What Is BMR (Basal Metabolic Rate)?

BMR stands for basal metabolic rate. It is the energy your body uses just to keep you alive while you rest. No walking, no exercise, no effort at all. Here is what that means, the formula behind it, and a worked example you can follow.

The short version: your body spends calories around the clock to run its basic systems. BMR puts a number on that baseline. It is a fact about how your body works, not a target to chase.

What BMR actually measures

Even when you are still, your body is busy. Your heart pumps, your lungs move air, your cells repair themselves, and your temperature holds steady. All of that costs energy, measured in calories. BMR is the sum of that resting work over a day.

For most people, this baseline is the largest share of the calories they use in a day. The rest comes from movement: standing, walking, chores, and exercise. So BMR is the floor, and daily activity builds on top of it.

The Mifflin-St Jeor formula

The most common way to estimate BMR is the Mifflin-St Jeor equation. It uses four inputs: sex, weight, height, and age. There is one version for men and one for women.

Men: (10 × kg) + (6.25 × cm) − (5 × age) + 5

Women: (10 × kg) + (6.25 × cm) − (5 × age) − 161

The weight and height terms add energy, since a larger body has more tissue to maintain. The age term subtracts a little each year, because resting energy use tends to ease down over time.

A worked example, step by step

Take a 40-year-old woman who weighs 68 kg and stands 165 cm tall. Use the women’s formula.

1. Weight term: 10 × 68 = 680.
2. Height term: 6.25 × 165 = 1,031.25.
3. Age term: 5 × 40 = 200.
4. Combine: 680 + 1,031.25 − 200 − 161 = 1,350.25.

So her estimated BMR is about 1,350 calories a day. That is what her body would use at complete rest. It is an estimate, since two people of the same size can differ by a few hundred calories.

BMR is not your daily total

BMR covers rest only. To reach the calories a full day uses, you multiply BMR by an activity factor. The result is often called total daily energy expenditure. A lightly active day uses a factor of about 1.375.

For the example above, 1,350 × 1.375 is roughly 1,860 calories a day. A more active week pushes the factor and the total higher. These multipliers are broad averages, so treat the figure as a guide rather than a precise reading.

What moves your BMR

• Body size. More tissue, including muscle, uses more energy at rest.
• Age. Resting energy use tends to drift down over the years.
• Sex. Average differences in body composition shift the figure, which is why the formula splits by sex.
• Genetics and hormones. These can raise or lower the number in ways a formula cannot capture.

Estimating versus measuring

The formula gives a quick estimate from four inputs. A lab can measure resting energy use directly, usually by tracking the air you breathe in a calm, fasted state. That reading is more exact, but it needs equipment and is rarely necessary for general interest.

For most people, the formula is close enough to understand their energy baseline. If you have a medical reason to know the precise figure, a clinic can test it. Otherwise, read the estimate as a well-grounded starting point rather than an exact reading.

Common mistakes to avoid

• Reading BMR as a daily calorie goal. It is the resting baseline, not the full picture.
• Mixing units. The formula uses kilograms and centimeters, so convert pounds and inches first.
• Treating the estimate as exact. Real resting energy use varies from person to person.
• Forgetting the activity factor when you want a full-day figure.

Frequently asked questions

What is the difference between BMR and TDEE? BMR is what you burn at rest. TDEE, total daily energy expenditure, is BMR plus all your daily movement. You find TDEE by multiplying BMR by an activity factor.

How accurate is the formula? Mifflin-St Jeor is one of the more reliable equations for healthy adults, but it is still an estimate. A clinic can measure resting energy use directly if you need precision.

Does BMR change with age? Yes. It tends to ease down as the years pass, partly because muscle mass often declines. The formula reflects this by subtracting a little for each year.

Is BMR useful for setting calorie goals? It is a helpful reference for understanding your energy needs. For a personal plan, especially with a health condition, a doctor or registered dietitian can account for your full situation.

Do I need to do this by hand? No. The formula is good to understand, but a tool is faster. Use the one below to get your estimate from four inputs in a second.