Mifflin-St Jeor Explained: The Formula Behind Your Calorie Needs

The Formula That Powers Every Calorie Goal

If you’ve ever wondered “how many calories should I eat in a day?”—the answer isn’t guesswork. It’s math.

Most modern calorie calculators use the Mifflin-St Jeor equation, a scientifically validated method for estimating your Basal Metabolic Rate (BMR)—sometimes called Resting Metabolic Rate (RMR) or Resting Energy Expenditure (REE). This is the number of calories your body burns at complete rest just to stay alive: breathing, pumping blood, and powering your brain.

The formula was introduced in 1990 by Dr. M.D. Mifflin and Dr. S.T. St Jeor in their landmark study, and it quickly became the gold standard for predicting calorie needs in both clinical and everyday use. Today, when you open a BMR calculator or TDEE calculator online, you’re almost always seeing the Mifflin-St Jeor math at work.

The Mifflin-St Jeor equation is a formula used to estimate how many calories your body burns at rest (RMR). It’s widely considered the most accurate calorie equation for healthy adults.

Quick Glossary: The Science-y Stuff Made Simple

Before we go further, this article gets a little…science’y. Don’t worry—we’ll keep it simple. Here are the acronyms you’ll see:

BMR (Basal Metabolic Rate): The calories your body burns at complete rest, just to stay alive.
RMR (Resting Metabolic Rate): Basically the same as BMR, but measured under slightly less strict lab conditions. Most calculators (including Mifflin-St Jeor) use this.
REE (Resting Energy Expenditure): Another name for RMR. Same concept, different wording.
TDEE (Total Daily Energy Expenditure): Your real-world calorie burn once you factor in activity—walking, workouts, chores, even fidgeting.
Mifflin-St Jeor Equation: A math formula created in 1990 that estimates your RMR and, with an activity multiplier, your TDEE. It’s considered the most accurate calorie equation for everyday use.
Harris-Benedict Equation: An older formula from 1919, revised in 1984. It was the standard before Mifflin-St Jeor but tends to overestimate calorie needs.

Mifflin-St Jeor vs Harris-Benedict: Why It Matters

Before Mifflin-St Jeor, the most common tool was the Harris-Benedict equation, first published in 1919 and later revised in 1984. While groundbreaking for its time, it often overestimated daily calorie needs.

A systematic review of predictive equations found that Mifflin-St Jeor was more accurate—predicting RMR within about 10% of lab-measured values more often than Harris-Benedict. This makes it the most reliable equation for most healthy and overweight adults.

That said, no formula is perfect. The Journal of the Academy of Nutrition and Dietetics points out that while Mifflin-St Jeor is useful for the majority of people, indirect calorimetry (a lab test that measures oxygen in vs. carbon dioxide out) remains the only way to directly measure calorie burn with precision. Experts also caution that predictive equations may be less accurate for older adults or populations not well-represented in original studies.

Breaking Down the Mifflin-St Jeor Formula

Here’s the equation in its simplest form:

For women:
RMR = (10 × weight in kg) + (6.25 × height in cm) – (5 × age) – 161

For men:
RMR = (10 × weight in kg) + (6.25 × height in cm) – (5 × age) + 5

That gives you your Resting Metabolic Rate (RMR). To adjust for activity, multiply by your lifestyle factor:

Sedentary × 1.2
Lightly active × 1.375
Moderately active × 1.55
Active × 1.725
Very active × 1.9

This step translates your RMR into Total Daily Energy Expenditure (TDEE)—the calories you burn in real life.

From there:

To maintain weight, eat around your TDEE.
To lose weight, subtract 500–1,000 calories per day for a safe pace of 1–2 lbs/week.
To gain weight, add calories instead of subtracting.

Bringing the Math to Life

Enough theory—let’s put the Mifflin-St Jeor equation to work. Here’s what it looks like when you apply it to actual people with everyday routines.

Sarah Whitaker, 35
Sarah is a project manager who juggles long hours at the office with evening dog walks.

Height: 165 cm (5'5")
Weight: 70 kg (154 lbs)
Activity: Lightly active
Calories to maintain: ~2,000/day
Calories for weight loss: ~1,500–1,600/day (steady loss of about 1 lb/week)

Michael Dunn, 28
Michael is a weekend warrior—he spends his weekdays at a desk but hits the basketball court or bike trails whenever he can.

Height: 180 cm (5'11")
Weight: 75 kg (165 lbs)
Activity: Moderately active
Calories to maintain: ~2,750/day
Calories for weight gain: ~3,000/day (slow, lean muscle gain)

How Hoot Makes It Effortless

Of course, you could pull out a calculator, crunch numbers, and adjust your intake by hand… but why make dieting feel like homework? Hoot does the heavy lifting for you.

When you sign up, you simply enter your age, weight, height, gender, activity level, and goal pace. Behind the scenes, Hoot quietly runs the Mifflin-St Jeor equation—the same one used in clinics and nutrition research—to set your personalized calorie target.

Then Hoot takes it further:

Adjusts your calories automatically as your weight and goals evolve
Builds a balanced macro plan (protein, fat, carbs) without the overwhelm
Updates targets in real time, so you don’t have to constantly recalculate
Adds playful feedback on each meal with a Nutrition Score, simple Pros & Cons, and gentle swap suggestions

The science is serious—but Hoot keeps the experience light. Think of it as having a smart, supportive sidekick that turns all the confusing math into simple, doable daily goals. Because diets are already hard; tracking them shouldn’t be.

