BMR Calculator

How to use this calculator#

1. 1
   Enter sex, age, height and weightMifflin-St Jeor needs all four. Weight carries the most influence — each extra kilogram adds 10 kcal — followed by height at 6.25 kcal per centimetre and age at −5 kcal per year. The sex constant is a flat +5 for men and −161 for women.
2. 2
   Use your current weight, not a goal weightThe equation estimates what your body burns today. Entering the weight you would like to be gives you the metabolic rate of a body you do not yet have, which then produces a calorie target that is too low from day one.
3. 3
   Add body fat percentage only if you actually measured itKatch-McArdle works from lean body mass and is the better equation when the body fat figure came from a DEXA scan, air displacement plethysmography or careful caliper work. A guessed percentage makes it worse than Mifflin-St Jeor, not better.
4. 4
   Compare the three results as a rangeMifflin-St Jeor, Harris-Benedict and Katch-McArdle will typically land within 3–5% of each other. Treat that spread as the honest precision of the estimate rather than picking the number you like best.
5. 5
   Validate it against the scale over two to three weeksEat at the calculated maintenance level, weigh yourself under identical conditions, and use the trend to correct the estimate. Real feedback beats any regression equation, which was fitted to a group average and can miss an individual by 10% or more.

The formula#

How the three BMR formulas differ#

Basal metabolic rate is the energy your body spends keeping your heart, brain, lungs and kidneys running while you lie completely still. This tool defaults to the Mifflin-St Jeor equation published in 1990: 10 x weight in kg, plus 6.25 x height in cm, minus 5 x age in years, then add 5 for men or subtract 161 for women.

A 30-year-old man at 180 cm and 80 kg works out to 800 + 1125 - 150 + 5 = 1780 kcal per day. The revised 1984 Harris-Benedict equation gives roughly 1854 kcal for the same person, and Katch-McArdle, which uses lean body mass instead of height, lands near 1840 kcal at 15% body fat. A spread of 3-5% between formulas is completely normal.

Turning BMR into a daily calorie number#

BMR is a floor, not a meal plan. It excludes digestion, fidgeting, walking to the bus and any deliberate exercise. Multiply it by an activity factor between 1.2 for desk-bound days and 1.9 for physical jobs or twice-daily training to get total daily energy expenditure, which is the number you actually eat against.

Katch-McArdle is the most accurate option if you have a reliable body fat measurement, because two people at the same weight can carry very different amounts of metabolically active muscle. Without a DEXA scan or careful caliper reading, stick with Mifflin-St Jeor rather than guessing a body fat number.

Limits of any predictive equation#

Published equations are regression fits to group averages and can miss an individual by 10% or more. Thyroid conditions, some medications, pregnancy, recent large weight loss and unusually high muscle mass all shift resting metabolism away from the prediction. Results here are estimates for general information and are not medical advice.

The practical fix is to treat the estimate as a starting point, hold your intake steady for two to three weeks and weigh yourself under the same conditions each time. If your weight moves sharply without any change in habits, or you feel persistently cold, exhausted or breathless, speak to a doctor or registered dietitian.

Worked examples#

Reference tables#

Common mistakes#

• Eating at BMR instead of at maintenanceBMR excludes digestion, walking, fidgeting and all exercise. Setting intake at BMR for a moderately active person is roughly a 700 kcal daily deficit before any deliberate cut, which is aggressive enough to cost muscle and make protein and micronutrient targets hard to hit.
• Guessing a body fat percentage to use Katch-McArdleThe equation is only better than Mifflin-St Jeor when the input is real. A guess that is 6 percentage points low on an 80 kg person inflates lean mass by nearly 5 kg and the BMR estimate by about 104 kcal a day, in the wrong direction.
• Assuming the equation is precise for youThese are regression fits to group averages. Individual resting metabolism can sit 10% or more either side of the prediction, and thyroid disorders, some medications, pregnancy, breastfeeding and substantial recent weight loss all shift it further. Use the number as a starting point and correct it from your own weight trend.
• Recalculating only when you remember toBMR falls as you lose weight, because there is less tissue to maintain. Losing 10 kg drops the Mifflin-St Jeor estimate by 100 kcal a day, so a deficit that worked at the start quietly becomes maintenance if the number is never revisited.

Frequently asked questions#

Key terms#

Basal metabolic rate (BMR)

Energy expended maintaining vital function at complete rest, measured after an overnight fast in a thermoneutral room having not moved. Typically 60–70% of total daily expenditure.

Resting metabolic rate (RMR)

A slightly less strict measurement than BMR, taken at rest but without the full overnight-fast and post-sleep protocol. RMR usually reads a few percent higher, and the two terms are often used interchangeably in practice.

TDEE (total daily energy expenditure)

BMR multiplied by an activity factor. It includes digestion, everyday movement and exercise, and it is the figure calorie targets are set against.

Lean body mass

Everything that is not fat — muscle, bone, organs, water. Weight × (1 − body fat% ÷ 100). It is the metabolically active tissue Katch-McArdle works from.

Thermic effect of food

The energy spent digesting and absorbing what you eat, roughly 10% of intake and higher for protein. It is not in BMR but is captured inside the activity multiplier.

Indirect calorimetry

The reference method behind these equations: measuring oxygen consumed and carbon dioxide produced to derive energy expenditure directly, rather than predicting it from body dimensions.

Sources#