How BMR Is Calculated: The Mifflin-St Jeor Formula Explained

Knowing how to calculate BMR — basal metabolic rate — is the starting point for almost any serious calorie or weight-management plan, because it answers a more fundamental question than “how many calories should I eat”: how many calories does my body burn just staying alive, before any deliberate activity is added on top.

The formula itself

The only difference between the two versions is a flat offset — men’s formula adds 5, women’s subtracts 161 — reflecting average differences in body composition between the sexes at a given height and weight. Everything else (the weight, height and age coefficients) is identical.

A worked example

For a 30-year-old woman at 65kg and 165cm: 10(65) + 6.25(165) − 5(30) − 161 = 650 + 1031.25 − 150 − 161 = 1,370 calories/day. The BMR Calculator runs this exact calculation — enter your own numbers and it returns the same formula instantly.

Why the formula uses these specific inputs and no others

It’s worth appreciating why the formula settled on exactly these four variables — weight, height, age and gender — rather than more or fewer. Each one captures a real, independent driver of resting metabolism: weight (and by extension, total tissue mass), height (a rough proxy for overall body surface area, which affects heat loss and therefore energy needs), age (correlating with the gradual muscle loss discussed above), and gender (approximating average body-composition differences). Adding more inputs — like a direct body-fat measurement — could improve accuracy but would also require equipment or testing most people don’t have easy access to, which is exactly the trade-off Mifflin-St Jeor was designed around: reasonable accuracy from inputs anyone can measure with a scale and a tape measure.

Why Mifflin-St Jeor replaced Harris-Benedict

The original Harris-Benedict equation was built from data collected in the early 20th century. When researchers Mifflin and St Jeor re-derived a resting-energy formula from a more contemporary sample in 1990 (published in the American Journal of Clinical Nutrition), it consistently outperformed Harris-Benedict for modern body compositions — which is why dietitians and nutrition software have largely shifted to it since.

What BMR doesn’t capture

Factor	Included in the formula?
Weight, height, age, gender	Yes
Muscle-to-fat ratio	No
Actual measured metabolism	No — this is a population-based estimate

Two people with identical height, weight, age and gender but different muscle mass will have somewhat different true BMRs — muscle tissue burns more calories at rest than fat tissue does — but the formula can’t see that difference, since it isn’t one of its four inputs. This is the same trade-off BMI makes: broad accessibility in exchange for individual precision.

Why the formula treats men and women differently

The offset difference (+5 for men, −161 for women) isn’t arbitrary — it approximates average differences in body composition between the sexes at a given height and weight. On average, women tend to carry a higher body-fat percentage than men at the same height and weight, and since fat tissue is less metabolically active than muscle, this pulls the average woman’s BMR somewhat below a same-sized man’s. The formula bakes this population-average difference in as a flat offset rather than requiring a separate body-composition input, which keeps the calculation simple at the cost of not capturing any individual’s actual deviation from that average.

How BMR changes across a lifetime

BMR doesn’t stay fixed as you age, and the formula reflects this directly through its age term — every additional year subtracts 5 from the result, for both men and women. This reflects a well-documented pattern: resting metabolic rate tends to decline gradually with age, partly due to a gradual loss of muscle mass (sarcopenia) that often accompanies aging, especially without deliberate resistance training to counteract it. A 30-year-old and a 60-year-old with identical weight and height will show a 150-calorie difference in estimated BMR purely from the age term (5 calories × 30 years) — a reminder that a calorie target calculated once in your twenties is unlikely to still be accurate decades later.

Other formulas you might encounter

Mifflin-St Jeor isn’t the only resting-metabolism formula in circulation. The Katch-McArdle formula takes a different approach entirely, estimating resting energy expenditure from lean body mass rather than total weight — which can be more accurate for people with an atypical body-fat percentage, since it directly accounts for the fact that muscle burns more calories at rest than fat does, rather than approximating that effect through a flat gender offset. The trade-off is that Katch-McArdle requires knowing (or estimating) your lean body mass first, an extra step Mifflin-St Jeor avoids by using only easily measured inputs.

Turning BMR into a daily calorie target

BMR alone isn’t a calorie target — it’s a baseline. The next step multiplies it by an activity factor (roughly 1.2 for sedentary, up to 1.9 for very active) to estimate total daily energy expenditure. From there, a macro split breaks that total into protein, carbs and fat, and if you’re specifically trying to preserve muscle while losing fat, checking your lean body mass alongside BMR gives a fuller picture than weight alone.

FAQ

Is Mifflin-St Jeor accurate for everyone? It’s a population-based estimate — individual results can vary by 10% or more, particularly for people with unusually high or low muscle mass relative to their height and weight.

Do I need to recalculate BMR as I lose or gain weight? Yes — since weight is a direct input, BMR shifts as body weight changes, so a number from months ago won’t reflect your current body accurately.

Is BMR the same as the calories I should eat to lose weight? No — for weight loss, total daily energy expenditure (BMR × activity factor) minus a deficit is the relevant target, not BMR on its own.

Why does age reduce BMR in the formula? It reflects a well-documented real-world pattern: resting metabolic rate tends to decline gradually with age, largely tied to gradual muscle loss that often accompanies aging without deliberate strength training to offset it.

Can strength training meaningfully change my BMR? Yes, indirectly — building muscle mass increases the amount of metabolically active tissue in your body, which raises true resting metabolism somewhat above what a formula based only on total weight would predict, since the formula can’t see the difference between a kilogram of muscle and a kilogram of fat.