How to Calculate Daily Calories With Mifflin St Jeor

To calculate your daily calories, you first estimate your basal metabolic rate with the Mifflin St Jeor equation, then multiply it by an activity factor and finally adjust the result for your goal. The number you get is not a medical truth. It is a starting point that you will correct with what you learn over the first few weeks.

What daily calories are and what they depend on

Your body spends energy every day in four ways. The largest share, 60 to 70% in many people, is your basal metabolic rate: the energy your organs use at rest. Next comes the thermic effect of food, meaning what it costs to digest it, which is around 10%. Then there is planned exercise and, lastly, everyday movement such as walking, climbing stairs or working on your feet.

The sum of all that is your total energy expenditure. If you eat what you burn, your weight tends to hold steady. If you eat less, it tends to drop, and if you eat more, it tends to rise. It sounds simple, but your expenditure changes with sleep, stress, muscle mass and how much you move, so no calculation replaces watching your own progress.

Working out your calories takes three steps: estimate your basal rate, apply your activity level and adjust for your goal. Let us take them one at a time.

Step 1: your basal metabolic rate with Mifflin St Jeor

The Mifflin St Jeor equation was published in 1990 and, according to a 2005 comparison of equations, it is one of the best at predicting resting expenditure in healthy adults compared with other common equations. Even so, it has an individual margin of error.

Here is the formula in words so you do not get lost:

1. Multiply your weight in kilograms by 10.
2. Multiply your height in centimeters by 6.25 and add it.
3. Multiply your age in years by 5 and subtract it.
4. If you are a man, add 5. If you are a woman, subtract 161.

The result is the kilocalories you would burn in a day of complete rest.

Example for a man

He is 30 years old, weighs 80 kg and is 180 cm tall.

1. 10 × 80 = 800.
2. 6.25 × 180 = 1,125, which added gives 1,925.
3. 5 × 30 = 150, which subtracted gives 1,775.
4. Plus 5 for being a man, 1,780 kcal.

His estimated basal metabolic rate is about 1,780 kcal per day.

Example for a woman

She is 30 years old, weighs 65 kg and is 165 cm tall.

1. 10 × 65 = 650.
2. 6.25 × 165 = 1,031.25, which added gives 1,681.25.
3. 5 × 30 = 150, which subtracted gives 1,531.25.
4. Minus 161 for being a woman, about 1,370 kcal.

Her estimated basal metabolic rate is about 1,370 kcal per day.

Step 2: the activity factor

Your basal rate only covers rest. To estimate what you really burn, you multiply it by a factor that reflects how much you move. The usual values in the literature run from 1.2 to 1.9.

1. 1.2 if you spend almost all day sitting and barely exercise.
2. 1.375 if you do light exercise 1 to 3 days a week.
3. 1.55 if you train 3 to 5 days a week at moderate intensity.
4. 1.725 if you train hard 6 to 7 days a week.
5. 1.9 if your job is physical and you also train hard, which is uncommon.

Be honest when you choose. It is the most frequent mistake: in general, people tend to place themselves one level above where they really are. If you hesitate between two, take the lower one and adjust later.

Following the examples, the man who trains 4 days a week uses 1.55: 1,780 × 1.55 is about 2,760 kcal for maintenance. The woman who does light exercise uses 1.375: 1,370 × 1.375 is about 1,885 kcal for maintenance.

If you want more context on the level of movement recommended for health, the World Health Organization recommends that adults get 150 to 300 minutes of moderate aerobic activity a week, plus muscle strengthening on at least 2 days. That gives you direction, but it does not replace your own measurement.

Step 3: adjust for your goal

Once you have maintenance, you decide where to go. To lose fat, you eat a little less than maintenance. To build muscle, a little more. And to maintain, what you calculated.

The changes should be moderate. An aggressive deficit is hard to sustain, raises hunger and can eat into muscle mass and energy. As a prudent guide, many guidelines put a reasonable loss at 0.5 to 1% of body weight per week, and a small surplus of 150 to 300 kcal to build muscle without gaining too much fat. If you want to go deeper on how that deficit is approached, see the guide to a fat loss app.

There are also safety floors. Do not go below your basal metabolic rate or below reasonable minimums: as a reference, not under about 1,500 kcal if you are a man or 1,200 if you are a woman. Those minimums are not a target, but a limit that is best not crossed without professional supervision.

After calories come macros. A widely used starting point is 1.6 g of protein per kilogram of body weight per day, a value supported by the 2018 meta analysis by Morton and colleagues, within a reasonable range. For an 80 kg person that would be about 128 g. The rest is split between fat, which should not drop too low, and carbohydrates. In the article on how much protein you need a day we go into it more calmly.

It is an estimate: how to check it

Here is the important part. Mifflin St Jeor is an estimate for the general population. Your real body can land 10% or more above or below it, because of genetics, body composition, medication or simple differences in unconscious movement.

The check is practical and simple:

1. Eat close to your calculated number for 2 weeks.
2. Weigh yourself daily or several times a week at the same time, and look at the weekly average, not a single day.
3. If your weight behaves as you expected, the calculation is good. If not, move the number by 100 to 200 kcal and observe again.

This does not work the same way in every situation. If you are pregnant, breastfeeding, have or have had an eating disorder, have a chronic illness or take medication that affects metabolism, do not use a generic calculation as your guide: talk to a doctor or a registered dietitian. And if you are under 18, this formula is not meant for you.

How it works in Elvyfit

In Elvyfit the calculation is done by code, not by the language model. It uses the Mifflin St Jeor equation and works out your expenditure in one of two ways: by components, using your steps, your type of job, your activities and your sessions, or with activity factors from 1.2 to 1.9. Based on your goal, the code applies limits: it never goes below the higher of your basal metabolic rate and 1,500 kcal for men or 1,200 for women, and those floors cannot be changed.

It then splits your macros from your weight, with 1.6 g of protein per kilogram as the base, and adjusts the diet using meal templates with a tolerance of plus or minus 5%. You can see how it is built on the AI nutrition plan page and keep your daily tracking with the meal and macro tracking. Remember that everything here is an estimate, and that the app is not a medical device and does not replace a professional. The app is currently available only in Spanish.