Understanding Your Basal Metabolic Rate (BMR)

Your Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest just to keep you alive — breathing, circulating blood, maintaining temperature, running your organs. For most people, it’s the single largest chunk of daily calorie expenditure, and understanding it clarifies everything else.

It is not the number you eat to: that is your TDEE, which adds digestion, daily movement and exercise on top. BMR is the foundation, usually 60–70% of the total.

What affects your BMR

• Body size: bigger bodies burn more at rest.
• Muscle mass: muscle is more metabolically active than fat, so more muscle raises BMR modestly.
• Age: BMR tends to decline with age, partly from muscle loss.
• Sex and genetics: these influence your baseline too.

How to estimate it

The Mifflin-St Jeor equation is the usual estimate. Men: 10 × weight in kg + 6.25 × height in cm − 5 × age + 5. Women: the same, minus 161 instead of plus 5. For a 40-year-old woman of 70kg and 165cm that gives about 1,370 calories. It is a solid starting point, though real BMRs vary around the prediction by a couple of hundred calories either way.

Can you “boost” your BMR?

Not dramatically, despite what supplements claim. The most legitimate lever is building muscle through strength training, which raises BMR modestly. Crash dieting does the opposite: it can lower BMR by costing you muscle.

Turning that number into a calorie target is the TDEE post’s job: it adds the activity factor and shows how to check the estimate against your own results.