BMR

BMR (Basal Metabolic Rate) is the energy your body uses at complete rest, lying down, awake, fasted, in a thermally-neutral environment. It accounts for the calories needed to sustain core biological functions — heart, lungs, brain, liver, kidneys — without any physical activity.

Typical BMR is 1200-1800 kcal/day for adult women, 1400-2200 for adult men, varying with weight, height, age, and muscle mass. The most accurate predictive formula for modern adults is Mifflin-St Jeor (1990):

Male: BMR = 10·weight(kg) + 6.25·height(cm) − 5·age + 5
Female: BMR = 10·weight(kg) + 6.25·height(cm) − 5·age − 161

BMR is the foundation for TDEE (total daily energy expenditure) — multiply BMR by an activity factor and you get the calories needed to maintain weight. Use our BMR calculator for both.

Worked example

A 35-year-old man, 180 cm, 80 kg. Mifflin-St Jeor: BMR = 10(80) + 6.25(180) − 5(35) + 5 = 800 + 1125 − 175 + 5 = 1755 kcal/day. To convert to TDEE for a sedentary office worker, multiply by 1.2: ~2,106 kcal/day. For light exercise 3-5 days/week, multiply by 1.55: ~2,720 kcal/day. To lose roughly half a kilo per week, eat ~500 kcal/day below TDEE — sustainable, evidence-based. Now compare with Harris-Benedict (revised 1984) for the same person: BMR = 88.4 + 13.4(80) + 4.8(180) − 5.7(35) = 88.4 + 1072 + 864 − 199.5 = 1825 kcal/day, about 4% higher. For a lean 80 kg man with 12% body fat (70 kg lean mass), Katch-McArdle gives 370 + 21.6(70) = 1882 kcal/day, higher still — closer to truth for the athletic case.

Three lifestyle factors shift BMR noticeably from the predicted value: chronic dieting (severe calorie restriction can drop BMR by 10-15% via metabolic adaptation, observed in the “Biggest Loser” follow-up study), thyroid function (hyperthyroidism raises BMR 20-50%, hypothyroidism lowers it similarly), and the menstrual cycle (BMR rises ~5-8% in the luteal phase). The formulas don’t account for any of these; treat their output as a baseline estimate to be adjusted from observed trends, not an oracle.

When and why it matters

BMR matters whenever calorie targets are being set: weight-loss diets, lean-bulk protocols for athletes, parenteral nutrition for hospitalised patients, and the “calories burned” estimates on every fitness watch. The most common mistake is using BMR alone as a daily calorie floor — BMR is the number you’d burn lying in a coma, not the number to eat. The second mistake is trusting a fitness tracker’s BMR figure unconditionally; smartwatch BMR estimates are typically Mifflin-St Jeor applied to user-entered weight and age, with no body-composition input, and they drift by 10-20% from measured metabolic-cart values. For clinical settings (cancer cachexia, ICU nutrition, post-bariatric tracking), indirect calorimetry — measuring actual oxygen consumption and CO₂ production — is the gold standard. For everyday weight-management goals, a Mifflin-St Jeor estimate plus 4-6 weeks of weight-trend feedback is accurate enough. Reference: USDA Nutrition.gov — Healthy Living Tools.

Mifflin-St Jeor vs Harris-Benedict vs Katch-McArdle: Mifflin-St Jeor is the consensus best predictor for adults of typical body composition — a 2005 American Dietetic Association review concluded it was within 10% of measured BMR more often than the 1919 Harris-Benedict equation. For lean, muscular individuals (athletes, bodybuilders), Mifflin underestimates because it has no body-composition input — Katch-McArdle (BMR = 370 + 21.6 · lean_mass_kg) uses lean mass directly and is more accurate when you have a DEXA or accurate body-fat reading. For children, neither applies; use the WHO/FAO age-specific equations.

What BMR doesn’t include — and why it matters: BMR is measured after 12 hours of fasting, lying still, in a thermoneutral room. The figure your calculator returns excludes the thermic effect of food (TEF, ~10% of intake), the energy of digestion, fidgeting (NEAT), and any deliberate movement. The slightly larger quantity that includes TEF and a small allowance for incidental movement is called Resting Metabolic Rate (RMR); RMR is what fitness trackers and indirect-calorimetry mouthpieces actually measure, and is typically 5-10% higher than true BMR. Many calculators report “BMR” when they mean RMR — the labels are used loosely in the consumer fitness space. Reference: Mifflin MD et al., American Journal of Clinical Nutrition, 1990.