BMR Calculator — Basal Metabolic Rate (Mifflin-St Jeor)

BMR Calculator

Your BMR is the energy your body uses just staying alive — breathing, circulation, cell repair — before any movement is counted.

The WHO adult BMI ranges. Where you land is a first look, not a full picture.

What BMR actually is

Basal metabolic rate is the energy your body spends keeping you alive at complete rest: running the brain, heart, liver and kidneys, maintaining temperature, repairing tissue. For most people it accounts for 60% to 70% of total daily energy use, which is why it dominates the calorie picture even for people who train regularly.

The formulas

The Mifflin-St Jeor equation, published in 1990, is the default here because it has been shown to predict measured resting energy expenditure more accurately than the older alternatives for most adults:

Men: 10w + 6.25h − 5a + 5
Women: 10w + 6.25h − 5a − 161

with weight in kilograms, height in centimetres and age in years. Harris-Benedict (revised 1984) is shown alongside for comparison. Katch-McArdle uses lean body mass instead of total weight and is the most accurate of the three if you know your body fat percentage — enter it and it appears.

Turning BMR into a daily calorie target

Multiply BMR by an activity factor to get total daily energy expenditure: roughly 1.2 for sedentary, 1.375 for light activity, 1.55 for moderate, 1.725 for very active. The calorie calculator does this and adds targets for losing or gaining weight.

Why individual results vary

These are population averages, and individual metabolic rates can sit 10% or more either side of the prediction. Thyroid function, medication, genetics, past dieting and body composition all shift it. Use the number as a starting point, then adjust based on what actually happens to your weight over a few weeks.

Frequently asked questions

What is the difference between BMR and RMR?

Resting metabolic rate is measured under less strict conditions than basal rate and usually comes out a few per cent higher. In everyday use the two terms are treated as interchangeable.

Can I eat below my BMR to lose weight faster?

It is generally discouraged. Very low intakes make adequate nutrition difficult, increase muscle loss and are hard to sustain. A moderate deficit below your total daily expenditure is the standard approach.

Does BMR fall with age?

Yes, gradually — the equations subtract roughly 5 kcal per year of age. Much of the decline is linked to lost muscle mass, which resistance training can slow.

Which formula should I use?

Mifflin-St Jeor for most people. Use Katch-McArdle instead if you have a reliable body fat measurement, particularly if you are very lean or very muscular.