BMR Calculator

BMR Calculator: How Many Calories Does Your Body Burn at Rest?

What Is BMR?

BMR stands for basal metabolic rate. It is the energy your body uses at complete rest to run its basic functions: breathing, pumping blood, keeping your brain working, repairing cells and holding your body temperature steady. Even if you stayed in bed all day, you would still burn this many calories.

For most people BMR makes up about 60% to 70% of the calories they burn in a whole day. The rest comes from moving around, exercising and digesting food.

How to Calculate Your BMR

BMR is estimated with an equation using your weight, height, age and sex. Weight is in kilograms, height in centimetres and age in years. There are three common equations.

Mifflin-St Jeor equation (1990)

This equation is widely considered the most accurate for healthy adults and is the one most calculators use.

Revised Harris-Benedict equation (1984)

An older equation that is still used for comparison. It tends to give slightly higher results than Mifflin-St Jeor.

Katch-McArdle formula

This formula uses lean body mass instead of total weight, so it can be more accurate for very lean or muscular people who know their body fat percentage.

BMR Calculator Examples

Example 1: a man. He is 45 years old, weighs 90 kg and is 180 cm tall.

Example 2: a woman. She is 40 years old, weighs 72 kg and is 168 cm tall.

Example 3: Katch-McArdle. A person with 55 kg of lean body mass has a BMR of 370 + 21.6 × 55 = about 1,558 calories.

The results differ a little between equations, which is normal. Every equation gives an estimate, and your real BMR can be roughly 10% higher or lower.

Average BMR by Body Weight

Approximate Mifflin-St Jeor BMR at age 30, for women who are 160 cm tall and men who are 175 cm tall:

Each extra kilogram of weight adds about 10 calories to BMR in this equation. Heavier bodies need more energy to run.

What Affects Your BMR?

• Body size: larger bodies burn more calories at rest.
• Muscle mass: muscle uses more energy than fat, although the difference is modest.
• Age: BMR slowly declines with age, mostly because people lose muscle.
• Sex: men usually have more muscle and a higher BMR than women of the same weight.
• Genes and hormones: an overactive thyroid can raise BMR and an underactive thyroid can lower it.
• Illness and fever: a body fighting an infection burns more energy.
• Very low-calorie diets: if you eat too little for a long time, your body can reduce the energy it uses.
• Pregnancy and breastfeeding: both raise calorie needs.

BMR During Pregnancy and Breastfeeding

Your BMR goes up gradually as pregnancy progresses, because your body is supporting a growing baby, a placenta and extra blood. Breastfeeding raises your energy needs as well. Because of this, standard BMR equations are only a rough guide in these stages, and they are not a reason to cut calories.

Don’t try to diet or lose weight while pregnant unless your doctor advises it. Your doctor or midwife can suggest how much you need to eat. If you are planning a pregnancy, check your starting category with our BMI calculator, and once you are pregnant follow your progress with the pregnancy calculator.

BMR vs RMR vs TDEE

Online calculators estimate BMR, and in everyday use BMR and RMR are close enough to be treated as the same number. TDEE is the figure you use to plan your daily food intake.

How to Use Your BMR

BMR is the base of your daily calorie needs, but it isn’t your daily calorie target. Multiply it by an activity factor to get your total daily needs, usually between 1.2 for a sedentary person and 1.9 for a very active one.

Eating only your BMR is usually too little for anyone who moves during the day. Health guidance generally says that women should not eat less than about 1,200 calories a day and men less than about 1,500 unless a doctor is supervising. Use a calorie calculator to find your daily needs.

Can You Increase Your BMR?

A little, yes, but many “metabolism boosters” are overrated. The most reliable steps are:

1. Strength training two to three times a week builds and protects muscle.
2. Eating enough protein helps you keep muscle, especially while losing weight.
3. Avoiding crash diets, which can lower your energy use and make weight easier to regain.
4. Sleeping seven to nine hours, because poor sleep disturbs appetite and activity levels.
5. Staying active through the day. This doesn’t change your BMR, but it raises your total daily burn.

How Accurate Is a BMR Calculator?

A BMR calculator is a good estimate with a typical error of about 10%. Two people with identical age, height, weight and sex can still differ by 10% to 15% in their real energy use. If you use BMR to plan a diet, track your weight for two to three weeks and adjust the plan based on what actually happens.

Recalculate your BMR whenever your weight changes by about 4 to 5 kg, as BMR falls with weight loss and rises with weight gain.

Common BMR Mistakes

• Eating your BMR instead of your total daily needs.
• Entering the wrong height or weight units.
• Using a goal weight instead of your current weight.
• Copying someone else’s calorie number.
• Never recalculating as your weight changes.

BMR Calculator FAQs

What is a normal BMR?

For many adults it is roughly 1,200 to 2,000 calories a day. Men generally have a higher BMR than women.

How do I calculate my BMR?

Use the Mifflin-St Jeor equation with your weight, height, age and sex, or enter them into a BMR calculator.

Which BMR formula is the most accurate?

For most healthy adults, Mifflin-St Jeor. Katch-McArdle can be better if you know your body fat percentage.

Should I eat my BMR to lose weight?

No. Start from your total daily calorie needs and subtract a moderate amount, not below the safe minimum.

Does muscle increase BMR?

Yes, but only modestly. Strength training still helps because it protects muscle while you lose fat.

Does BMR change with age?

Yes. It slowly declines as you age, mainly due to loss of muscle mass.

Is a low BMR the reason I can’t lose weight?

Rarely. Most plateaus come from underestimating food intake or overestimating activity. See a doctor if you suspect a thyroid problem.

Does BMR change during pregnancy?

Yes, it rises as pregnancy progresses. Ask your doctor or midwife how much you need to eat rather than relying on a standard calculator.

How often should I recalculate my BMR?

Whenever your weight changes by about 4 to 5 kg, or if your body composition changes significantly.

Final Thoughts

Your BMR is the starting point for understanding how your body uses energy. Use it as an estimate, combine it with your activity level to find your daily needs, and adjust based on real results over a few weeks. If you have a medical condition or are pregnant, ask your doctor for personal advice.