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Mifflin-St Jeor vs Harris-Benedict vs Katch-McArdle

Here is a full explanation of the three most common BMR formulas, how each one works, why the results can be different, and which equation is best for your TDEE.

If you’ve ever used more than one calorie calculator and noticed different results, the reason usually comes down to the BMR formula being used. Understanding how these equations work helps you interpret your TDEE estimate correctly instead of chasing a “perfect” number.

Want to see how these formulas affect your calories? Use our free TDEE calculator to compare results, then come back to understand what the numbers mean.

Why BMR Formulas Matter

Basal Metabolic Rate (BMR) represents the calories your body burns at rest. Because we can’t easily measure metabolism outside of a laboratory, researchers use equations to estimate it.

The formula chosen sets the foundation for your Total Daily Energy Expenditure (TDEE). Even small differences in BMR can lead to meaningful changes in calorie targets over time.

For a full overview of how BMR fits into daily calorie needs, start with Understanding TDEE (Total Daily Energy Expenditure).

Harris-Benedict Equation

Developed in the early 1900s, the Harris-Benedict equation was one of the first attempts to estimate resting calorie needs using height, weight, age, and sex.

While groundbreaking at the time, it was built on small sample sizes and a population that lived very differently than people do today.

  • Does not account for body composition
  • Tends to overestimate calorie needs
  • Still used in older calculators

Mifflin-St Jeor Equation

Introduced in 1990, the Mifflin-St Jeor equation uses more modern population data and consistently outperforms Harris-Benedict in research studies.

Because it balances simplicity with accuracy, it’s the default choice for most modern calorie calculators.

  • Best accuracy for general population
  • No body fat input required
  • Widely recommended in nutrition research

Katch-McArdle Equation

The Katch-McArdle formula estimates BMR using lean body mass rather than total body weight. This can be useful for athletes or very lean individuals.

However, its accuracy depends entirely on how precisely body fat percentage is measured.

  • Requires accurate body fat measurement
  • Can outperform other formulas in athletic populations
  • Highly sensitive to estimation error

Which BMR Formula Should You Use?

No equation is perfect. All BMR formulas are estimates, not direct measurements of metabolism. The goal is consistency and context.

  • Most people: Mifflin-St Jeor
  • Accurate body fat data: Katch-McArdle
  • Legacy comparisons: Harris-Benedict

The best approach is to pick one method, track results over time, and adjust intake based on real-world outcomes rather than chasing formula changes.