BMR Calculator: Basal Metabolic Rate on Five Equations

How to use this tool

Choose male or female, then give your age, weight and height in whichever units you use. The result appears as you type, with Mifflin St Jeor as the headline and the other four equations laid out beside it. Switching units converts what you have typed, so nothing needs to be entered twice.

Leave the body fat box empty unless you have a measured figure from a scan, calipers or a tape measure estimate you trust. It only feeds the Katch McArdle and Cunningham equations, which work from lean mass, and with a guessed figure they are no more accurate than the others. The headline equation box changes which figure leads the page, the charts and the shareable link.

What the calculation is doing

Basal metabolic rate is the energy your body uses to stay alive when it is doing nothing else: keeping the heart beating, the lungs working, the brain running and every cell pumping ions across its membrane. It is measured, rather than estimated, by indirect calorimetry. You lie still under a hood after a night’s sleep and a fast, and the machine measures the oxygen you use and the carbon dioxide you breathe out. Weir’s equation from 1949 turns those into energy: 3.941 kcal for every litre of oxygen plus 1.106 kcal for every litre of carbon dioxide, which comes to roughly 4.8 kcal per litre of oxygen on a normal diet.

The equations on this page are regressions fitted to groups of people who were measured that way. Each one takes size, sex and age, or lean mass, and predicts what the hood would have shown. That is why they disagree a little with each other and with any one person: they describe an average body of your dimensions, not yours.

The five equations

These are the equations the calculator runs, with the figure each gives for the same man. Mifflin St Jeor, fitted on 498 people in 1990, is the one most dietitians use. The original Harris Benedict equations from 1919 were fitted on 239 people and tend to run high on modern bodies. The lean mass equations ignore height, age and sex altogether and are only as good as the body fat figure behind them.

Weight in kilograms, height in centimetres, age in years, result in kcal a day. The lean mass figures in the last column use the Boer estimate of 61.5 kg, because no body fat was measured.

Why BMR falls with age, and why men run higher

Most of the difference between two people of the same weight comes down to lean tissue. Organs and muscle burn energy at rest; fat barely does. Men carry more lean mass than women at the same height and weight, which is where the 166 kcal gap in Mifflin St Jeor comes from, and all of us lose some muscle with age, which is why every height and weight equation subtracts calories for each year.

The age term in Mifflin St Jeor is 5 kcal a year, so the same body burns about 150 kcal less at 60 than at 30. Some of that is muscle loss, and resistance training slows it. Some of it is the organs themselves running a little slower, which training does not change.

Mifflin St Jeor, kcal a day. Each figure opens the calculator on that person.

A worked check you can repeat on paper

Take the starting values on the page: a 30 year old man, 5 ft 10 in and 180 lb. In metric that is 177.8 cm and 81.65 kg. Mifflin St Jeor is 10 times the weight, 816.5, plus 6.25 times the height, 1,111.25, minus 5 times the age, 150, plus 5 for a man. That comes to 1,783 kcal a day.

Divide by 24 for 74.3 kcal an hour. A kilocalorie is 4,184 joules and a day is 86,400 seconds, so the same figure is 86.3 watts, a little less than an old 100 watt light bulb burning day and night. A woman of 30 at 5 ft 4 in and 150 lb comes to 1,385 kcal a day on the same equation.

Mistakes that catch people out

Treating BMR as the number to eat. It is what you would burn lying in bed all day. Your daily need is BMR times an activity factor, which the table on this page and the TDEE calculator work out.

Typing a guessed body fat figure into the lean mass equations. Katch McArdle is excellent with a DEXA result and no better than the rest with a guess. An error of five points in body fat moves its answer by about 90 kcal for an average man.

Expecting the equations to agree to the calorie. A spread of 100 to 200 kcal between them is normal, and any one of them can be 10 percent out for an individual. Use the result as a starting point and adjust from what the scale does over a few weeks.

