BMR calculator: the energy your body uses at rest

01What basal metabolic rate actually is

BMR is the energy your body uses at complete rest, awake but doing nothing: no digestion in progress, no movement, a comfortable room. It covers the invisible work of staying alive: circulation, breathing, brain activity, maintaining body temperature, building and repairing tissue. For most people it accounts for the largest portion of daily energy use, more than exercise does.

Strictly speaking, laboratories measure BMR under tight fasting and rest conditions; what equations like this one estimate is closer to resting energy expenditure, a slightly less strict cousin. For everyday planning the distinction rarely matters, which is why the two terms travel together.

02The exact formula behind the result

This calculator uses the Mifflin-St Jeor equation. It takes your weight in kilograms, your height in centimetres (converted for you if you enter feet and inches) and your age in years, then computes: 10 × weight + 6.25 × height − 5 × age, plus 5 for men or minus 161 for women. The output is resting energy expenditure in kilocalories per day. Under the result the calculator shows that working line by line with your own figures, and the same rate as kilocalories per hour.

Nothing else feeds the number: no activity level, no body-fat estimate, no adjustment for climate or cuisine. That simplicity is deliberate: the equation asks only for measurements you can take reliably at home, and it behaves predictably across the adult range of heights and weights.

03Why Mifflin-St Jeor, and the limits of any equation

The Harris-Benedict equation, published in 1919, held the field for decades, but it was built on a small early-twentieth-century sample and tends to overestimate resting energy for many adults today. Mifflin-St Jeor was derived later, validated on modern populations, and carries a smaller average error, which is why it has become the standard choice for adults and the one this calculator uses.

The Katch-McArdle formula takes a different route, working from lean body mass, and can suit very muscular people better. But it needs a body-fat percentage, and most people do not have a reliable one. A guessed body-fat figure fed into a precise formula produces a precise-looking wrong answer. Mifflin-St Jeor asks only for what you actually know.

Any prediction equation describes an average body, not yours. Two people with identical age, height and weight can differ meaningfully in resting energy, because the formula cannot see body composition, hormonal health, pregnancy or the adaptation that follows hard dieting, and it was not built for children. Expect the true figure to sit somewhat above or below the estimate, and treat it as a starting point rather than a verdict.

04A baseline, not an eating target

Do not eat to your BMR. The figure describes a body at complete rest, and you do not live at complete rest: you commute, cook, climb stairs, digest, fidget. Your true daily requirement is BMR multiplied by an activity factor, which the TDEE calculator works out for you.

Eating at or below BMR for long stretches is a common self-inflicted mistake: energy drops, meals become joyless, and the deficit rarely holds. If your goal is weight change, start from the day total and set a moderate deficit or surplus. The calorie calculator covers that step.

05What a trained practitioner does with this number

For a nutrition counsellor, BMR is the first line of a longer calculation, not the conclusion. A practitioner estimates the day's total requirement, sets a target that fits the client's goal, then translates it into actual meals: dal, rice, roti, vegetables, portions that survive a festival week. They also know when the equation misleads: for the very muscular, the elderly, or someone with a thyroid condition, the estimate needs professional judgement around it.

That movement from a formula to a workable plan is what NNWA teaches. The Diploma in Nutrition builds the full method from first principles, and the weight management course applies it to fat loss specifically.