BMR calculator:
how many calories you burn at rest

What BMR means and what it includes

Basal metabolic rate (BMR) is an estimate of the energy your body needs at complete rest, just to keep breathing, circulating blood, regulating temperature and running your organs. It is measured in kilocalories (kcal) per day.

Strictly, BMR is measured under tightly controlled conditions after a night's sleep and a long fast. What most online tools give you is closer to resting metabolic rate, estimated from your size and age with an equation.

It does not include walking, work, exercise or digesting food. That is why BMR is always lower than the calories you actually burn in a normal day.

Several things set the figure:

• Body size. A larger body has more tissue to maintain, so it needs more energy.
• Body composition. Lean tissue such as muscle uses more energy at rest than fat tissue does.
• Age. Estimates drift downwards across adulthood, partly through loss of muscle.
• Sex. On average men have more lean mass, which the equation reflects with a higher constant.
• Health and hormones. Thyroid conditions, illness, pregnancy and some medicines change resting needs and are not captured by a formula.

BMR formula: the Mifflin-St Jeor equation

The Mifflin-St Jeor equation is widely used for adults because it relies on three simple inputs: weight, height and age. There are two versions, one for men and one for women.

Each extra kilogram adds about 10 kcal a day, each extra centimetre of height adds about 6 kcal, and each year of age subtracts 5 kcal. The result is an estimate for the average person with those measurements.

How to calculate BMR: two worked examples

Use the same four steps each time:

1. 1Multiply your weight in kilograms by 10.
2. 2Multiply your height in centimetres by 6.25 and add it to step 1.
3. 3Subtract five times your age in years.
4. 4Add 5 if you are male, or subtract 161 if you are female.

If your measurements are in feet and pounds, convert first. A height of 5 ft 6 in is 66 × 2.54 = 167.6 cm, and a weight of 154 lb is 154 × 0.45359237 = 69.9 kg. Then apply the equation as above.

Round the answer to the nearest 10 or so. The equation cannot be more precise than the inputs, and real individuals vary by well over ten calories either way.

How BMR changes with weight and age

Sex shows up through the constant. For the same weight, height and age, the female equation gives 166 kcal less than the male one (5 minus −161). In the table the gap is larger, about 260 kcal, because the woman is also 15 cm shorter.

Weight has the clearest effect. In the table above, a 40 kg difference in body weight moves the estimate by about 400 kcal a day.

The equation lowers the estimate by 25 kcal for every five years of age. That is a population average; your own change depends on muscle mass and activity.

Height works in the opposite direction to age. Between two people of the same weight and age, the taller one has the higher estimate, by about 6 kcal for each extra centimetre, because a taller frame has more body surface and tissue to support.

How accurate is a BMR estimate?

Every predictive equation is an average drawn from groups of people. Yours may be higher or lower by a couple of hundred calories without anything being wrong, because muscle mass, genetics, hormones and recent illness all play a part.

The three results sit within about 120 kcal of each other, which is typical. A sensible working assumption is a range of roughly 10% either side rather than one exact number.

The practical check is slow but reliable. Log what you eat and your weight for two to three weeks. If your weight holds steady, your average intake is your maintenance level, whatever any equation predicted.

Mifflin-St Jeor is the one used on this page because it has generally performed well in comparisons with older equations for healthy adults. The older Harris-Benedict equation tends to give slightly higher results for the same person.

Why BMR is not your daily calorie target

BMR is the floor of your energy use, not the amount you eat. To estimate what you burn in a normal day, multiply BMR by an activity factor, which gives your total daily energy expenditure (TDEE).

For the 70 kg man above, a lightly active factor of 1.375 turns 1,649 into about 2,267 kcal a day. The TDEE calculator on this page applies the multiplier for you.

Once you have a TDEE, the choice of food intake follows your goal: eat near it to hold weight, somewhat below it to lose, somewhat above it to gain. The calorie deficit and daily calorie needs guides on this site take that step.

If you know your lean body mass, the Katch-McArdle equation, 370 + 21.6 × lean mass in kg, can be more suitable for very muscular or very lean people. Mifflin-St Jeor does not use body composition at all.

Mistakes people make with BMR

• Entering pounds or inches into a formula built for kilograms and centimetres.
• Treating BMR as an eating target. Eating at or below BMR for long periods is not advisable without medical supervision.
• Expecting a wearable device to match. Watches and rings use their own models and rarely agree with an equation.
• Blaming a low number on a 'slow metabolism'. The equation reflects size and age, not a fault.
• Using the formula for children, pregnancy or medical conditions, where needs differ.

A last point on rounding and false precision: 1,648.75 is not more informative than 1,650. Quoting decimals suggests an accuracy that the method does not have.