To calculate your daily calorie intake, you need two things: an estimate of how many calories your body burns at rest, and a multiplier that accounts for how active you are. The most widely recommended formula for that first number is the Mifflin-St Jeor equation, which uses your weight, height, age, and sex to estimate your resting metabolism. From there, you adjust based on your goals.
The Mifflin-St Jeor Equation
This formula estimates your basal metabolic rate (BMR), the calories your body needs just to keep you alive: breathing, circulating blood, maintaining body temperature. It doesn’t account for any movement at all.
For males: (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) + 5
For females: (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) – 161
If you’re working 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 35-year-old woman who weighs 150 pounds (68 kg) and stands 5’6″ (168 cm) would calculate: (10 × 68) + (6.25 × 168) – (5 × 35) – 161, which comes out to about 1,394 calories per day at rest.
Factoring In Your Activity Level
Your BMR only covers what your body burns while doing nothing. Your actual daily burn, often called total daily energy expenditure (TDEE), includes everything else: walking, exercising, fidgeting, even digesting food. Digestion alone accounts for roughly 10% of your total daily calorie burn. Your resting metabolism is by far the largest component, but physical activity can push your total significantly higher.
To estimate your TDEE, multiply your BMR by an activity factor:
- Sedentary (desk job, little exercise): BMR × 1.2
- Lightly active (light exercise 1-3 days per week): BMR × 1.375
- Moderately active (moderate exercise 3-5 days per week): BMR × 1.55
- Very active (hard exercise 6-7 days per week): BMR × 1.725
- Extremely active (physical job plus intense training): BMR × 1.9
Using the example above, that 35-year-old woman with a BMR of 1,394 who exercises moderately three to five days a week would multiply by 1.55, giving a TDEE of roughly 2,161 calories. That’s her estimated maintenance intake, the number that should keep her weight stable over time.
If You Know Your Body Fat Percentage
Standard formulas treat all body weight the same, but muscle tissue burns considerably more energy than fat tissue. If two people weigh 180 pounds but one carries significantly more muscle, their calorie needs will differ. The Katch-McArdle formula addresses this by using lean body mass instead of total weight:
BMR = 370 + (21.6 × lean body mass in kg)
To use it, you need a reasonable estimate of your body fat percentage. If you weigh 180 pounds and your body fat is 20%, your lean mass is 144 pounds, or about 65.5 kg. That gives a BMR of around 1,785 calories. This formula tends to be more accurate for people who are notably muscular or notably lean, since the standard equations can underestimate their needs.
Adjusting for Weight Loss or Gain
Once you have your TDEE, adjusting for your goals is straightforward math. To lose weight, you eat below your TDEE. To gain, you eat above it. A common starting point for weight loss is cutting about 500 calories per day from your maintenance number, which typically produces a loss of roughly half a pound to one pound per week.
That 500-calorie figure isn’t a hard rule. A smaller person with a lower TDEE might find a 500-calorie cut leaves them too little food to get adequate nutrition. In that case, a 250 to 300 calorie deficit is more sustainable and still produces gradual loss. What matters most is consistency over weeks and months, not the size of any single day’s deficit.
For weight gain, adding 250 to 500 calories above maintenance is a typical approach. Larger surpluses speed up the process but tend to add more fat relative to muscle.
Why Your Calculation Is an Estimate
Every calorie formula is a starting point, not a precise measurement. Several factors can create a gap between your calculated number and your actual needs.
One is metabolic adaptation. When you lose weight, your body doesn’t just burn fewer calories because it’s smaller. It also becomes slightly more efficient, so your metabolism drops a bit more than the weight loss alone would predict. Research suggests this effect is relatively modest: after a month of stabilization following weight loss, the gap between expected and actual calorie burn averages only a few dozen calories per day. It’s real, but it’s not the dramatic “starvation mode” that popular culture sometimes describes.
Part of the reason your calorie needs drop after weight loss is that internal organs actually shrink. Your heart, kidneys, and pancreas all get somewhat smaller when you lose significant weight, and organs burn calories at a rate up to 20 times higher than muscle tissue per unit of mass. So even a small reduction in organ size has a measurable metabolic impact.
The other major source of error is on the intake side. People consistently underestimate how much they eat. Studies using precise laboratory methods to measure actual energy expenditure have found that people underreport their calorie intake by 25% to 34% on average. That’s not a rounding error. Someone who believes they’re eating 1,800 calories a day may actually be consuming 2,400 or more. Portion sizes, cooking oils, sauces, and “forgotten” snacks are the usual culprits.
Tracking Tools and Their Limits
Calorie tracking apps like MyFitnessPal or Cronometer make logging easier, but they can’t fix the estimation problem. You’re still guessing portion sizes unless you weigh your food with a kitchen scale. If you’re serious about accuracy, a digital food scale is the single most useful investment you can make. Weigh ingredients raw whenever possible, since cooked weights change depending on water loss.
Fitness trackers and smartwatches are tempting as a shortcut for the “calories out” side of the equation, but their accuracy for energy expenditure is poor. Systematic reviews of commercial wearables have found that no brand was accurate for estimating calorie burn. Most devices tend to underestimate energy expenditure, though the size and direction of the error varies by activity type and brand. Use your wearable for trends and motivation, but don’t treat its calorie numbers as reliable enough to eat back after a workout.
Putting It Into Practice
The most reliable approach is to calculate your TDEE, set an intake target based on your goal, track what you eat for two to three weeks, and then compare your results against what actually happens on the scale. If your weight isn’t moving in the expected direction, adjust your intake by 100 to 200 calories and reassess after another two weeks. This iterative process matters more than getting the initial formula perfectly right, because no equation can account for your individual metabolism, daily movement patterns, and the dozens of small choices you make around food every day.
A food scale, a tracking app, and a regular weigh-in schedule (same time of day, averaging weekly rather than fixating on daily fluctuations) give you the feedback loop you need. The formula gets you in the neighborhood. Your real-world data gets you to the right address.