BMR Calculator: Your Basal Metabolic Rate (Mifflin-St Jeor)
Your Basal Metabolic Rate (BMR) is the energy your body burns just to stay alive — heartbeats, breathing, brain activity, and cell repair. It typically represents 60–70% of your total daily calories. This free BMR calculator uses the Mifflin-St Jeor equation, the formula the Academy of Nutrition and Dietetics recognizes as most accurate for the general population.
How the BMR Calculator Works
Mifflin-St Jeor (1990):
BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + s
where s is +5 for men and −161 for women. Enter your measurements in either imperial or metric units — the calculator converts everything internally and displays results in kcal/day.
Worked Example
A 30-year-old woman, 5'6" (165 cm), 140 lb (63.5 kg):
- Weight term: 10 × 63.5 = 635
- Height term: 6.25 × 165 = 1,031
- Age term: 5 × 30 = 150
- Sex term: −161
- BMR = 635 + 1,031 − 150 − 161 = 1,355 kcal/day
This is what her body burns during 24 hours of complete bed rest. Real daily burn — her TDEE — is higher and depends on movement.
Why BMR Matters for Weight Management
Every diet works by manipulating the gap between intake and expenditure, and expenditure starts with BMR. Knowing yours answers practical questions:
- Setting a floor — eating far below BMR for weeks accelerates muscle loss and triggers adaptive thermogenesis (your body quietly burns less)
- Sanity-checking diets — any plan prescribing less than your BMR deserves skepticism
- Understanding plateaus — as you lose weight, BMR falls slightly (smaller body = smaller engine), so targets must be recalculated
Mifflin-St Jeor vs Other Formulas
| Formula | Year | Notes | |---|---|---| | Mifflin-St Jeor | 1990 | Most accurate for the general public (±10%) | | Harris-Benedict (rev.) | 1984 | The classic; overestimates by ~5% | | Katch-McArdle | — | Best if you know body fat % (uses lean mass) |
If you have a body fat estimate, Katch-McArdle computes BMR as 370 + 21.6 × lean mass (kg) — try our body fat calculator to get lean mass first.
What Changes Your BMR
- Age — BMR drops ~1–2% per decade after 30, mostly from muscle loss (much of it preventable with resistance training)
- Muscle mass — each kg of muscle burns ~13 kcal/day at rest; gaining muscle raises your floor permanently
- Body size — taller and heavier bodies burn more, which is why formulas scale with height and weight
- Illness and climate — fever can raise BMR 7–13% per °C; extreme cold adds shivering cost
From BMR to Actual Daily Calories
BMR alone never equals your daily needs. Multiply by an activity factor — 1.2 (sedentary) up to 1.9 (athlete) — to get TDEE. Our TDEE calculator does this in one step and suggests intake targets for losing, maintaining, or gaining weight.
BMR estimates are statistical, not prescriptions. If you have a medical condition affecting metabolism (thyroid disorder, PCOS), discuss calorie targets with a clinician.