How to Calculate Your BMR (Basal Metabolic Rate)

What Is Basal Metabolic Rate?

Basal Metabolic Rate (BMR) is the number of calories your body requires to sustain its basic physiological functions at complete rest — no digestion, no movement, thermoneutral environment. Think of it as the energy cost of simply existing: keeping your heart beating, your lungs breathing, your brain active, your kidneys filtering, and your cells repairing themselves around the clock.

For most adults, BMR accounts for 60–70% of Total Daily Energy Expenditure (TDEE). It is the largest single component of your energy budget, which is why even small changes to the factors that drive BMR — particularly muscle mass — have meaningful long-term effects on body composition.

The Mifflin-St Jeor Formula

The most widely validated BMR equation in current clinical use is the Mifflin-St Jeor equation, published in 1990. It was developed from measurements on 498 healthy adults and has been shown in subsequent studies to predict resting energy expenditure within about 10% for most people — outperforming the older Harris-Benedict equation.

The formulas differ by sex:

The only difference between the two is the final constant: +5 for males and−161 for females. This 166 kcal gap reflects the average difference in lean body mass and hormonal metabolism between sexes at equivalent height and weight.

Worked Example

A 35-year-old woman who weighs 68 kg and stands 165 cm tall:

BMR = (10 × 68) + (6.25 × 165) − (5 × 35) − 161 = 680 + 1,031.25 − 175 − 161 = 1,375 kcal/day

This is the energy she would need if she were confined to complete bed rest. In practice, with normal daily movement, her actual calorie needs will be considerably higher.

Four Factors That Drive BMR

1. Muscle Mass

Skeletal muscle is metabolically active tissue — it burns calories even at rest, unlike fat tissue which is largely inert. Research suggests that each kilogram of muscle burns approximately 13 kcal per day at rest, while each kilogram of fat burns only about 4.5 kcal. This is why resistance training has long-term metabolic benefits well beyond the calories burned during the workout itself: gaining muscle permanently raises your BMR.

2. Age

BMR declines with age, and the Mifflin formula captures this with its age term. The decline is partly explained by the natural loss of muscle mass (sarcopenia) that begins in the 30s and accelerates after 60, and partly by hormonal changes. Studies suggest BMR falls by roughly 1–2% per decade in adults who maintain a stable body composition, but the muscle loss component is modifiable with resistance exercise.

3. Height and Weight

Larger bodies have more cells to maintain and more surface area to keep warm, so they burn more calories at rest. Height influences BMR partly through its relationship with lean body mass — taller people tend to carry more muscle and organ mass. Weight matters because it includes all metabolically active tissue. Two people of identical weight but different body composition will have different BMRs: the more muscular individual burns more at rest.

4. Sex

On average, males have higher BMRs than females at equivalent heights and weights, primarily because males carry a greater proportion of lean muscle mass. Hormonal differences also play a role — testosterone promotes muscle protein synthesis, while estrogen has different effects on fat distribution and metabolic rate. These differences are real at the population level but overlap considerably between individuals.

Why BMR Alone Is Not Enough for Nutrition Planning

Eating at your BMR would be a serious mistake. BMR represents your energy needs at complete bed rest — not your actual daily needs as a functioning person. Even a sedentary desk worker who never intentionally exercises will burn 20% more than their BMR just from digesting food and moving around their home. The correct number to use as your calorie baseline is your TDEE (Total Daily Energy Expenditure), which multiplies BMR by an activity factor to reflect your real movement pattern.

BMR is useful as a floor — eating significantly below your BMR is rarely advisable because it risks muscle loss, hormonal disruption, and nutrient deficiency. Most clinicians recommend never eating below 1,200 kcal for women or 1,500 kcal for men, which in practice means staying above or at BMR for most people.