Before you can lose fat, gain muscle, or simply maintain your weight, you need to know one number: how many calories your body actually burns every day. Most people guess - and that guess is usually off by 300-500 calories. Here's the science behind an accurate calculation.
What Is BMR - and Why It's Just the Starting Point
Your Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest - basically, just to keep your heart beating, lungs breathing, and organs functioning. It accounts for roughly 60-70% of your total daily energy expenditure.
BMR on its own is not the number you should eat at. Even a sedentary person burns additional calories through daily movement, digestion (the thermic effect of food), and exercise. That's where Total Daily Energy Expenditure (TDEE) comes in.
Key distinction: BMR = what you burn doing nothing. TDEE = what you actually burn in a full day. Always base your calorie goal on TDEE, not BMR.
The Mifflin-St Jeor Formula
There are several formulas for calculating BMR (Harris-Benedict, Katch-McArdle, others), but the Mifflin-St Jeor formula has been consistently shown in peer-reviewed research to be the most accurate for most people. It's the formula used by Slymo, registered dietitians, and major health apps worldwide.
BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
For women:
BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161
Step-by-Step Example
Let's calculate for a 30-year-old woman who is 168 cm tall and weighs 68 kg:
- 10 × 68 = 680
- 6.25 × 168 = 1,050
- 5 × 30 = 150
- BMR = 680 + 1,050 − 150 − 161 = 1,419 kcal/day
This means at complete rest, her body requires approximately 1,419 calories just to survive. But she doesn't spend all day in bed, so we need to apply an activity multiplier.
From BMR to TDEE: Activity Multipliers
Multiply your BMR by the factor that best describes your typical week. Most people overestimate their activity level - when in doubt, go one level lower.
| Activity Level | Description | Multiplier |
|---|---|---|
| Sedentary | Desk job, little to no exercise | × 1.2 |
| Lightly active | Light exercise 1-3 days/week | × 1.375 |
| Moderately active | Moderate exercise 3-5 days/week | × 1.55 |
| Very active | Hard exercise 6-7 days/week | × 1.725 |
| Extra active | Physical job + hard daily training | × 1.9 |
Back to our example: if she exercises moderately 3-4 times a week, her TDEE is:
1,419 × 1.55 = ~2,199 kcal/day. Eating exactly this amount, her weight stays stable over time.
Setting Your Goal: Deficit vs. Surplus
Fat Loss - Create a Deficit
Subtract 300-500 kcal/day from your TDEE for a moderate, sustainable deficit. This theoretically produces ~0.3-0.5 kg of fat loss per week. Avoid deficits larger than 750 kcal/day - aggressive restriction increases muscle loss and makes long-term adherence almost impossible.
- Gentle cut (−200 to −300 kcal): Ideal if you're close to goal or want to preserve muscle
- Standard cut (−300 to −500 kcal): Sweet spot - noticeable progress, still fuelling performance
- Aggressive cut (−500 to −750 kcal): Only short-term; requires high protein to protect muscle
Muscle Gain - Create a Surplus
Add 200-400 kcal/day above TDEE for a lean bulk. The body can only synthesize about 0.25-0.5 kg of new muscle per week under optimal conditions - anything beyond that tends to be stored as fat.
Why the Formula Is an Estimate - and How to Refine It
The Mifflin-St Jeor formula is based on population-level data. Individual metabolic rates can vary by ±10-15% due to genetics, hormones, and lean body mass. Here's how to calibrate over time:
- Track your calories consistently for 2-3 weeks using an app like Slymo.
- Weigh yourself daily (morning, after bathroom, before eating) and take the 7-day average.
- Compare expected vs. actual weight change. Losing faster? Your TDEE is likely higher. Slower? Lower.
- Adjust by 100-150 kcal and repeat. After 6-8 weeks, you'll have a personalized TDEE far more accurate than any formula.
Pro tip: As you lose weight, your BMR decreases because there's less body mass to maintain. Recalculate your TDEE every 5-7 kg of weight change to keep your targets accurate.
How Slymo Handles This For You
When you set up Slymo, it asks for your age, gender, height, weight, activity level, and goal. It then runs the Mifflin-St Jeor calculation and sets your daily calorie and macro targets automatically. If you connect Apple Health or Google Health Connect, Slymo even adjusts your daily calorie budget upward on days when you burn more than usual-so you always hit your deficit without under-fuelling training.
Let Slymo Do the Math
Download Slymo free - it calculates your personal calorie goal using Mifflin-St Jeor and lets you log any meal in under 10 seconds with AI.
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