"How many calories should I eat?" is the most-searched nutrition question on the internet — and it is also a math question. Behind every calorie target sits one formula, the Mifflin-St Jeor equation, plus one multiplier for how active you are. Here is the whole calculation, worked out by hand, so you can check any number an app gives you.
Step 1: Your BMR — What You Burn Doing Nothing
Your basal metabolic rate (BMR) is the energy your body needs just to stay alive: breathing, keeping warm, running your organs. The Mifflin-St Jeor equation, the version dietitians use most, is:
Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
Men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
Notice the only difference between the two formulas is the final constant: −161 for women, +5 for men. Everything else — weight, height, age — works the same way.
Worked Example: A 30-Year-Old Woman
Take a woman who is 30 years old, weighs 70 kg, and is 165 cm tall.
- Weight term: 10 × 70 = 700
- Height term: 6.25 × 165 = 1,031.25
- Age term: 5 × 30 = 150
- Put it together: 700 + 1,031.25 − 150 − 161 = 1,420.25
Her BMR is about 1,420 calories per day. That is what her body burns lying still all day — before a single step, chore, or workout.
Step 2: Your TDEE — BMR × Activity
Nobody lies still all day, so BMR is only half the answer. Total daily energy expenditure (TDEE) multiplies BMR by an activity factor:
| Lifestyle | Factor |
|---|---|
| Sedentary (desk job, little 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 |
| Extremely active (physical job + training) | 1.9 |
Our example woman exercises moderately, so her factor is 1.55:
TDEE = 1,420.25 × 1.55 = 2,201 calories per day
That 2,201 is her maintenance level: eat that much, and her weight stays roughly the same.
Step 3: Calories to Lose Weight
Weight loss is a deficit: eat less than your TDEE. The standard, sustainable deficit is 500 calories a day, which adds up to 3,500 calories a week — roughly 0.45 kg (1 lb) of body fat per week.
Target = 2,201 − 500 = 1,701 calories per day
So her weight-loss target is about 1,700 calories a day. Not 1,200, not 2,500 — 1,700, derived from her own numbers. Skip the arithmetic anytime with the free calorie calculator — it runs the same Mifflin-St Jeor math and lets you test different activity levels instantly.
A Second Example, to Prove the Pattern
Take a 35-year-old man, 80 kg, 180 cm, lightly active (1.375):
- BMR = 10 × 80 + 6.25 × 180 − 5 × 35 + 5 = 800 + 1,125 − 175 + 5 = 1,755
- TDEE = 1,755 × 1.375 = 2,413
- Weight-loss target = 2,413 − 500 = 1,913 calories per day
Same formula, same three steps, different person, different answer. That is the whole point: there is no universal calorie number, only your number.
5 Mistakes That Break the Calculation
- Using pounds and inches in the formula. Mifflin-St Jeor needs kilograms and centimetres. 154 lb is 70 kg; 5 ft 5 in is 165 cm. Convert first.
- Picking the wrong activity factor. Most people overestimate — a desk job with three gym sessions is "moderately active" (1.55), not "very active" (1.725).
- Confusing BMR with TDEE. BMR is your floor, not your target. Eating at BMR while exercising daily is an accidental crash diet.
- Cutting more than 500 calories. A 1,000-calorie deficit sounds faster, but it is hard to sustain and usually costs muscle along with fat.
- Forgetting the target moves. Lose 5 kg and your BMR drops — recalculate every few weeks, or your deficit quietly disappears.
Why 500? The 3,500-Calorie Rule
The 500-calorie deficit is not arbitrary. One pound of body fat stores roughly 3,500 calories of energy, so a daily deficit of 500 calories adds up to 3,500 over a week — about 0.45 kg (1 lb) lost per week. That pace matters: faster loss usually means losing muscle alongside fat, and crash deficits are the ones people abandon. A 300-calorie deficit works too, just slower (about 0.27 kg a week); a 750-calorie deficit is faster but harder to sustain and only sensible for people with a lot to lose. The formula stays the same either way — only the final subtraction changes. Pick the deficit you can actually live with for months, because consistency beats intensity here.
How Often Should You Recalculate?
Your target is not permanent. Lose 5 kg and your BMR drops, because a lighter body burns less energy just existing. Our example woman at 65 kg instead of 70 kg: BMR = 700 − 50 = 1,370.25 (the weight term falls by 50), TDEE = 1,370.25 × 1.55 = 2,124, and her new target is 1,624 — about 77 calories lower than before. Recalculate every 4–6 weeks, or every time you lose 5 kg, or your deficit quietly shrinks to nothing and progress stalls for no visible reason. This is also why "eat 1,700 calories forever" fails: the number that worked in January is not the number that works in June.
The Bottom Line
"How many calories should I eat" has a real answer, and it is three steps: BMR from Mifflin-St Jeor, TDEE from your activity factor, then minus 500 for steady weight loss. Do it once by hand and no app's number will ever look like magic again. For the instant version, use the free calorie calculator — and if you want to know how many calories should i eat at a different activity level, just change the factor and watch the target move.
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