How many calories should I eat a day? The Mifflin-St Jeor formula, explained
There's no single right number of calories for everyone. What you need depends on your body, how active you are and what you want to achieve. Still, a well-tested formula gets most people close, and a few weeks of tracking show how to fine-tune it.
This is how Biscai calculates your daily target.
Step 1: Resting metabolism (BMR)
Your basal metabolic rate is the energy your body uses at rest: breathing, keeping warm, keeping your organs running. Biscai uses the Mifflin-St Jeor equation (1990):
- Men: 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + 5
- Women: 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) − 161
A 2005 systematic review in the Journal of the American Dietetic Association compared the common equations. Mifflin-St Jeor predicted resting energy use within 10% of the measured value for more people than any other. That's why most nutrition tools use it today.
Step 2: Activity (TDEE)
Next, BMR is multiplied by an activity factor. The result is your total daily energy expenditure, the calories you burn on a normal day:
| Activity level | What it means | Factor |
|---|---|---|
| Sedentary | Mostly sitting, no workouts | 1.2 |
| Lightly active | Walks or 1–3 light workouts a week | 1.375 |
| Moderately active | 3–5 workouts a week | 1.55 |
| Very active | 6–7 workouts a week | 1.725 |
| Extremely active | Physical job or hard daily training | 1.9 |
If you're unsure, pick the lower level. It's easier to raise your target later than to wonder why nothing changes.
Step 3: Your goal
- Maintain: eat about your TDEE.
- Lose weight: TDEE minus 500 kcal a day. As a rule of thumb, that's roughly 0.5 kg a week at first. In practice, weight loss slows over time as your body adapts.
- Gain weight: TDEE plus 300 kcal a day, a modest surplus.
Step 4: A safety floor
A deficit never takes the target below 1,200 kcal for women or 1,500 kcal for men, the minimums common in clinical guidance. The result is rounded to the nearest 50 kcal. Anything more exact would only look precise.
Two examples
Anna, 35, 165 cm, 68 kg, lightly active, wants to lose weight
- BMR: 10 × 68 + 6.25 × 165 − 5 × 35 − 161 = 1,375 kcal
- TDEE: 1,375 × 1.375 = 1,891 kcal
- Goal: 1,891 − 500 = 1,391 → 1,400 kcal a day
Mark, 40, 180 cm, 85 kg, moderately active, wants to keep his weight
- BMR: 10 × 85 + 6.25 × 180 − 5 × 40 + 5 = 1,780 kcal
- TDEE: 1,780 × 1.55 = 2,759 kcal
- Goal: maintain → 2,750 kcal a day
A starting point, not a verdict
Any formula describes the average person with your numbers. Your real needs can be a few hundred calories higher or lower. So:
- Eat at your target for two to three weeks and log honestly.
- Watch the trend of your weight, not single weigh-ins. Day-to-day weight jumps with water and salt.
- If the trend doesn't match your goal, adjust the target by 100–200 kcal and repeat.
Biscai is for people aged 16 and over. If you're pregnant or breastfeeding, have a medical condition or a history of disordered eating, talk to a doctor or a registered dietitian before you set a calorie target.
Sources
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition, 1990.
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic Association, 2005.
