Whatever your goal — losing weight, gaining it, or holding steady — it all comes back to one number: how many calories your body burns in a day. That number has two parts. Your basal metabolic rate (BMR) is what you burn just existing. Your total daily energy expenditure (TDEE) adds activity on top. Here are the formulas, and more usefully, how to put them to work without getting lost in the decimals.
The energy ledger
Your weight follows a simple balance:
weight_change = calories_in − calories_out- calories_in = everything you eat and drink.
- calories_out = everything your body burns by breathing, moving, exercising, and maintaining itself.
Burn more than you eat and you lose stored fat; eat more than you burn and you store the rest. The idea is simple — the hard part is estimating the out side accurately, and that’s exactly what BMR and TDEE are for.
What BMR actually measures
BMR stands for basal metabolic rate — the calories your body needs at complete rest just to keep your heart, lungs, brain, and cells running. For most people it accounts for 60–75% of the day’s burn, which is why it’s the natural starting point.
The standard estimate is the Mifflin-St Jeor equation:
- Men:
BMR = 10 × weight_kg + 6.25 × height_cm − 5 × age + 5 - Women:
BMR = 10 × weight_kg + 6.25 × height_cm − 5 × age − 161
Example: a 30-year-old woman, 5’6” (168 cm), 140 lb (64 kg):
BMR = 10×64 + 6.25×168 − 5×30 − 161
BMR ≈ 640 + 1050 − 150 − 161 = 1,379 kcal/dayThat’s her maintenance floor: to survive doing nothing, her body uses roughly 1,380 calories a day.
The catch: you are rarely at rest
TDEE adds activity on top of BMR, using an activity multiplier:
| Activity level | Description | Multiplier |
|---|---|---|
| Sedentary | Desk job, little exercise | × 1.2 |
| Lightly active | Exercise 1–3 days/week | × 1.375 |
| Moderately active | Exercise 3–5 days/week | × 1.55 |
| Very active | Hard exercise 6–7 days/week | × 1.725 |
| Extra active | Physical job plus training | × 1.9 |
TDEE = BMR × activity multiplierSame woman, working out 4 days a week:
TDEE = 1,379 × 1.55 ≈ 2,137 calories/dayThat’s her maintenance number — eating exactly that means she neither gains nor loses.
Using the numbers: maintenance, deficit, surplus
- Maintenance — eat ~TDEE, hold body weight.
- Fat loss — eat below TDEE, with a sensible deficit around 200–500 calories per day. A pound of fat holds about 3,500 calories, so a 500/day deficit removes roughly a pound a week — the rate most guidelines call safe.
- Weight gain — eat above TDEE with a 200–400 calorie surplus plus training, to build muscle instead of just fat.
Worth keeping straight: the deficit always targets your TDEE. A “1,500 calorie” number someone quoted you doesn’t transfer — TDEE depends on your size, age, and activity. It’s personal.
Why your number isn’t exact
The formulas are estimates, not measurements. Your true TDEE will differ. Watch for:
- Equation drift. BMR formulas assume an average body composition; lean people tend to need more than these estimates say.
- Activity guess error. The “moderately active” row lumps in a lot of different weeks.
- Scale noise. Sweat, salt, meals, and glycogen swing the scale 2–4 lb in a single day. Judge yourself on a 7-day AVERAGE, not one morning.
- Tracking error. Most people underlog by 10–30%. Weigh food for the first few weeks while you calibrate.
Don’t fight these — plan for them. Start at your computed number, hold it for a week, and adjust in ±100-calorie steps based on your measured 7-day trend.
FAQ
What’s the difference between BMR and TDEE? BMR is the energy you’d burn at complete rest. TDEE = BMR × activity multiplier — the full burn of a normal day. Use TDEE, not BMR, to set your maintenance and deficit targets.
How many calories should I eat to lose weight? Start at TDEE minus 300–500. Dramatic deficits shed muscle and tank energy; gentler ones are easier to maintain.
How do I measure mine, or is BMR just an estimate? BMR estimates are equations, not measurements. A true reading takes a lab test (indirect calorimetry) — but the formula’s number is close enough to start your plan and adjust from what the scale actually does.
Why am I not losing weight on a “deficit”? Most often because your actual TDEE is lower than the formula suggests (activity multiplier set too high), or your logged portions are a lot larger than reality shows. Try a lower multiplier, weigh food, and give the 7-day average a couple of weeks to speak.
How much protein do I need while cutting? Roughly 0.7–1 g per pound of body weight helps protect muscle during a deficit. Whatever the rest of your calories, aim to keep protein up.