What is BMR?
BMR stands for Basal Metabolic Rate. It's the number of calories your body burns just to stay alive — breathing, circulating blood, growing cells, keeping your brain running. If you lay in bed all day doing absolutely nothing, this is roughly what you'd burn. It's not glamorous, but it's the calorie foundation everything else builds on.
For most people, BMR accounts for 60-75% of total daily calorie burn. The less active you are, the larger that percentage becomes.
The Mifflin-St Jeor Equation
The most accurate way to estimate BMR for most people is the Mifflin-St Jeor equation, developed in 1990. It's considered more accurate than the older Harris-Benedict formula for modern populations.
For men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
For women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161
Try our BMR calculator to get your number instantly.
What Actually Affects Your BMR
Your BMR isn't fixed. It changes based on several factors:
- Muscle mass — More muscle means higher BMR. Muscle tissue burns calories even at rest.
- Age — BMR drops about 1-2% per decade after age 20. This is one reason people gain weight as they get older.
- Sex — Men typically have higher BMR than women because they carry more muscle mass on average.
- Genetics — Some people are born with faster or slower metabolisms. This is harder to change than you'd think.
- Hormones — Thyroid hormones regulate metabolism. Conditions like hypothyroidism lower BMR significantly.
- Temperature — Your body burns more calories in cold environments trying to maintain core temperature.
BMR vs RMR vs TDEE
These terms get confused a lot. BMR (Basal) is measured after 12 hours of fasting in a lab. RMR (Resting) is similar but measured under less strict conditions — close enough for practical purposes. TDEE (Total Daily Energy Expenditure) is what you actually burn in a day including activity. That's BMR multiplied by an activity factor.
Understanding BMR is the foundation of any weight goal. To lose weight you need to eat below your TDEE. To gain muscle you need to eat above it with enough protein and training stimulus.