What BMR actually means
Your Basal Metabolic Rate (BMR) is an estimate of how many calories your body burns in a day doing absolutely nothing no walking, no digesting a meal, not even sitting up. It's the energy your body needs just to keep your heart beating, your lungs breathing, and your brain running while you're at complete rest. Every calorie you burn on top of that comes from movement, digestion, and daily activity.
The Mifflin-St Jeor formula
There are several BMR formulas in circulation, but Mifflin-St Jeor (published in 1990) is the one most modern nutrition guidelines and calculators including ours use, since it's been shown to be more accurate for the general population than the older Harris-Benedict equation. It uses your weight, height, age, and sex:
Men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + 5
Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) − 161
The only difference between the two is the final constant men get +5, women get −161. That gap exists because, on average, men carry more muscle mass relative to body weight than women, and muscle burns more calories at rest than fat does.
Worked example
Let's calculate BMR for a 30-year-old woman who weighs 65 kg and is 165 cm tall:
BMR = (10 × 65) + (6.25 × 165) − (5 × 30) − 161
BMR = 650 + 1,031.25 − 150 − 161
BMR = 1,370.25 calories/day
That's roughly how many calories her body would burn in 24 hours lying perfectly still. Now let's try a 25-year-old man who weighs 80 kg and is 180 cm tall:
BMR = (10 × 80) + (6.25 × 180) − (5 × 25) + 5
BMR = 800 + 1,125 − 125 + 5
BMR = 1,805 calories/day
Turning BMR into a real calorie target
BMR alone isn't very useful on its own, since almost nobody actually lies motionless all day. The next step is multiplying it by an activity multiplier to get your Total Daily Energy Expenditure (TDEE) the number most people actually want when they're planning their diet:
- Sedentary (little to no exercise): BMR × 1.2
- Lightly active (light exercise 1–3 days/week): BMR × 1.375
- Moderately active (moderate exercise 3–5 days/week): BMR × 1.55
- Very active (hard exercise 6–7 days/week): BMR × 1.725
- Extremely active (physical job or twice-daily training): BMR × 1.9
Going back to our 30-year-old woman with a BMR of 1,370.25: if she's moderately active, her TDEE is 1,370.25 × 1.55 ≈ 2,124 calories/day that's roughly how many calories she'd need to eat to maintain her current weight. Eating consistently below that number leads to weight loss; eating above it leads to weight gain.
BMR Calculator → plug in your own numbers and get your BMR and TDEE instantly, no math required.A few caveats
Mifflin-St Jeor is an estimate based on population averages, not a precise measurement of your individual metabolism actual BMR varies by genetics, muscle mass, and hormone levels, and can only be measured exactly with a lab test (indirect calorimetry). It's a solid starting point for planning, not a medical diagnosis.