Every diet, bulk, and “why am I not losing weight” conversation runs on three numbers: BMR, the energy your body burns doing nothing; TDEE, what it burns in your actual life; and the calorie balance between TDEE and what you eat. Get those three straight and most nutrition arithmetic becomes simple subtraction. This guide walks through the whole system with worked numbers, and our free BMR and TDEE calculator runs your own in seconds.
In this guide
BMR: the engine at idle
Basal metabolic rate is the energy your body spends just staying alive: heartbeat, breathing, brain, temperature, cell repair. It is what you would burn lying still all day, and for most people it is the single largest share of daily energy use. BMR scales mostly with body size, especially lean mass, which is why it appears in formulas as a function of weight, height, age, and sex rather than willpower.
The Mifflin-St Jeor formula, worked
The most widely used modern estimate:
- Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
- Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161
A 30-year-old man, 80 kg, 180 cm: 10 × 80 + 6.25 × 180 − 5 × 30 + 5 = 800 + 1,125 − 150 + 5 = 1,780 kcal/day. A 28-year-old woman, 65 kg, 165 cm: 650 + 1,031 − 140 − 161 = 1,380 kcal/day. These are estimates with real individual spread, typically within about 10% for most people, but they are the right starting point, and far better than guessing.
TDEE: multiplying by your real life
Total daily energy expenditure adds everything else on top of BMR: walking, training, fidgeting, digesting. The standard estimate multiplies BMR by an activity factor:
| Activity level | Multiplier | TDEE (BMR 1,780) |
|---|---|---|
| Sedentary (desk job, little exercise) | 1.2 | 2,136 |
| Light (1 to 3 workouts/week) | 1.375 | 2,448 |
| Moderate (3 to 5 workouts/week) | 1.55 | 2,759 |
| Active (6 to 7 workouts/week) | 1.725 | 3,070 |
| Very active (physical job + training) | 1.9 | 3,382 |
The honest weakness of the whole system lives in this table: people routinely overrate their activity level by one row. Picking “moderate” when your life is “light” inflates the estimate by about 300 kcal/day, which is enough to erase a planned deficit entirely. When in doubt, pick the lower row; we cover choosing and then correcting the multiplier from real-world data in the TDEE deep dive.
The calorie balance: the only lever
Against your TDEE, intake does exactly one of three things:
- Eat less than TDEE: the gap comes out of stored energy, mostly fat. A sustained 500 kcal/day gap is roughly 3,500 kcal/week, about half a kilogram of fat.
- Eat at TDEE: weight holds.
- Eat above TDEE: the surplus is stored.
Every diet that works, works through the first line, whatever its branding says about timing, food groups, or fasting windows. The structure of the diet decides how sustainable the gap is; the gap decides the result. How large that gap should be is its own question with real trade-offs, covered in the calorie deficit guide.
One person, the full chain
The 28-year-old woman from above, training moderately:
- BMR (Mifflin-St Jeor): 1,380 kcal
- TDEE = 1,380 × 1.55 = 2,139 kcal
- Goal: lose fat at a moderate pace → 20% deficit = 428 kcal → target intake about 1,711 kcal/day
- Expected pace: 428 × 7 = about 3,000 kcal/week, a bit under 0.4 kg of fat.
Four lines, no mystery. The same chain with a surplus instead of a deficit is a lean-gain plan. The calorie calculator automates the chain end to end, including pace targets.
Why the numbers feel wrong sometimes
- Formulas estimate averages. Two people with identical stats can differ by a couple hundred kcal/day. The fix is feedback: track weight trend for two or three weeks and adjust intake, rather than re-running formulas.
- Intake errors dwarf formula errors. Untracked oils, sauces, and weekend meals routinely add hundreds of kcal. When “eating at a deficit” produces no change for weeks, measurement is the first suspect.
- Adaptation is real but modest. Bodies in a prolonged deficit trim energy use somewhat, through less fidgeting and lighter body mass burning less. It slows progress; it does not stop arithmetic.
- Water hides fat. Day-to-day scale moves are mostly water and food in transit. Judge weekly averages, not mornings.
Run your own numbers
- BMR and TDEE calculator: the formulas above, with activity levels.
- Calorie calculator: target intake for loss, maintenance, or gain at a chosen pace.
- BMI calculator: the screening metric, explained fully in our BMI guide.
- Body fat calculator: the tape-measure method, a better progress metric than weight alone; the full method is in our Navy body fat guide.
- Ideal weight calculator: the classic reference formulas side by side, compared honestly in our ideal weight guide.
- Pace calculator: race times and training paces, with the full math in our pace guide.
Frequently asked questions
Is BMR the number of calories I should eat?
No. BMR is what the body burns at complete rest; you live above that. Targets are set against TDEE. Eating at or below BMR is an aggressive deficit that most people cannot and should not sustain without professional guidance.
Why do different calculators give different numbers?
Different formulas (Mifflin-St Jeor, Harris-Benedict, Katch-McArdle) and different rounding. The spread is usually under 10%, smaller than the error in most people’s intake tracking. Pick one, stay consistent, and correct from real results. Mifflin-St Jeor is used here because validation studies favor it for the general population; the original 1990 study is on PubMed.
Does muscle really burn more than fat?
Yes, lean tissue is metabolically more expensive, which is one reason BMR formulas using lean mass (like Katch-McArdle) can be more accurate for very lean or very muscular people. The day-to-day difference is modest, though; training matters more through the activity side of TDEE.
Do I need to hit the target exactly every day?
The weekly average is what your body experiences. A day 200 over and a day 200 under cancel out. Consistency over weeks beats precision on any single day.
Is this medical advice?
No. These are standard estimation formulas for general information. Anyone with a medical condition, a history of disordered eating, or significant weight to lose should plan with a qualified professional.