What Is BMR? Basal Metabolic Rate Made Simple

Your basal metabolic rate (BMR) is the number of calories your body needs to stay alive at complete rest. It covers the quiet, constant work you never think about: breathing, pumping blood, keeping warm, repairing cells and running your brain, liver and kidneys. For most people it’s the largest single part of daily energy use, and you can estimate it with a straightforward equation.

BMR vs RMR

You’ll often see two similar terms:

• BMR (basal metabolic rate) is measured under strict lab conditions: right after a night’s sleep, after fasting, lying still in a comfortably warm room.
• RMR (resting metabolic rate) is measured under looser conditions, so it often comes out slightly higher.

The gap between them is small, and most calculators use the terms interchangeably. Either one describes your energy use before you add digestion and movement.

What affects your BMR

BMR isn’t a fixed trait. It depends on:

• Body size. Larger bodies need more energy to maintain.
• Lean mass. Muscle and organs use more energy than fat tissue, so people with more lean mass tend to have a higher BMR.
• Age. BMR usually drifts down slowly through adulthood, partly because many people lose muscle over the years.
• Sex. On average, men have higher BMRs, mostly because of differences in body size and composition.
• Genetics. Some people simply run a bit higher or lower than predicted.
• Hormones. An underactive thyroid can lower BMR, while an overactive one can raise it.
• Life stage and health. Pregnancy, fever and recovery from injury all increase energy needs.

How to estimate BMR with Mifflin-St Jeor

The Mifflin-St Jeor equation is among the most commonly used formulas today. It uses weight in kilograms, height in centimeters and age in years:

• Men: BMR = (10 × weight) + (6.25 × height) − (5 × age) + 5
• Women: BMR = (10 × weight) + (6.25 × height) − (5 × age) − 161

To convert, divide pounds by 2.2 to get kilograms and multiply inches by 2.54 to get centimeters.

These examples show why no single average fits everyone. Height, weight and age each pull the result in different directions.

Measured vs estimated BMR

Equations are built from data on large groups, so your true BMR could be noticeably higher or lower than the formula says. Some clinics, hospitals and research labs measure it directly with a method called indirect calorimetry, which analyzes the oxygen you breathe in and the carbon dioxide you breathe out. For everyday purposes, an equation plus some real-world observation is usually enough.

Can you raise your BMR?

You can nudge it, though not dramatically:

• Strength training helps you build or preserve muscle, which supports a higher resting burn over time.
• Eating enough protein helps maintain muscle, especially as you get older.
• Avoiding extreme calorie cuts matters, because very aggressive dieting can prompt your body to conserve energy.
• Getting enough sleep supports the hormones involved in appetite and energy balance.

Be wary of products or foods that promise to “boost your metabolism.” Chili peppers, green tea and caffeine have only small, short-lived effects at best.

How BMR fits into the bigger picture

BMR is only one piece of the energy you use each day. Add the energy spent digesting food and moving around, and you get your total daily energy expenditure. Our guide to TDEE shows how to multiply BMR by an activity factor to estimate it. That larger number is the useful one for planning meals, whether you want to maintain your weight or create a gentle calorie deficit. If you’re curious about the unit behind all of this, see what a calorie is.

The bottom line

BMR is the energy your body uses at complete rest to keep you alive and functioning. The Mifflin-St Jeor equation gives a reasonable estimate from your weight, height, age and sex, though your real number may differ. You can support a healthy metabolism with strength training, adequate protein and good sleep, and by steering clear of extreme diets.