Your body burns calories even if you do absolutely nothing — lying still in bed all day, your heart, brain, lungs, liver, and every cell keep consuming energy. That background burn is your basal metabolic rate (BMR): the number of calories your body needs in 24 hours to maintain its most basic functions at complete rest.
BMR typically accounts for 60–75% of your total daily calorie burn, which is why it sits at the foundation of every calorie goal — weight loss, muscle gain, or maintenance. This guide covers the formula, works examples, and shows how BMR turns into your real daily target.
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What BMR actually covers
BMR is measured under strict laboratory conditions: you are awake, lying down, at complete rest, in a thermoneutral room, 12 hours after eating. It powers breathing, circulation, brain activity, cell repair, and hormone production. It does not include digesting food, walking to the kitchen, fidgeting, or workouts — all of that sits on top of BMR.
The Mifflin-St Jeor formula
The equation most dietitians use today comes from Mifflin and St Jeor's 1990 study, validated against indirect calorimetry:
Men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
The older Harris-Benedict equations (originally from 1918–1919, revised 1984) still appear in textbooks, but research consistently finds Mifflin-St Jeor more accurate for healthy adults, which is why the Academy of Nutrition and Dietetics adopted it. Expect roughly ±10% error for any individual — good enough for planning, not lab-grade.
Worked example
A 35-year-old woman weighing 65 kg at 165 cm tall:
- Weight term: 10 × 65 = 650
- Height term: 6.25 × 165 = 1,031.25
- Age term: 5 × 35 = 175
- BMR = 650 + 1,031.25 − 175 − 161 = 1,345 calories/day
A 35-year-old man at the same weight and height: 650 + 1,031.25 − 175 + 5 = 1,511 calories/day. The sex difference reflects average differences in lean mass — it is not a judgment, just the math.
From BMR to your real daily target: TDEE
BMR is only the starting point. Your total daily energy expenditure (TDEE) adds everything else: digesting food (~10% of intake), all movement, and exercise. The standard shortcut multiplies BMR by an activity factor:
| Activity level | Factor | Example |
|---|---|---|
| Sedentary | 1.2 | Desk job, little exercise |
| Lightly active | 1.375 | Light exercise 1–3 days/week |
| Moderately active | 1.55 | Moderate exercise 3–5 days/week |
| Very active | 1.725 | Hard exercise 6–7 days/week |
| Extra active | 1.9 | Physical job plus training |
Our woman above at moderate activity: 1,345 × 1.55 ≈ 2,085 calories/day to maintain her weight. Weight goals then adjust from TDEE: roughly 300–500 fewer calories for steady fat loss (about 0.5 lb per week at a 500-calorie deficit), or 250–500 more for muscle gain. See how many calories per day for the full goal-setting walkthrough.
What raises or lowers your BMR
- More lean mass raises it — muscle burns roughly 6 kcal per pound per day at rest, fat about 2.
- Age lowers it gradually, about 1–2% per decade after the 20s, mostly via muscle loss.
- Extreme dieting can suppress it (adaptive thermogenesis) — another reason crash diets backfire.
- Thyroid conditions, pregnancy, fever, and extreme cold all shift it measurably.
The most reliable way to nudge BMR up is resistance training: adding 10 lb of muscle adds only ~40–60 kcal/day at rest, but the activity of maintaining it — and the afterburn of training — matters far more than the BMR bump itself.
Frequently asked questions
BMR is measured under strict laboratory conditions — complete rest, fasting, in a dark room. RMR (resting metabolic rate) is measured under looser real-world conditions and typically reads 10–15% higher. For everyday calorie planning, the difference barely matters; both estimate your at-rest burn.
It is the most accurate predictive equation validated against indirect calorimetry in studies of healthy adults, which is why the Academy of Nutrition and Dietetics adopted it. Expect roughly 10% error for individuals — close enough for calorie planning, not lab-grade precision.
Eating below BMR means eating less than your body needs for basic functions even with zero activity — a very aggressive target. Most guidelines suggest cutting 300–500 calories from your total daily expenditure (TDEE), not from BMR, and discussing larger deficits with a professional.
Yes, but modestly. Muscle burns roughly 6 calories per pound per day at rest versus about 2 for fat. Adding 10 pounds of muscle raises BMR by only around 40–60 calories a day — helpful, but nowhere near what popular claims suggest.
Disclaimer: Calculator health content is for education only — not medical advice. Body metrics are screening tools, not diagnoses. If you have concerns about your health, speak with a qualified health professional.