Your daily calorie requirement is your basal metabolic rate (the calories your body burns at rest) multiplied by an activity factor that accounts for how much you move. For most people, the Mifflin-St Jeor equation is the most accurate starting formula, predicting resting metabolism within a reasonable range for about 80% of non-obese adults. Here’s how to use it step by step.
Step 1: Estimate Your Resting Metabolic Rate
Your resting metabolic rate (RMR) is the energy your body uses just to keep you alive: breathing, circulating blood, maintaining body temperature, and running your organs. For most people, this accounts for the largest share of daily calorie burn. The Mifflin-St Jeor equation calculates it using your weight in kilograms, height in centimeters, and age in years.
For men: (10 × weight) + (6.25 × height) – (5 × age) + 5
For women: (10 × weight) + (6.25 × height) – (5 × age) – 161
So a 35-year-old woman who weighs 70 kg (154 lbs) and stands 165 cm (5’5″) would calculate: (10 × 70) + (6.25 × 165) – (5 × 35) – 161 = 700 + 1,031 – 175 – 161 = 1,395 calories per day at rest. A man of the same size and age would get 1,561 calories, since the equation uses +5 instead of -161.
To convert pounds to kilograms, divide by 2.2. To convert inches to centimeters, multiply by 2.54.
Why This Equation Over Others
Several formulas exist for estimating metabolic rate, but they’re not equally reliable. The older Harris-Benedict equation, developed in 1919, has been shown to accurately predict resting metabolism in only about 70% of people. The Mifflin-St Jeor equation, published in 1990, hits around 82% accuracy in non-obese adults and 75% in obese adults when compared against indirect calorimetry, the gold-standard lab test that measures actual oxygen consumption.
Research comparing these equations to lab measurements found the Mifflin-St Jeor equation was essentially unbiased, with errors averaging close to zero (within about 17 calories per day in either direction). Other popular equations tended to consistently overestimate or underestimate, which compounds into meaningful error when you’re using the number to plan your diet over weeks or months.
If You Know Your Body Fat Percentage
Standard equations use total body weight, but muscle tissue burns significantly more calories than fat tissue. If two people weigh the same but one carries more muscle, the muscular person has a higher metabolic rate. The Katch-McArdle formula addresses this by using lean body mass instead of total weight:
BMR = 370 + (21.6 × lean body mass in kg)
Lean body mass is simply your total weight minus your fat weight. If you weigh 80 kg and your body fat is 20%, your fat mass is 16 kg and your lean mass is 64 kg. Your estimated BMR would be 370 + (21.6 × 64) = 1,752 calories. This formula works the same for men and women, since it already accounts for body composition differences by using lean mass directly.
The catch is that you need a reasonably accurate body fat measurement. Bathroom scales with bioelectrical impedance can be off by 5% or more. DEXA scans or hydrostatic weighing give better numbers but require a visit to a clinic or university lab. If you don’t have a reliable body fat reading, stick with Mifflin-St Jeor.
Step 2: Multiply by Your Activity Level
Your resting metabolic rate only tells you what your body burns doing nothing. To get your total daily energy expenditure (TDEE), you multiply by a physical activity level (PAL) factor. The Food and Agriculture Organization of the United Nations classifies these into three broad ranges:
- Sedentary or light activity (1.4 to 1.69): Office work, minimal exercise, mostly sitting or standing throughout the day. Most desk workers fall here.
- Active or moderately active (1.70 to 1.99): Jobs that involve regular movement, or sedentary jobs combined with consistent exercise like running, cycling, or weight training several times a week.
- Vigorous activity (2.0 to 2.4): Physically demanding jobs like construction or farming, or athletes training heavily. Sustaining a PAL above 2.4 over long periods is extremely difficult.
Using the earlier example of the 35-year-old woman with an RMR of 1,395 calories: if she works a desk job but exercises moderately three to four days a week, a multiplier of about 1.55 puts her total daily need around 2,162 calories. If she trained intensely most days, a multiplier closer to 1.85 would give her about 2,581 calories.
Choosing your multiplier honestly matters more than which equation you use. People consistently overestimate how active they are. If you’re unsure, start with the lower end of the range that seems to fit your life and adjust based on what actually happens to your weight over two to three weeks.
What Else Burns Calories
Beyond your resting metabolism and physical activity, your body also burns energy digesting food. This is called the thermic effect of food, and it typically accounts for about 10% of your total calorie intake. Protein-rich foods require more energy to digest than fats or carbohydrates, which is one reason high-protein diets can have a slight metabolic edge. But for the purpose of calculating your calorie requirement, this 10% is already baked into the activity multipliers most calculators use, so you don’t need to add it separately.
Adjusting for Weight Loss or Gain
Once you have your TDEE, that number represents maintenance: the calories needed to keep your weight stable. To lose weight, you eat below it. To gain, you eat above it.
A deficit of about 500 calories per day produces roughly one pound of weight loss per week. This is a widely recommended starting point because it’s aggressive enough to produce visible results but moderate enough to sustain without excessive hunger or muscle loss. So if your TDEE is 2,200 calories, a target of 1,700 calories per day would put you on track for about a pound per week.
For weight gain, a surplus of 250 to 500 calories per day is typical, depending on whether you’re trying to build muscle (which requires a smaller surplus combined with resistance training) or simply need to increase body weight.
Why Your Real Number May Differ
Every equation gives you an estimate, not a measurement. Several factors can shift your actual calorie needs away from the calculated number. Genetics influence metabolic rate by a few hundred calories in either direction. Hormonal conditions, particularly thyroid disorders, can raise or lower it significantly. Chronic dieting can temporarily reduce metabolic rate as your body adapts to lower calorie intake. Medications, sleep quality, and stress also play roles that no equation captures.
The practical solution is to treat your calculated number as a starting point and then track what happens. Weigh yourself under consistent conditions (same time of day, same clothing) and look at the trend over two to three weeks rather than day-to-day fluctuations. If your weight isn’t moving in the direction you want, adjust by 100 to 200 calories and reassess. The equation gets you in the right neighborhood. Real-world tracking gets you to the right address.
For the most accurate result without lab testing, calculate your TDEE using Mifflin-St Jeor, eat at that level for two weeks while tracking your intake carefully, and then adjust based on scale trends. A food scale and a tracking app remove most of the guesswork from the intake side of the equation, which is where most errors actually occur.