BMR is the floor, not the target
Your Basal Metabolic Rate is the calories your body burns over 24 hours if you did literally nothing — no walking, no thinking-but-sitting, no digesting. Just heartbeat, breathing, cell repair, brain idle.
Eat at or below BMR for long stretches and your body starts cannibalising lean tissue to keep critical systems running. That’s why the often-cited “1,200 kcal women / 1,500 kcal men” minimums exist — they’re the floor below which the body assumes a famine and starts breaking itself down to compensate.
TDEE is what you actually burn
Total Daily Energy Expenditure = BMR × an activity multiplier. The multipliers come from the original Harris-Benedict / WHO work:
| Activity level | Multiplier | What it means |
|---|---|---|
| Sedentary | 1.2 | Desk job, little or no deliberate exercise |
| Light | 1.375 | Exercise 1–3 days/week |
| Moderate | 1.55 | Exercise 3–5 days/week |
| Very Active | 1.725 | Hard exercise 6–7 days/week |
| Extra Active | 1.9 | Physical job + hard daily training, or 2× training a day |
For a typical 30-year-old, 70 kg, 175 cm male, that means:
- BMR (kcal/day, at rest)
- TDEE @ Moderate
- TDEE @ Extra Active
The 1,500 kcal/day difference between a desk job and a manual job is the entire reason “I work out a lot but can’t lose weight” is so common — most people overestimate their activity multiplier by a full notch.
If you walk to the kitchen, take the stairs, and “try to be active”, you’re sedentary (1.2). If you walk 30 min/day deliberately AND go to the gym 3×/week, you’re light to moderate. “Very active” is reserved for people who train more than 1 hour, almost every day, plus a non-sedentary job.
The three formulas
Mifflin-St Jeor (1990) — the default
The Academy of Nutrition and Dietetics’ recommendation. ~5% more accurate than Harris-Benedict for the modern population (which is heavier and less active than the 1919 population Harris-Benedict was calibrated on).
Male: BMR = 10 × weight_kg + 6.25 × height_cm − 5 × age + 5
Female: BMR = 10 × weight_kg + 6.25 × height_cm − 5 × age − 161The +5 / −161 split is what encodes the average difference in lean-mass-fraction between sexes at the same weight.
Harris-Benedict (revised 1984) — still everywhere
The grandparent equation. Calibrated in 1919, revised in 1984 (the version most calculators still use). Tends to overestimate modern adults by ~5%.
Male: BMR = 88.362 + 13.397 × weight_kg + 4.799 × height_cm − 5.677 × age
Female: BMR = 447.593 + 9.247 × weight_kg + 3.098 × height_cm − 4.330 × ageIf you’re comparing against an older app, textbook, or fitness tracker, this is the equation they’re using.
Katch-McArdle — best if you know body fat
Sidesteps the sex split entirely by computing lean body mass (LBM) first, then deriving BMR from LBM. Most accurate of the three if you have a real body-fat measurement (DEXA, hydrostatic, BodPod, skinfold).
LBM = weight_kg × (1 − bodyFat% / 100)
BMR = 370 + 21.6 × LBMSmart-scale body-fat estimates are too noisy for this — they can be off by 10+ percentage points day-to-day with hydration. Use Katch-McArdle only with a measured value.
Why your smart scale disagrees
Most smart scales display a “BMR” number that comes from bioelectrical impedance — running a small current through your body to estimate fat-free mass, then plugging into a proprietary equation. Three problems:
- Impedance is noisy — depends on hydration, time of day, recent food, foot moisture. ±15% day-to-day on the body-fat output isn’t unusual.
- The proprietary equations are not validated — every manufacturer rolls their own; they don’t publish them; they don’t have to.
- The “BMR” they show isn’t always BMR — some show resting energy expenditure (REE, slightly higher than BMR), some show “average daily burn” without explaining.
Putting BMR / TDEE into a calorie target
Once you have your TDEE, the math for a goal is simple:
- Maintain weight — eat at TDEE
- Lose fat sustainably — eat at TDEE minus 300–500 kcal (about a 10–15% deficit). Expected loss: 0.3–0.7 kg/week. Bigger deficits accelerate muscle loss and metabolic adaptation.
- Lean bulk — eat at TDEE plus 200–300 kcal. Expected gain: 0.2–0.4 kg/week. Bigger surpluses just add fat.
These numbers come from the published surplus/deficit-vs-body-composition meta-analyses (e.g. Helms 2014, Garthe 2011). They are starting points, not magic — recalculate every 3–6 weeks as your weight changes.
Set your target from the calculator, hold it for 14 days, weigh yourself daily (same time, same conditions), then average. If the trend isn’t moving the way the math predicts, adjust your intake by ±100–200 kcal. Don’t tweak before you have 14 days of data — daily weight noise will mislead you.
What this calculator doesn’t tell you
- Macronutrient split — protein/carbs/fat. The calorie target alone won’t tell you whether to eat 100 g of protein or 200 g; that’s a separate decision driven by lean mass and goals.
- Meal timing — pre/post workout, intermittent fasting windows. Calories matter most; timing matters at the margins.
- Adherence — the best calorie target is the one you’ll actually hit. A “perfect” plan you abandon in 3 weeks loses to an “okay” plan you keep for 6 months.
Try the calculator
Open the BMR Calculator, enter your details, and try clicking through the activity rows on the right — you’ll see TDEE swing by ~700 kcal between sedentary and extra active. That swing is the gap between “what your formula says” and “what your day actually burns” — and it’s the most common reason a calorie target doesn’t work the way someone expected.