Calorie needs and TDEE explained

BMR, activity multipliers, and why most online calorie targets are wrong by a few hundred a day.

"How many calories should I eat?" is the most-Googled diet question of all time, and the answer most calculators give you is probably wrong by a few hundred calories a day in either direction. The formulas are reliable; the numbers you plug in usually aren't. Here's what's actually going on under the hood.

Two numbers, not one

Your daily calorie need is built from two parts:

  • BMR (basal metabolic rate) — the calories your body burns at complete rest, just to run organs, repair tissue, and stay warm. This is roughly 60–70% of total daily burn for most people.
  • TDEE (total daily energy expenditure) — BMR plus everything else: walking around, exercise, fidgeting, digesting food. This is the number you actually want for diet planning.

Estimating BMR: the Mifflin–St Jeor equation

The most accurate widely-used BMR formula is Mifflin–St Jeor (1990). For metric units: 

Men:   BMR = 10·weight(kg) + 6.25·height(cm) − 5·age + 5
Women: BMR = 10·weight(kg) + 6.25·height(cm) − 5·age − 161

For a 30-year-old woman, 165 cm, 65 kg: 

BMR = 650 + 1031.25 − 150 − 161 = 1370 calories/day

Mifflin–St Jeor was derived from a few hundred adults in indirect calorimetry studies. It's accurate to within about ±10% for the average person. That ±10% is roughly ±150 calories on a 1500-calorie BMR. Worth remembering when you obsess over a 50-calorie difference in your meal log.

Activity multipliers: the source of most error

TDEE is BMR multiplied by an "activity factor" that estimates how much you move during the day. The standard ladder:

  • Sedentary (desk job, no exercise): BMR × 1.2
  • Lightly active (light exercise 1–3 days/week): BMR × 1.375
  • Moderately active (moderate exercise 3–5 days/week): BMR × 1.55
  • Very active (hard exercise 6–7 days/week): BMR × 1.725
  • Extra active (physical job + training): BMR × 1.9

Here's the catch: almost everyone overestimates their activity level. "Moderately active" sounds like a normal person who exercises a few times a week. In reality, that multiplier was calibrated against people who are genuinely burning meaningful calories — long sessions of structured activity, not three 30-minute peloton classes.

The other catch: even a "sedentary" person with a desk job who walks the dog and runs errands burns more than BMR × 1.2 in practice. The real spread is wider than the categories let on.

If you're estimating, err one notch lower than the description suggests. That alone gets the answer closer for most people.

A more accurate path: track and adjust

The formula gives you a starting point. Real TDEE varies by ±300 calories from formula predictions for the same person. The fix is simple: log honestly for two to three weeks, weigh yourself daily, and watch what your body actually does.

Each pound of body weight is roughly 3,500 calories of stored energy. So:

  • If your weight is stable on a daily-average basis, your intake equals your TDEE. Whatever you've been eating is your maintenance number.
  • If you're losing 1 lb per week, you're eating ~500 calories under TDEE per day. Add 500 to your average intake to find maintenance.
  • If you're gaining 1 lb per week, you're eating ~500 calories over TDEE per day. Subtract 500.

Use the formula to set the starting line. Use the scale to find the truth.

Why the scale doesn't move on day 4

Body weight on any given day is a noisy signal. Daily fluctuation of 1–3 lb is normal — it's mostly water, glycogen, and gut content responding to sodium, carbs, hydration, exercise, and the menstrual cycle. Real fat loss is signal under that noise.

Don't compare today to yesterday. Compare this week's average to last week's. After 3–4 weeks the trend line is usually clear.

Targeting weight loss or gain

Once you know your maintenance:

  • Sustainable cut — eat 300–500 calories under maintenance. This averages 0.5–1 lb of fat loss per week. Most people can sustain this without losing significant muscle if protein is high enough (~0.7–1 g per pound of body weight).
  • Aggressive cut — 750+ calories under maintenance. Faster loss but harder to sustain, more muscle risk, more rebound risk. Reserve for short pushes.
  • Lean bulk — eat 200–300 calories over maintenance. About 0.5 lb of weight gain per week, most of which can be muscle if you train hard.

Macros, briefly

Calories determine weight change. Macros (protein, fat, carbs) determine body composition and how you feel doing it.

  • Protein: 0.7–1 g per pound of body weight. Preserves muscle, keeps you full.
  • Fat: at least 0.3 g per pound of body weight. Hormone production needs it.
  • Carbs: whatever's left. They fuel training and brain function.

The short version

  • BMR is the baseline burn at rest. TDEE is total burn including activity.
  • Mifflin–St Jeor gives a good BMR estimate, ±10%.
  • Activity multipliers are usually overestimated — pick one notch lower than you think.
  • Use the formula as a starting point, then adjust based on actual scale trend over 3+ weeks.
  • Daily weight is noise. Weekly average is signal.

Run your own numbers in our calorie calculator and macro/TDEE calculator.