BMR Calculator

What is BMR?

Basal metabolic rate (BMR) is the energy your body spends just to stay alive while doing nothing: keeping the heart beating, the lungs moving, the brain and kidneys working and body temperature steady. It is measured in kilocalories per day, which food labels in the US simply call Calories.

In the laboratory BMR is measured after a night's sleep and a 12-hour fast, lying awake and relaxed in a room at a comfortable temperature. Outside the lab it is estimated from an equation fitted to measured data, which is what this calculator does. For most people BMR is the largest single part of daily energy use, typically 60 to 75 percent, so it is the natural starting point for any calorie target.

How to use the calculator

1. Choose US units or Metric units. This changes how height is entered (feet and inches, or centimetres and metres).
2. Enter your age in years (the minimum is 1) and pick male or female.
3. Enter your height. In US mode, feet or inches may be left blank; in metric mode pick centimetres or metres.
4. Enter your weight and pick kilograms or pounds with the selector. Weight and height are both required.
5. Press Calculate.

You get two lines. BMR is the daily figure in Calories. BMR (scientific) expresses the same energy per square metre of body surface per hour, which is the form used in physiology papers.

The equation behind the number

The calculator uses the Mifflin-St Jeor equation, published in 1990 from measurements on roughly 500 healthy adults. In later comparisons of the common equations it predicted resting energy within 10 percent of measured values for more people than its older rivals, which is why dietitians now favour it.

BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + ss = +5 for men and −161 for women

Example, man. Age 30, height 177.8 cm (5 ft 10 in), weight 77.11 kg (170 lb): 10 × 77.11 + 6.25 × 177.8 − 5 × 30 + 5 = 771.1 + 1,111.3 − 150 + 5 ≈ 1,737 Calories per day.

Example, woman. Age 25, height 160 cm, weight 60 kg: 600 + 1,000 − 125 − 161 = 1,314 Calories per day.

The "scientific" BMR line

Metabolic rate scales with body surface more closely than with weight, so researchers divide the daily figure by 24 to get Calories per hour and then by body surface area. This calculator uses the Du Bois surface area formula (see the body surface area calculator).

For the 30-year-old man above: 1,737 ÷ 24 ≈ 72.4 Calories per hour, his surface area is about 1.948 m², and 72.4 ÷ 1.948 ≈ 37.2 kCal per m² per hour. For the woman, 1,314 ÷ 24 ÷ 1.622 ≈ 33.8. Healthy adults mostly fall between roughly 30 and 40.

From BMR to daily calorie needs

BMR is the floor, not the budget. Movement, exercise and digesting food add to it. The usual shortcut is to multiply BMR by an activity factor to get total daily energy expenditure (the TDEE calculator does this for you).

Eating below the maintenance figure produces weight loss and above it weight gain, but as a rule of thumb never eat far below your BMR without medical supervision. The weight loss calorie calculator and the calorie calculator turn these numbers into a plan.

What raises or lowers BMR

• Body size and composition. Bigger bodies and bodies with more lean mass burn more. Muscle and organs are metabolically active, fat tissue much less so.
• Age. BMR drifts downward through adulthood, partly from loss of lean mass. The equation builds in 5 Calories less for every year.
• Sex. Men average a higher BMR than women of the same weight and height, mostly because they carry more lean mass.
• Hormones and health. An overactive thyroid raises BMR, an underactive one lowers it. Fever, pregnancy and breastfeeding all increase it.
• Dieting. Prolonged calorie restriction and weight loss lower BMR somewhat, by more than the lost tissue alone would explain.
• Climate. Living in cold conditions can raise BMR slightly.

To see how the age slope plays out, the table applies the equation to a reference man (178 cm, 80 kg) and woman (165 cm, 65 kg) at different ages. Real BMR does not fall in a straight line, so treat it as an illustration of the equation rather than a measurement.

Accuracy and limits

Even a good equation is an average. For most people the estimate lands within about 10 percent of the true value, and some people are further off. Use the number as a starting point, track your weight over a few weeks and adjust.

The equation was built on adults. It is not suitable for children, pregnant women or people with serious illness. Input far outside normal human ranges, such as a very low weight at a great age, can even produce a negative result, which is a sign the equation does not apply.

Results are estimates for general information, not medical or nutritional advice. If you have a medical condition or want to lose weight fast, talk to a doctor or registered dietitian. See the disclaimer.