Calculating your metabolism and total energy expenditure
Updated: October 9, 2026
Have you been counting calories for weeks, eating "reasonably," and yet the scale just won't budge? The problem often comes down to a number you've never actually calculated: your basal metabolic rate. Without it, there's no way to know how many calories you're really burning each day, which means you can't set a deficit or surplus that actually makes sense. Let's break it all down, with real formulas and concrete examples.
What exactly is your basal metabolic rate?
Your basal metabolic rate (BMR) is the energy your body uses at total rest, just to keep your vital functions going: breathing, blood circulation, temperature regulation, and the work your brain and organs do around the clock. It's basically the number of calories you'd burn if you stayed in bed all day without moving a muscle.
For most people, BMR makes up the biggest share of total energy expenditure, often around two thirds. That makes it the foundation of any serious calorie calculation, way before you even start factoring in workouts or daily activity.
How to calculate your BMR: the Mifflin-St Jeor formula
There are several formulas out there (Harris-Benedict, Katch-McArdle...), but the Mifflin-St Jeor equation is now considered the most reliable for the general population, since it's been validated on modern cohorts and matches today's metabolic reality more closely.
The formula
- For men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
- For women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161
A real-life example
Let's take a 30-year-old woman, 65 kg (143 lbs), 168 cm (5'6"):
- (10 × 65) = 650
- (6.25 × 168) = 1050
- (5 × 30) = 150
- Calculation: 650 + 1050 − 150 − 161 = 1389 kcal
Those 1389 kcal are what her body burns just to function, before she even gets out of bed. It's a theoretical number, but it's pretty close to reality for someone within the average range, without any extreme hormonal or muscular traits.
Total daily energy expenditure (TDEE): the number that really matters
Your BMR alone won't get you very far when it comes to building your diet. What you actually need is your total daily energy expenditure (TDEE), which adds up:
- Your basal metabolic rate (BMR)
- The thermic effect of food (digesting food costs energy, roughly 10%)
- NEAT, meaning all your non-exercise activity: walking, taking the stairs, fidgeting, doing chores
- The energy you burn during your workouts
To estimate your TDEE from your BMR, you apply an activity multiplier:
- Sedentary (little to no exercise, desk job): BMR × 1.2
- Lightly active (exercise 1 to 3x/week): BMR × 1.375
- Moderately active (exercise 3 to 5x/week): BMR × 1.55
- Very active (intense exercise 6 to 7x/week): BMR × 1.725
- Extremely active (physical job + daily exercise): BMR × 1.9
Back to our example: a lightly active woman with a BMR of 1389 kcal would have a TDEE of roughly 1389 × 1.375 = 1910 kcal. That's the number that should serve as your baseline for setting a calorie deficit or surplus, not the BMR on its own.
Why these numbers shift over time
Your metabolism isn't set in stone. It changes based on several factors, and this is exactly where a lot of people mess up by sticking to the same calorie target for months on end:
- Weight loss naturally lowers your BMR: less body mass to run means fewer calories burned at rest.
- Metabolic adaptation: during prolonged restriction, your body becomes more energy-efficient (lower body temperature, an unconscious drop in NEAT). That's why a deficit that worked great at first can stall out after 2-3 months.
- Building muscle slightly boosts your BMR, since muscle tissue burns more energy at rest than fat does.
- Aging naturally lowers your BMR, largely due to muscle loss if it isn't actively maintained.
In practice, this means a calculation you do once is only accurate for a limited window of time. You need to revisit it regularly, especially if you're in a significant weight loss or weight gain phase.
How to use these numbers to hit your goal
Once you've calculated your TDEE, everything gets a lot simpler:
- Fat loss: aim for a deficit of 15 to 25% below your TDEE. A deficit that's too aggressive (over 30%) speeds up muscle loss and metabolic adaptation.
- Muscle gain: aim for a slight surplus of 10 to 15% above your TDEE, so you can build muscle without piling on too much fat along the way.
- Maintenance / getting back in shape: stay close to your TDEE, with balanced macros: enough protein to maintain your muscle mass, fat that doesn't drop too low, and carbs to fill the rest depending on your activity.
Calibrating the calculation against your scale
However solid it is, a formula is still an estimate: it knows neither your actual NEAT nor how accurate your food tracking is. The real judge is the trend in your weight over several weeks.
- Weigh yourself under the same conditions: in the morning, on an empty stomach, after using the bathroom, several times a week.
- Compare averages, not single weigh-ins: water, salt and whatever is in your digestive tract make the scale move from one day to the next, with no connection to your body fat.
- Give it at least two weeks before drawing conclusions: over a few days, those swings hide the trend.
- Adjust in small steps: if the trend isn't heading toward your goal, move by 100 to 150 kcal a day, then watch again. Cutting 500 kcal in one go risks tipping you into the overly aggressive deficit we talked about above.
How Kaizmax runs this calculation
Kaizmax applies this method to your questionnaire answers, with one difference on activity:
- Basal metabolic rate: the Mifflin-St Jeor formula, exactly as detailed above.
- Activity: a factor that only covers your day-to-day life outside of exercise, from 1.2 for a desk job to 1.65 for a physical job. Your workouts are added separately based on how many you do and how long they last, and so is your club sport if you play one. The classic multipliers already include exercise: adding your workout calories on top would count them twice.
- Goal: −20% of TDEE for fat loss, TDEE as is for maintenance, +10% for muscle gain, never dropping below 1200 kcal for a woman or 1500 kcal for a man.
- Macros: protein at 1.8 to 2.0 g per kg depending on your goal (with safeguards for higher body weights), fat around 28% of calories and never below 0.6 g per kg, carbs for the rest.
This calculation is done when your program is generated (the first generation is included in the free version; regenerating is a Premium feature), and your weigh-ins draw your weight curve so you can keep an eye on it yourself. Recalibrating is the job of the adaptive coach, a Premium feature (first adjustment free): at most once every 7 days, and never before your program is 7 days old, it compares the trend of your weigh-ins over 14 days (at least two weigh-ins, 5 or more days apart) with the expected range for your goal. If it falls outside that range, it adjusts your calories by 100 to 125 kcal through carbs and fat, without touching your protein, then rebuilds your week of meals around these new targets; your workouts stay as they are.
This article is informational and not a substitute for medical advice. In case of a health condition, pregnancy or doubt, consult a healthcare professional.
Want to go further? Browse all the Kaizmax guides. And for your tailored diet + training plan: download Kaizmax for free — a short questionnaire, ready in 2 minutes.