BMR formulas and calculator methodology

This page lists the equations, units, study populations, and calculation rules used by the adult BMR calculator.

Primary equation: Mifflin–St Jeor

The calculator uses the widely used simplified Mifflin–St Jeor form as its primary general-adult prediction:

The original 1990 study included 498 healthy adults, 251 men and 247 women, aged 19–78, including normal-weight and obese participants. A 2005 systematic review of common equations found Mifflin–St Jeor the most reliable overall among the equations it compared in healthy non-obese and obese adults, while also showing that no prediction equation is exact for every person.

Why Revised Harris–Benedict is shown separately

Roza and Shizgal re-evaluated the Harris–Benedict data in 1984. The calculator shows that equation as a separate comparison because validation results vary across populations. Each equation keeps its own result.

Optional body-composition estimate: Cunningham

Fat-free mass is a major determinant of resting expenditure. If the user supplies a body-fat percentage, the calculator derives fat-free mass and adds the 1991 Cunningham estimate:

This does not automatically make the result more accurate. If body-fat percentage is poorly measured, the resulting fat-free-mass estimate is also poor. For that reason Cunningham is presented as an optional comparison, not as an automatic replacement for Mifflin.

Optional athlete estimate: Ten Haaf

General-population equations can perform differently in athletes. The Ten Haaf equation was developed in recreational athletes aged 18–35, and a later systematic review/meta-analysis of athlete studies found it among the strongest-performing equations overall.

If an athlete is older than 35, the site still allows the comparison but explicitly marks it as outside the original derivation age range.

Age handling

This version accepts adults from age 18. Mifflin’s original sample covered ages 19–78, so age 18 and ages above 78 receive a study-range note. People under 18 are outside this release because pediatric and adolescent energy estimation uses different age-specific methods and growth considerations.

Why BMI is calculated only as context

BMI is not used to alter the Mifflin formula. It is calculated only to trigger interpretation notes for low BMI, obesity, and severe obesity. Reviews in obesity show that no single equation is uniformly accurate and precise, and individual errors remain meaningful. BMI can also misclassify muscular people, so athlete users receive an additional caution.

Special circumstances

The form allows users to flag pregnancy/lactation, serious illness or major injury, and recent major weight loss/prolonged restriction. These flags add interpretation warnings. The calculator does not apply an unvalidated correction factor for those situations.

• Pregnancy/lactation: energy expenditure changes across pregnancy, and total requirements involve more than resting expenditure.
• Serious illness/critical care/major trauma: clinical nutrition guidelines favor measured expenditure such as indirect calorimetry when feasible in relevant high-risk settings.
• Weight loss/energy restriction: adaptive thermogenesis can occur, with substantial between-person and between-study variability.

Why results are rounded

Displaying a result such as 1,648.75 kcal/day creates false precision. Prediction-equation error is far larger than a fraction of one calorie, so the site displays whole calories while retaining full numerical precision internally.

Why there is no “±10% confidence interval”

Validation papers frequently call a prediction “accurate” when it falls within ±10% of measured expenditure. That is a study scoring rule, not a guarantee that an individual user’s true expenditure lies within ±10% of the estimate. The calculator therefore does not present ±10% as a personalized confidence interval.

Measured metabolism

When individual precision matters, indirect calorimetry is more informative than a population equation. It estimates resting energy expenditure from respiratory-gas exchange under standardized resting conditions. Measurement itself still depends on protocol, preparation, and equipment quality, but it is a direct physiological assessment rather than a demographic regression estimate.

Why this calculator does not output TDEE

Total daily energy expenditure includes resting expenditure, physical activity, and the thermic effect of food. Turning one resting estimate into a diet prescription with a generic activity multiplier adds another layer of uncertainty. A future TDEE/EER calculator should use its own methodology rather than silently extending this one.