Mixing up pounds and kilograms. Entering 180 kg by mistake adds about 1,000 kcal on Mifflin St Jeor. The unit box sits right next to the number for that reason.

Believing muscle burns 50 calories a pound. The measured figure is about 6 kcal per pound a day at rest. Muscle is worth building for many reasons, but a big jump in BMR is not one of them.

Popular calculations

Each opens the calculator on that person with its own address, and every value can be changed once it is open.

Women, age 30

Men, age 30

By age

Metric

Other equations

How this was checked

Each equation was coded from its published coefficients and tested in Node against hand calculations before it went near the page. For the 30 year old man at 180 lb and 5 ft 10 in the results are 1,783 on Mifflin St Jeor, 1,865 on the 1984 Harris Benedict and 1,876 on the 1919 original, and the gap between men and women of the same size on Mifflin St Jeor is exactly 166 kcal, as the constants require. The engine is the same one behind the TDEE calculator on this site, so the two pages always agree.

Every address in the lists above was then opened and checked for a distinct title, the right answer, charts that render, and clean CSV, image and print output containing only that calculation, at desktop, tablet and phone widths.

Common questions

What is a normal BMR?

It depends almost entirely on size, sex and age. On Mifflin St Jeor a 40 year old woman of 5 ft 4 in and 170 lb comes out at about 1,426 kcal a day, and a 40 year old man of 5 ft 9 in and 200 lb at about 1,808. Most adults fall somewhere between 1,200 and 2,200.

How do I calculate my BMR by hand?

With Mifflin St Jeor: 10 times your weight in kilograms, plus 6.25 times your height in centimetres, minus 5 times your age, then add 5 for a man or subtract 161 for a woman. For a 30 year old man of 180 lb (81.65 kg) and 5 ft 10 in (177.8 cm) that is 1,783 kcal a day.

What is the difference between BMR and TDEE?

BMR is what your body burns at complete rest. TDEE, total daily energy expenditure, adds digestion and every bit of movement on top, and is usually 1.2 to 1.9 times BMR. TDEE is the number to eat to if you want to hold your weight; BMR is only the floor under it.

Does building muscle raise BMR much?

Less than the fitness folklore suggests. Resting skeletal muscle burns about 13 kcal per kilogram a day, so adding 5 kg of muscle raises BMR by roughly 65 kcal a day. Muscle matters for BMR because there is so much of it, not because each kilogram burns a lot. The heart and kidneys burn over thirty times as much per kilogram.

Is it bad to eat less than my BMR?

For a day, no. As a sustained diet it is a very large deficit for most people, it makes it hard to get enough protein and nutrients, and it tends to cost muscle as well as fat. A deficit of 300 to 500 calories below your daily burn, not your BMR, is the usual starting point.

How accurate are BMR calculators?

For an individual, roughly within 10 percent most of the time. A 2005 systematic review for the American Dietetic Association found Mifflin St Jeor came within 10 percent of measured resting rate for more people than any other equation, but noted that individual errors can be larger. The only way to know yours exactly is indirect calorimetry, which measures the oxygen you use.

Related Calculators and Reference

• TDEE Calculator: BMR times your activity, the number to eat to if you want your weight to hold.
• Calorie Deficit Calculator: a daily target and a date for a goal weight, with sensible limits.
• Body Fat Calculator: a tape measure estimate of body fat, which unlocks the lean mass equations here.
• BMI Calculator: where your height and weight sit on the adult categories.
• Macro Calculator: protein, carbohydrate and fat once you know your calories.
• Protein Intake Calculator: grams of protein a day for your weight and goal.
• Ideal Weight Calculator: ideal body weight for your height.
• Sleep Calculator: bedtimes and wake times by sleep cycle.
• Lean Body Mass Calculator: lean body mass, the input the Katch McArdle formula needs.
• Mifflin and others, American Journal of Clinical Nutrition 1990: the paper behind the headline equation.