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Understanding the Mifflin-St Jeor Equation

The Mifflin-St Jeor equation is the gold standard for estimating your daily calorie needs. This article explains how it works, how Hoot applies it, and what you should know about its strengths and limitations.

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Written by Patrick McCarthy
Updated this week

What is the Mifflin-St Jeor Equation?

The Mifflin-St Jeor equation is a scientifically validated formula used to estimate your Resting Metabolic Rate (RMR)—the calories your body burns at rest just to stay alive (breathing, circulating blood, powering your brain).

When multiplied by an activity factor, the result becomes your Total Daily Energy Expenditure (TDEE), or the total calories you burn in everyday life. This number forms the foundation for setting calorie targets to maintain, lose, or gain weight.

The formula was introduced in 1990 by Dr. Mifflin and Dr. St Jeor, and studies show it is more accurate than older equations like Harris-Benedict. Today, it’s the most widely used method in both clinical settings and fitness apps.


The Formula

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

This result gives you your Resting Metabolic Rate. To adjust for activity, multiply by a lifestyle factor:

Hoot’s Activity Levels

To keep things simple, Hoot uses four activity choices:

  • Sedentary (×1.2): Little to no exercise, mostly sitting.

  • Lightly Active (×1.375): Light exercise or activity 1–3 days per week.

  • Active (×1.55): Moderate exercise or physical activity most days.

  • Very Active (×1.725): Intense exercise or physical job, 6–7 days per week.

This step translates your RMR into your TDEE—the calories you burn in real life.


How Hoot Uses the Equation

When you sign up for Hoot, you enter your age, weight, height, gender, activity level, and goal pace. Behind the scenes, Hoot applies the Mifflin-St Jeor formula to calculate your personalized calorie target.

From there, Hoot makes it easier:

  • Adjusts calories automatically as your weight and goals change

  • Builds a balanced macro plan (protein, fat, carbs)

  • Ensures calories never drop below safe thresholds (1,200/day for women, 1,500/day for men)

  • Provides daily guidance through Nutrition Scores and meal feedback

The science is serious—but Hoot keeps the experience light, simple, and motivating.


Strengths and Limitations

Strengths:

  • Validated in research as one of the most accurate predictive equations

  • More precise than older formulas like Harris-Benedict

  • Works well for most healthy adults

Limitations:

  • Still an estimate, not a direct measurement

  • May be less accurate for people with unusually high muscle mass, older adults, or populations not well-represented in the original study

  • Daily calorie needs also fluctuate with stress, sleep, and other factors that the formula doesn’t capture

For maximum accuracy, labs can measure calorie burn using indirect calorimetry (tracking oxygen in vs. carbon dioxide out). But for everyday use, Mifflin-St Jeor is the most practical, trusted tool available.


References

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