carbohydrates cost 5–10%, and fats cost 0–3%. This is one reason higher-protein diets can feel more effective for weight management: you’re burning more calories just processing the food.
For most people, digestion accounts for roughly 10% of total daily energy expenditure. It’s built into the activity multipliers to some degree, but it’s worth knowing that what you eat, not just how much, subtly shifts the equation.
Adjusting for Weight Loss
If your goal is to lose weight, you need to eat fewer calories than your TDEE. A common starting point is cutting about 500 calories per day from your maintenance number, which typically produces a loss of about half a pound to one pound per week. The old rule that 3,500 calories equals one pound of fat is a rough guide, but the Mayo Clinic notes it doesn’t hold perfectly for everyone. Your actual rate of loss depends on your starting weight, sex, activity level, and how long you’ve been dieting.
One thing to watch for: when you lose weight, your calorie needs drop more than the math predicts. This is called metabolic adaptation. If someone at 220 pounds needs 2,500 calories a day and loses 22 pounds, you’d expect their needs to fall to around 2,200 calories. But measured in a metabolic chamber, their actual expenditure might be closer to 2,000. Part of the reason is that weight loss reduces the size of metabolically active organs, including the heart, kidneys, and pancreas, which burn calories at rates up to 20 times higher than muscle tissue per unit of mass.
The encouraging news is that this adaptation isn’t permanent. Research suggests that if you give your body about a month to stabilize after losing weight, the gap between expected and actual calorie burn shrinks to only a few dozen calories per day. Periodic diet breaks, where you eat at maintenance for a week or two, can help manage this effect.
Adjusting for Muscle Gain
Building muscle requires eating more than your TDEE. The recommended surplus is 10–20% above your maintenance calories, which supports an average weight gain of about 0.25–0.5% of body weight per week. For someone maintaining at 2,500 calories, that means eating 2,750 to 3,000 calories daily.
Going higher than 20% above maintenance doesn’t accelerate muscle growth. It just increases fat gain alongside it. Keeping the surplus modest and pairing it with consistent resistance training gives your body enough raw material to build tissue without excessive fat storage.
Does Metabolism Slow With Age?
Less than you think. A large-scale study published in Science found that total and resting energy expenditure remain essentially stable from age 20 to 60, regardless of sex. The gradual weight gain most people experience in their 30s, 40s, and 50s is more likely driven by changes in activity and eating habits than by a declining metabolism. After 60, metabolic rate does begin to decrease, but at a slow and modest pace. The takeaway: if you’re gaining weight in middle age, recalculating your activity level is probably more useful than blaming your metabolism.
Putting It All Together
Here’s the practical workflow. Calculate your RMR with the Mifflin-St Jeor formula. Multiply by the activity factor that honestly reflects your week. That gives you a maintenance estimate. Subtract 500 calories if you want to lose weight gradually, or add 10–20% if you want to build muscle. Track your weight for two to three weeks. If it’s not moving in the direction you expected, adjust by 100–200 calories and reassess. No formula is perfectly accurate for every individual, but this process gets you close, and real-world tracking closes the remaining gap.