BMR vs TDEE: the calorie numbers that matter

Basal Metabolic Rate is the energy your body uses at complete rest — lying still in a thermoneutral room, digesting nothing, just keeping you alive. Heart, lungs, kidneys, brain, cell turnover. For most adults it sits between 1,300 and 1,800 kilocalories per day.

BMR scales primarily with body weight, and within weight, with lean mass. A 75 kg athlete with 12% body fat has a higher BMR than a 75 kg sedentary person with 30% body fat, because muscle burns more calories at rest than fat does. Height, age, and sex also matter — BMR declines slowly with age (about 1–2% per decade after 60, according to the 2021 Pontzer study in Science), and declines with weight loss because there is less tissue to power.

BMR matters because it is the floor below which chronic eating becomes counterproductive. The body responds to sustained sub-BMR intake with adaptive thermogenesis — slowing the metabolic rate, raising hunger hormones, preferentially burning muscle over fat. You lose weight fast in the first weeks, then plateau at a lower body composition, and often regain.

Total Daily Energy Expenditure is BMR plus everything else you do in a day. It has four components: BMR itself (roughly 60–70% of TDEE), the thermic effect of food (TEF — the energy used to digest what you eat, about 10%), non-exercise activity thermogenesis (NEAT — fidgeting, walking the dog, standing at a desk, roughly 15–25%), and exercise activity thermogenesis (EAT — intentional workouts, 5–15% depending on how much you train).

TDEE is what weight loss and weight gain revolve around. Eat below your TDEE and you lose. Eat at TDEE and you hold steady. Eat above TDEE and you gain. The arithmetic is simple; the execution is where everything breaks.

TDEE is traditionally estimated by multiplying BMR by an activity factor on a five-level Harris-Benedict scale: 1.2 sedentary, 1.375 lightly active, 1.55 moderately active, 1.725 very active, 1.9 extremely active. The hard part is picking the right level honestly. Most office workers who "go to the gym three times a week" underestimate their true sedentary-desk-time and should use lightly active (1.375), not moderate (1.55).

Mifflin-St Jeor (1990). The modern gold standard. Developed on a representative US sample, validated in dozens of follow-up studies, now the default formula used in clinical practice. For men: 10 × kg + 6.25 × cm − 5 × age + 5. For women: 10 × kg + 6.25 × cm − 5 × age − 161.

Harris-Benedict revised (1984). The classic formula, revised by Roza and Shizgal. Still used in older nutrition software. Tends to overestimate modern bodies by about 5% because the original 1919 sample skewed leaner than contemporary averages.

Katch-McArdle (1977). Uses lean body mass directly: 370 + 21.6 × LBM(kg), where LBM is weight × (1 − body fat %). Identical for men and women. Most accurate of the three if you know your body fat percentage from a reliable source (DEXA, BodPod, or calibrated bioimpedance).

A ±100–150 calorie spread between formulas is normal. The calculator runs all three, highlights Katch-McArdle when you provide body fat, and falls back to Mifflin when you don't.

Maintain. Eat at your TDEE. The scale stays flat within its normal 1–2 kg weekly noise from water and glycogen.

Lose weight (moderate). TDEE − 500 kcal/day. Expect roughly 0.4 kg/week (about 1 lb) of fat loss. Sustainable for months. This is the safest deficit for someone who wants steady, long-term change.

Lose weight (aggressive). TDEE − 750 kcal/day. About 0.6–0.7 kg/week. Only appropriate when the resulting calorie target stays above your BMR. The calculator shows a warning if your aggressive target dips below your BMR and recommends consulting a registered dietitian before going lower.

Gain slowly. TDEE + 250 kcal/day. About 0.25 kg/week, suited to lean bulking — most of the gain is muscle if training is on point.

Gain fast. TDEE + 500 kcal/day. Roughly 0.5 kg/week. Muscle-to-fat ratio starts to deteriorate above this surplus unless the trainee is a beginner or returning from a layoff.

Once you have a calorie target, macronutrients split it into protein, carbs, and fat. The calculator offers three common presets.

Balanced 30/40/30. Thirty percent of calories from protein, forty from carbs, thirty from fat. General-purpose split that suits most goals.

High protein 40/30/30. Useful in a cut — protein is the most satiating macronutrient and protects lean mass during a deficit.

Low carb 35/20/45. Lower carb, higher fat. Fits anyone who finds it easier to stick with satiety-first eating patterns; also the default for anyone managing insulin sensitivity.

Convert percentages to grams with: protein × 4, carbs × 4, fat × 9 kcal per gram. At a 2,500 kcal maintenance goal on a balanced split, that is 188 g protein, 250 g carbs, 83 g fat daily — which is enough specificity to build a real meal plan around.

Formulas are estimates, not measurements. Individual metabolism varies by ±15% from formula predictions even among healthy adults. Use the calculator as a starting point, then adjust based on how your weight actually moves across a 3–4 week block.

Age and condition matter. The formulas were validated on adults 18–65. Pediatric and geriatric metabolism differ. Pregnancy and breastfeeding add 300–500 kcal/day with specific nutrient priorities. Thyroid disease, Cushing, and other endocrine conditions can shift BMR by 30%.

YMYL disclaimer. This content is for education, not medical advice. For weight loss below BMR, medical conditions affecting metabolism, or aggressive cutting phases, consult a registered dietitian or physician. The calculator page includes a prominent medical disclaimer covering this.

This article is educational only and not medical advice. Individual metabolism varies significantly, and underlying health conditions can affect calorie needs. Consult a registered dietitian or physician before making substantial changes to your diet or exercise routine, and especially before adopting aggressive calorie deficits.

Plug in your weight, height, age, sex, and activity level into the calculator. Add body fat percentage if you have a recent measurement from DEXA or bioimpedance — that unlocks Katch-McArdle and gives a more accurate BMR. Pick a goal (maintain, cut, bulk) and a macro split. The calculator returns your target calories, grams of each macronutrient, and a warning if the goal target drops below your BMR.

Re-run every 4–5 kg of weight change. BMR drops as body weight drops, so a 500-calorie deficit at 90 kg is not the same as a 500-calorie deficit at 80 kg. Recalibrating quarterly keeps the math honest as your body changes.