From Hoot's Help Center: Calorie Needs and Weight Loss: How Hoot Uses Standard Calculation Methods

FAQs About the Mifflin-St Jeor Formula

1. What is the Mifflin-St Jeor equation used for?
The Mifflin-St Jeor equation is used to estimate your Resting Metabolic Rate (RMR), also called Resting Energy Expenditure (REE). This represents the calories your body burns while at complete rest—enough to power essential functions like breathing, blood circulation, and organ activity. Once multiplied by an activity factor, it becomes your Total Daily Energy Expenditure (TDEE), the basis for weight maintenance, loss, or gain.

2. What is the formula for the Mifflin-St Jeor equation?
The formula differs slightly for men and women:

For women: (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) – 161
For men: (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) + 5

After calculating RMR, multiply the result by an activity multiplier (ranging from 1.2 for sedentary to 1.9 for very active) to estimate your total daily calorie needs.

3. How accurate is the Mifflin-St Jeor equation?
Research shows that the Mifflin-St Jeor equation predicts RMR within 10% of measured values for most healthy adults. This makes it more accurate than older formulas like Harris-Benedict. However, it’s still an estimate, not a direct measurement—your true calorie burn can vary based on factors like muscle mass, genetics, or health status.

4. Is the Mifflin-St Jeor formula accurate?
Yes, it’s considered the gold standard for calorie estimation in both clinical and consumer use. That said, no equation is perfect. The Journal of the Academy of Nutrition and Dietetics highlights that while Mifflin-St Jeor is generally reliable, certain populations—such as older adults, children, or ethnic groups underrepresented in validation studies—may see larger errors compared to lab-measured values.

5. What are the limitations of the Mifflin-St Jeor equation?
The biggest limitation is that it provides an average prediction, not a personalized measurement. For individuals with unusually high muscle mass, hormonal imbalances, or chronic illness, the estimate may be off. It also doesn’t account for daily variations in metabolism caused by stress, sleep, or diet. In clinical settings, indirect calorimetry (a test that measures oxygen and carbon dioxide exchange) is still the gold standard for accuracy.

6. How is Mifflin-St Jeor different from the Harris-Benedict equation?
The Harris-Benedict formula (1919, revised 1984) was the original standard but often overestimated calorie needs. In contrast, the Mifflin-St Jeor equation (1990) has been shown to be more accurate for modern populations, particularly in overweight or obese adults. That’s why most nutrition apps and clinicians now prefer it.

7. What is the most accurate BMR formula?
For most healthy adults, Mifflin-St Jeor is the most accurate widely used predictive equation. However, the most accurate method overall is not an equation—it’s indirect calorimetry, which directly measures your energy expenditure in a lab. Equations are best for convenience; calorimetry is best for precision.

8. Who is Mifflin-St Jeor?
The equation was developed by Dr. M.D. Mifflin and Dr. S.T. St Jeor, researchers who published their work in 1990 in the American Journal of Clinical Nutrition. Their study compared existing formulas and introduced a new one that more closely matched measured energy expenditure in healthy individuals.

9. Does the Mifflin-St Jeor equation work for weight gain as well as weight loss?
Yes. The equation provides your baseline calorie needs, which you can adjust depending on your goal. To lose weight, subtract 500–1,000 calories per day (for about 1–2 lbs/week). To gain weight, add calories on top of your maintenance number. The flexibility of the equation makes it useful in both directions.

10. Can the formula adapt to lifestyle changes?
Yes. If your weight, activity level, or age changes, your calorie needs shift too. Recalculating (or letting an app like Hoot recalculate automatically) ensures your targets stay accurate.

11. Does muscle mass affect the equation?
Not directly. The formula uses weight, not body composition. Since muscle burns more calories than fat, people with high lean mass may have a higher metabolism than the equation predicts. In these cases, lab measurement or careful tracking may provide better accuracy.

12. Is it the same as counting macros?
Not quite. The equation gives you total calorie needs, while macros (protein, carbs, fat) are how those calories are distributed. Apps like Hoot use your Mifflin-St Jeor calories as a starting point, then assign macros for better nutrition balance.

13. How often should I recalculate my calorie needs?
It’s smart to recalculate whenever you lose or gain more than about 5–10 lbs, or if your lifestyle changes significantly (e.g., going from sedentary to active). Age-related metabolic changes also make recalculation useful every few years.

14. Can stress or sleep affect how accurate the formula is?
Yes. While the equation doesn’t factor in stress or sleep, both can influence energy expenditure and appetite. Poor sleep can lower metabolism and increase cravings, while high stress can alter hormonal balance. These factors mean your real-world calorie needs might fluctuate beyond the formula’s estimate.

15. Why does Hoot use the Mifflin-St Jeor formula?
Because it balances scientific credibility with simplicity. It’s widely validated, easy to calculate, and reliable for most users. Best of all, Hoot automates the math, adjusts your targets as you progress, and provides context (like Nutrition Scores and feedback) so you don’t have to think about equations—you just focus on building habits.