Understanding BMR and TDEE
What you'd burn lying still, plus everything else.
The two numbers nutrition apps rely on, the formulas that estimate them, and the activity multipliers that always overstate.
BMR — basal metabolic rate.
The energy your body burns at rest, doing nothing, in a thermoneutral environment. Just keeping you alive — heart beating, lungs moving, organs running, brain firing. For a sedentary adult, BMR accounts for 60-70 % of total daily energy expenditure. Measured precisely in a metabolic chamber; estimated from population formulas everywhere else.
The Mifflin-St Jeor formula.
The 1990 formula now considered the gold standard for BMR estimation (replacing the older Harris-Benedict formula): men: BMR = 10·weight(kg) + 6.25·height(cm) − 5·age + 5 and women: BMR = 10·weight(kg) + 6.25·height(cm) − 5·age − 161. The constants come from regression on a thousand-person calorimeter dataset. Typical accuracy: ±10 % for any individual, better on average across a group.
BMR ≈ 10·kg + 6.25·cm − 5·age ± 161 (sex)
TDEE — total daily energy expenditure.
BMR plus everything else: thermic effect of food (digestion, ~10 % of intake), non-exercise activity thermogenesis (NEAT — fidgeting, walking around the office, standing, ~15 % typical), exercise activity thermogenesis (the gym session, the run, ~5-15 %). Apps compute TDEE by multiplying BMR by an activity factor: sedentary 1.2, light 1.375, moderate 1.55, very active 1.725, extremely active 1.9.
A worked example.
A 35-year-old woman, 65 kg, 165 cm, moderately active. BMR = 10×65 + 6.25×165 − 5×35 − 161 = 650 + 1031 − 175 − 161 = 1345 kcal. TDEE at 1.55 multiplier = 1345 × 1.55 ≈ 2085 kcal. Eating at this number maintains weight; deficit (-500) loses ~0.5 kg/week; surplus (+300) gains lean mass slowly. Adjust based on actual outcome after two weeks; formulas are estimates.
Female, 35, 65 kg, 165 cm, moderate
Mifflin-St Jeor × 1.55
Compute BMR, multiply by activity factor.
(650 + 1031 − 175 − 161) × 1.55
= ≈ 2085 kcal TDEE
The activity multiplier is usually wrong.
The 1.55 "moderately active" multiplier corresponds to ~30-60 minutes of moderate exercise 3-5 days per week. Almost everyone overestimates their own activity level. The honest baseline is 1.2 (truly sedentary, desk job, no exercise) or 1.375 (lightly active, walking around, occasional activity). If your TDEE estimate produces weight gain at maintenance, you picked too high a multiplier; recalibrate downward.
Body composition changes the math.
Mifflin-St Jeor uses weight as a proxy for lean body mass. For people with unusual body composition — high muscle mass (bodybuilders), high fat mass (obesity) — the formula skews. The Katch-McArdle formula uses lean body mass directly: BMR = 370 + 21.6 × LBM(kg). More accurate when you know LBM (via DEXA, bio-impedance); not knowable for most people, so Mifflin remains the default.
Adjust to your actual outcome.
Treat the formula's TDEE as a starting point. Track intake and weight for two weeks. If you maintain at the predicted number, the formula nailed it. If you gain on it, your real TDEE is ~200 kcal lower; adjust down. If you lose, adjust up. The body's actual metabolism is the only ground truth; formulas are the prior, your data is the posterior.