Your body burns calories even when you're doing absolutely nothing — scrolling through your phone in bed, watching TV, or sleeping. That baseline burn is called your Basal Metabolic Rate (BMR).
The Simple Definition
BMR is the number of calories your body needs to perform basic life-sustaining functions: breathing, circulating blood, regulating body temperature, and keeping your cells running. Think of it as your body's "idle speed" — the minimum fuel it needs just to exist.
For the average adult, BMR accounts for 60–75% of total daily calorie expenditure. That's a surprisingly large chunk, considering it requires zero effort on your part.
How BMR Is Calculated
The most widely used and research-backed formula is the Mifflin-St Jeor equation, developed in 1990:
For men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
For women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161
Example: A 30-year-old woman who is 165 cm tall and weighs 65 kg has a BMR of approximately 1,445 calories/day.
What Affects Your BMR?
Several factors raise or lower your BMR significantly:
- Body size: Larger bodies have more tissue to maintain, so they burn more calories at rest.
- Muscle mass: Muscle tissue is metabolically active — it burns more calories than fat even at rest. This is why strength training helps long-term weight management.
- Age: BMR decreases by roughly 1–2% per decade after age 20, largely due to gradual muscle loss.
- Sex: Men typically have higher BMRs than women of the same weight, primarily because of greater average muscle mass.
- Thyroid function: The thyroid gland directly controls metabolic rate. Hypothyroidism can lower BMR by 30–40%.
- Body temperature / illness: Fever raises BMR by approximately 7% for every 1°C increase in body temperature.
BMR vs. TDEE: What's the Difference?
BMR is just your resting burn. Your Total Daily Energy Expenditure (TDEE) adds everything else: walking, working, exercising, and even digesting food (a process called the thermic effect of food, which accounts for roughly 10% of TDEE).
TDEE = BMR × Activity Factor
If your BMR is 1,500 calories and you're moderately active, your TDEE is around 2,325 calories — meaning you need 2,325 calories per day just to maintain your current weight.
Why This Actually Matters
Knowing your BMR gives you a solid starting point for any nutrition goal:
- Eating below your BMR for extended periods is risky. Your body may downregulate metabolism and break down muscle for energy.
- A safe calorie deficit is typically 250–500 calories below TDEE — not below BMR.
- Strength training preserves and builds muscle, keeping your BMR higher as you age or lose weight.
The numbers are estimates — individual variation exists — but they give you a meaningful baseline that's far better than guessing.
Calculate Yours Now
Use our free BMR and TDEE calculator to get your personalized numbers in seconds. Enter your weight, height, age, sex, and activity level — and we'll calculate your BMR, TDEE, and daily calorie targets for weight loss, maintenance, and muscle gain.