Basal Metabolic Rate Calculator

Your body spends most of its energy on tasks you never notice: pumping blood, filtering waste through your kidneys, maintaining your body temperature, and keeping every cell alive. Even lying completely still in a dark room, you burn a substantial number of calories just sustaining these processes. That baseline is your Basal Metabolic Rate, and it sets the floor for every calorie calculation in nutrition and fitness.

The Mifflin-St Jeor equation translates four inputs — weight, height, age, and sex — into an estimate of that floor. It was built from metabolic chamber studies where researchers measured how much oxygen participants consumed at rest, which directly reflects calorie burn. The equation is a regression model fitted to those measurements, not a theoretical derivation. Its coefficients represent statistical best-fit values across a large and varied sample.

Because BMR is your resting baseline, it does not account for any activity. Walking to the kitchen, sitting upright, fidgeting, exercising — all of these burn additional calories on top of your BMR. To estimate total daily calorie needs, most practitioners multiply BMR by an activity factor. The moderate activity estimate shown alongside your BMR uses a conventional multiplier of 1.55, representing a person who exercises a few times per week. That number is a starting reference, not a precise prescription.

Use this calculator when you are setting a starting calorie target for a diet or fitness plan, comparing your estimated resting needs before and after a significant weight change, or explaining to someone else why two people of different sizes need different calorie intakes. It is also a quick sanity check: if a diet plan prescribes calories dramatically below your BMR, that is a signal to ask harder questions.

Do not use this as a clinical nutrition prescription if you have a metabolic condition (hypothyroidism, PCOS, or similar), have recently undergone significant weight loss surgery, or are in a state of extreme calorie restriction under medical supervision. In those contexts, indirect calorimetry — a clinical test that measures your actual oxygen consumption — provides a measurement rather than an estimate, and a registered dietitian should be involved in any calorie targets.

Also recognize that BMR accuracy degrades at the extremes of body composition. Very lean athletes and individuals with high body-fat percentages both see larger gaps between estimated and measured BMR than people in the typical weight range. The equation was built on a population sample and reflects population averages — your individual result may differ by more than the commonly cited error margin.

Treating BMR as your daily calorie target. The most common mistake is eating at or near your BMR on the assumption that this produces weight loss. BMR is your resting floor — if you move at all during the day, your actual calorie need is higher. Eating at BMR typically creates too large a deficit, which can cause muscle loss rather than fat loss, slowing your metabolism further.

Ignoring how much body composition matters. The Mifflin-St Jeor equation uses total body weight, not lean mass. Two people with the same weight, height, age, and sex can have meaningfully different BMRs if one carries significantly more muscle. A 75 kg male with 35 kg of lean mass burns more at rest than a 75 kg male with 25 kg of lean mass — but the calculator returns the same number for both. If you have had body composition measured, the Katch-McArdle formula (which uses lean mass directly) may produce a more accurate result.

Recalculating and adjusting too frequently. BMR changes as weight changes, so some people recalculate after every few pounds lost and immediately adjust their targets. In practice, metabolic adaptation — your body's tendency to reduce energy output during sustained calorie restriction — means the equation's estimate can diverge from your actual expenditure during active weight loss phases. Use BMR as a planning anchor, not a daily adjustment trigger.

The Mifflin-St Jeor equation for males is: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) + 5. For females, the final term changes: instead of adding 5, you subtract 161. Each coefficient emerged from regression analysis of measured resting metabolic rates.

For the example of a 30-year-old male at 75 kg and 180 cm: the weight term contributes (10 x 75), the height term adds (6.25 x 180), the age term subtracts (5 x 30), and the sex constant adds 5. These terms sum to 1,730 kcal/day. Each term has a distinct biological rationale: larger bodies require proportionally more energy, taller individuals have more surface area and organ mass, metabolism declines with age, and males carry more metabolically active muscle on average.

When imperial inputs are used, the tool converts pounds to kilograms using the standard factor and converts feet and inches to centimeters before applying the formula. The conversion and the formula together produce the same result you would get entering metric values directly — the equation always operates in metric units internally, regardless of which unit system the user selects.

The Mifflin-St Jeor equation assumes a fixed relationship between body weight and metabolic rate, but that relationship is mediated by lean mass — which the formula cannot see. At equivalent weights, an athlete with 40 kg of lean mass will have a measurably higher BMR than a sedentary individual with 28 kg of lean mass, because skeletal muscle at rest consumes more energy per kilogram than adipose tissue. This means the formula systematically underestimates BMR in highly muscled individuals and overestimates it in those with a high fat-to-lean ratio at any given total weight. Practitioners working with competitive athletes or post-bariatric patients routinely switch to the Katch-McArdle or Cunningham equations, which accept lean body mass as a direct input and sidestep this limitation entirely.