What BMR means
Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest to maintain basic physiological functions — breathing, circulation, organ function, temperature regulation, and cellular repair. It is measured under strict conditions: lying still, in a thermoneutral environment, after 12 hours of fasting.
For most people, BMR accounts for 60–75% of total daily energy expenditure. Even on your laziest day, your body is burning calories at this baseline rate. You cannot go below your BMR without serious health consequences.
How BMR is calculated: the Mifflin-St Jeor formula
The Mifflin-St Jeor equation (1990) is the most accurate BMR formula for most adults in normal weight ranges. Validated in multiple meta-analyses, it is now preferred over the older Harris-Benedict formula.
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
Woman, 30 years old, 65 kg, 165 cm:
BMR = (10 × 65) + (6.25 × 165) − (5 × 30) − 161
BMR = 650 + 1031.25 − 150 − 161 = 1,370 kcal/day
BMR vs TDEE: the difference that matters for dieting
BMR is only the starting point. Your actual daily calorie need is your Total Daily Energy Expenditure (TDEE), which is BMR multiplied by an activity factor. Sedentary people have a TDEE roughly 1.2–1.3× their BMR; very active people can reach 1.7–1.9×.
The critical rule: never eat below your BMR for extended periods. Doing so can cause muscle loss, hormonal disruption, immune suppression, and metabolic slowdown. Most nutritionists recommend keeping intake at or above BMR, with a safe deficit achieved by reducing calories from TDEE, not below BMR.
What affects your BMR?
Several factors raise or lower your BMR significantly:
- Lean muscle mass — muscle burns roughly 3× more calories at rest than fat. More muscle = higher BMR.
- Age — BMR typically decreases 1–2% per decade after age 30, primarily from muscle loss.
- Sex — men average 5–10% higher BMR than women due to greater muscle mass and lower body fat percentage.
- Body size — larger bodies require more energy to maintain; BMR rises with weight and height.
- Thyroid function — hypothyroidism can reduce BMR significantly, causing unexplained weight gain.
- Extended calorie restriction — severe dieting reduces BMR beyond what weight loss alone would predict (adaptive thermogenesis).
How to use your BMR practically
Calculate your BMR, then multiply by your activity factor to get TDEE. Subtract 300–500 kcal from TDEE to set a fat loss target. This gives you a number grounded in your actual physiology rather than a generic recommendation like "eat 1,200 calories."
Re-check your BMR every 4–8 weeks as your weight changes. A 5kg weight loss lowers BMR by approximately 50–100 kcal/day, so your targets should be recalculated regularly to maintain the intended deficit.