BMR Calculator

What is BMR?

Basal metabolic rate (BMR) is the energy your body uses to stay alive at complete rest: breathing, pumping blood, keeping warm, repairing cells and running your brain. It is measured in kilocalories (kcal) per day.

For most adults, BMR is the largest part of the calories they burn each day, often around 60 to 70% of the total. The rest comes from movement, exercise and digesting food. Knowing your BMR is the first step to working out how much you should eat.

How is BMR calculated?

This calculator uses the Mifflin-St Jeor equation, published in 1990. A review of the common equations found it predicted measured resting metabolism within 10% more often than the others, which is why dietitians use it most.

For example, a 30-year-old man who is 175 cm tall and weighs 70 kg has a BMR of 700 + 1,093.75 − 150 + 5 = 1,648.75, or about 1,649 kcal a day.

Older equations such as Harris-Benedict give slightly different figures. The Katch-McArdle equation uses lean body mass instead, so it needs an accurate body fat measurement.

BMR and your daily calorie needs

Your total daily energy expenditure (TDEE) is your BMR multiplied by an activity factor. The calculator shows all five levels so you can pick the one that matches a typical week.

Most people who work at a desk and exercise a few times a week fall between lightly and moderately active. If you are unsure, choose the lower level: it is easy to overestimate how active you are.

What affects your BMR?

• Muscle: muscle burns more energy than fat, even at rest, so people with more muscle have a higher BMR.
• Age: BMR tends to fall as you get older, mainly because muscle mass drops.
• Sex: men usually have a higher BMR than women of the same size because they carry more muscle.
• Body size: taller and heavier people burn more energy to run a larger body.
• Hormones: an underactive thyroid lowers BMR and an overactive one raises it.
• Illness, fever and pregnancy raise energy needs, while crash dieting can lower BMR as the body tries to save energy.

Using BMR to manage your weight

• To keep your weight steady, eat around the calories shown for your activity level.
• To lose about half a kilo a week, eat roughly 500 kcal a day less than that figure.
• To gain weight, add around 300 to 500 kcal a day and pair it with strength training so more of the gain is muscle.
• Avoid eating below your BMR for long periods without medical advice. It can slow your metabolism and leave you short of nutrients.
• Recalculate after every few kilos of change, because your BMR changes with your weight.

How accurate is a BMR calculator?

Equations predict the average for people with your age, sex, height and weight. Your real BMR can be 10% or more above or below that. Very muscular people usually burn more than predicted, and some people with obesity or medical conditions burn less.

The only way to know your exact figure is a test called indirect calorimetry, which measures the oxygen you use while resting. For everyday planning, the estimate is close enough: track your weight for a few weeks and adjust your intake if it moves in the wrong direction.

When a slow metabolism needs a doctor

Gaining weight without eating more, feeling tired and cold all the time, or losing weight you cannot explain can all point to a thyroid problem or another condition. A simple blood test can check your thyroid levels.

Many health insurance plans include a free yearly health check-up, which often covers basic blood tests. It is worth checking what your policy offers before paying for tests yourself.

Frequently asked questions