What Is the Mifflin-St Jeor Equation? (Formula Explained)

If you've ever used an online calorie or BMR calculator, there's a good chance it was built on the Mifflin-St Jeor equation — the formula most doctors, dietitians, and fitness apps consider the gold standard for estimating how many calories your body burns at rest. This guide breaks down exactly where the formula came from, how it works, and how to calculate it yourself with a real example.

A Quick History: Who Created the Mifflin-St Jeor Equation?

The Mifflin-St Jeor equation was developed in 1990 by Dr. Mark D. Mifflin and Dr. Sachiko T. St Jeor, along with a team of researchers, and published in the American Journal of Clinical Nutrition. At the time, the most widely used formula for estimating Basal Metabolic Rate (BMR) was the Harris-Benedict equation, originally created in 1919 and revised in 1984.

The problem? Harris-Benedict was built using data from a population with very different body compositions and lifestyles than the modern general public, which led it to consistently overestimate calorie needs for many people — especially those who were overweight or less active.

Mifflin and St Jeor re-derived the formula using a larger, more representative sample of men and women, and the result was a noticeably more accurate predictor of resting metabolic rate. Since then, it has become the equation recommended by the Academy of Nutrition and Dietetics as the preferred method for estimating BMR in healthy adults.

The Mifflin-St Jeor Formula

The formula uses four simple inputs: weight, height, age, and biological sex. Here it is, broken down:

For men:

BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5

For women:

BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161

The output, BMR, is the number of calories your body burns in 24 hours just to keep you alive — breathing, circulating blood, regulating temperature, and running basic cell functions — before any movement, exercise, or digestion is factored in.

Why This Formula Replaced Harris-Benedict

Three main reasons the Mifflin-St Jeor equation is now preferred over the older Harris-Benedict formula:

1. More accurate for the general population Multiple validation studies have found Mifflin-St Jeor predicts resting metabolic rate within about 10% of measured values for most healthy adults, compared to a wider margin of error for Harris-Benedict.

2. Less likely to overestimate calorie needs Harris-Benedict tends to run high, which can lead to overeating if someone uses it to plan a diet.

3. Simpler, more current dataset The original Harris-Benedict data is over a century old; Mifflin-St Jeor was built using a more modern, diverse sample.

This doesn't mean Mifflin-St Jeor is perfect — no formula can perfectly predict an individual's metabolism, since things like genetics, muscle mass, hormone levels, and medical conditions all play a role. But among the major equations available, it's currently considered the most reliable starting point for the average person.

Worked Example: Calculating BMR Step by Step

Let's say we're calculating BMR for a 27-year-old woman who weighs 58 kg and is 160 cm tall.

Step 1: Multiply weight by 10 10 × 58 = 580

Step 2: Multiply height by 6.25 6.25 × 160 = 1000

Step 3: Multiply age by 5 5 × 27 = 135

Step 4: Add and subtract according to the formula for women 580 + 1000 − 135 − 161 = 1284

Her estimated BMR is approximately 1,284 calories per day.

This is the number of calories her body would burn if she stayed in bed all day doing nothing. To find out how many calories she actually burns including daily activity and exercise — known as Total Daily Energy Expenditure (TDEE) — this BMR number gets multiplied by an activity factor.

A Second Example: Calculating BMR for a Man

To see the formula applied with the male constant, let's calculate BMR for a 40-year-old man who weighs 80 kg and is 178 cm tall.

Step 1: Multiply weight by 10 10 × 80 = 800

Step 2: Multiply height by 6.25 6.25 × 178 = 1112.5

Step 3: Multiply age by 5 5 × 40 = 200

Step 4: Add everything and apply the male constant (+5) 800 + 1112.5 − 200 + 5 = 1717.5

His estimated BMR is approximately 1,718 calories per day.

Notice that even though his weight is higher, the constant difference (+5 for men vs. −161 for women) reflects the average difference in lean muscle mass between sexes, since muscle tissue burns more calories at rest than fat tissue.

How Accurate Is the Mifflin-St Jeor Equation, Really?

Accuracy is the most important question for any predictive formula, and this is where Mifflin-St Jeor has earned its reputation. Validation research comparing predicted BMR against indirect calorimetry (the lab-grade method of directly measuring metabolic rate through oxygen consumption) has repeatedly found that Mifflin-St Jeor produces estimates within roughly 10% of measured values for the majority of healthy, non-obese adults — a tighter margin than older formulas like Harris-Benedict or the Owen equation.

That said, accuracy does shift depending on the population:

Average-weight adults Most accurate group; predictions are usually very close to measured BMR.

Individuals with obesity Still considered one of the better-performing formulas, though it can slightly underestimate BMR in some cases.

Very muscular individuals (athletes, bodybuilders) Since the formula doesn't account for lean body mass directly, it can underestimate BMR for people with significantly above-average muscle mass. In these cases, a formula like Katch-McArdle (which factors in body fat percentage) may be more precise.

Older adults The age term in the formula does adjust for the natural metabolic slowdown that comes with aging, but individual variation increases with age due to changes in muscle mass and hormone levels.

This is why most clinicians treat the Mifflin-St Jeor result as a reliable starting point rather than a fixed number — it gets you very close, and then real-world tracking over 2–3 weeks lets you fine-tune it to your actual metabolism.

Quick FAQ

Is Mifflin-St Jeor better than just guessing my calories?

Yes. Even with its margin of error, it gives a far more reliable estimate than a generic guess like "2,000 calories a day" that ignores your individual stats.

Do I need to recalculate my BMR often?

Recalculate whenever your weight changes by more than a few kilograms, or roughly every 4–6 weeks if you're actively trying to lose or gain weight.

Does the formula work the same for everyone, regardless of body type?

It's designed for the general adult population. People with high muscle mass, certain medical conditions, or who are pregnant or breastfeeding may need a healthcare provider's personalized assessment instead.

What Mifflin-St Jeor Does NOT Account For

It's worth understanding the formula's limitations:

Body composition Two people with the same weight but very different muscle-to-fat ratios will have different actual metabolic rates, but the formula treats them the same.

Medical conditions Thyroid disorders, PCOS, and other conditions can significantly raise or lower metabolism beyond what the formula predicts.

Genetics Individual metabolic rate can vary by as much as 10–15% from the formula's estimate due to genetic factors alone.

This is why the result should be treated as a starting estimate, not an exact number — something to refine over a few weeks by tracking your actual progress.