Compare your Basal Metabolic Rate across all four major medical formulas simultaneously. Isolate formula variance in seconds.
Biological Profile
Estimated Body Fat PercentageUnlocks Katch-McArdle
Leave blank to skip. Body fat % is required only for the Katch-McArdle formula.
Formula Comparison Matrix
Average BMR (active formulas)
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kcal / day
Mifflin-St Jeor
Modern standard (1990)
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kcal / day
Harris-Benedict
Original 1919 equation
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kcal / day
Harris-Benedict
Revised - Roza and Shizgal (1984)
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kcal / day
Katch-McArdle
Lean Body Mass based
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kcal / day
Enter Body Fat % above to unlock this formula.
Variance Readout
Max Variance
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calories / day between highest and lowest
Highest Formula
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Lowest Formula
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Key Terms Explained
Basal Metabolic Rate (BMR)The minimum calories your body burns at complete rest to sustain core functions: heartbeat, breathing, cell repair, and temperature regulation. Measured under strict fasting conditions.
Resting Metabolic Rate (RMR)Similar to BMR but measured under less strict conditions. RMR is typically 10 to 20 percent higher than true BMR and is the practical value used in nutrition clinics.
Mifflin-St Jeor EquationA predictive formula developed in 1990 using weight, height, age, and sex. It is currently the most widely recommended formula for estimating BMR in healthy adults.
Harris-Benedict EquationThe earliest widely adopted BMR formula, first published in 1919 by Harris and Benedict. The constants were revised in 1984 by Roza and Shizgal to improve accuracy on modern populations.
Katch-McArdle FormulaA formula that calculates BMR exclusively from Lean Body Mass, bypassing sex as a variable. It is often more accurate for lean and athletic individuals whose body composition differs from average study populations.
Lean Body Mass (LBM)Total body weight minus fat mass. It represents the metabolically active portion of the body: muscle, bone, water, and organs. LBM is the core variable in the Katch-McArdle formula.
ThermogenesisHeat production by the body. Diet-induced thermogenesis (the thermic effect of food) accounts for roughly 10 percent of daily calorie burn on top of BMR and activity.
Total Daily Energy Expenditure (TDEE)Your BMR multiplied by an activity factor. TDEE represents the total calories you need each day to maintain your current weight, including exercise, walking, and all daily movement.
The Complete Guide to BMR Formula Comparison
Your Basal Metabolic Rate is the foundation of every calorie calculation in nutrition science. But there is no single universally accepted formula for calculating it. The four major medical equations in this tool each represent a different decade of research, a different study population, and a different set of input variables. Understanding where they agree and where they diverge is essential for anyone who wants to move past a single number and think critically about their metabolic baseline.
How to use this tool
Enter your age, weight, and height. The three standard formulas (Mifflin-St Jeor, Original Harris-Benedict, and Revised Harris-Benedict) will calculate instantly. To unlock the Katch-McArdle card, drag the Body Fat slider or type your estimated body fat percentage. The Average BMR updates in real time based on all active formulas. Use the Copy Results button to capture your full profile for tracking or sharing.
Why the formulas differ
Each formula was developed using regression analysis on a specific group of research subjects. The 1919 Harris-Benedict study used a relatively small sample of young, healthy, and predominantly white Americans. The 1984 Roza and Shizgal revision drew on a larger and more diverse hospital patient dataset. The Mifflin-St Jeor study (1990) enrolled both obese and non-obese adults to improve accuracy across a wider range of body types. Katch and McArdle took a fundamentally different approach: instead of predicting BMR from demographic variables, they derived it from lean tissue mass directly.
Because the regression constants differ, the same person run through all four formulas will get four slightly different answers. Variance of 50 to 200 calories per day between formulas is normal. This tool makes that variance visible so you can see which formulas cluster together and which outlier formula may or may not apply to your body type.
When each formula has an advantage
The Mifflin-St Jeor equation is the best general-purpose starting point for the majority of adults. The Revised Harris-Benedict is a reasonable alternative and produces similar results. The Original Harris-Benedict is included primarily for historical context and to show how much the science has moved. The Katch-McArdle formula shines for athletes, bodybuilders, and very lean individuals whose fat mass is low enough that total body weight becomes a misleading predictor of metabolic output.
Frequently Asked Questions
Basal Metabolic Rate (BMR) is the calories your body burns at absolute rest in a temperature-controlled environment after a 12-hour fast. Resting Metabolic Rate (RMR) is measured under less strict conditions and is typically 10 to 20 percent higher than BMR. In everyday practice the two terms are used interchangeably, but BMR is the true physiological baseline, while RMR is the practical measurement used in clinical and fitness settings.
The Mifflin-St Jeor equation (1990) is currently the most widely endorsed formula for general populations. Multiple validation studies, including a 2005 review published in the Journal of the American Dietetic Association, found it predicted RMR within 10 percent for most non-obese adults. The Katch-McArdle formula can be more accurate for lean, athletic individuals because it factors in actual lean body mass rather than using total body weight.
The Katch-McArdle formula calculates BMR from Lean Body Mass (LBM) rather than total body weight, age, or sex. Fat tissue is metabolically less active than muscle and organ tissue, so stripping out fat mass produces a more direct estimate of your active metabolic tissue. Body fat percentage is the only way to mathematically derive LBM from total weight, which is why it is a required input for this formula.
The Original Harris-Benedict equation was published in 1919 and was derived from a relatively small study. In 1984, Roza and Shizgal revised the constants using a larger and more diverse dataset, producing what is now called the Revised Harris-Benedict equation. The Revised version is generally considered more accurate and is the standard when the Harris-Benedict method is still used today.
Each formula was derived from a different study population, using different measurement methods and statistical regression techniques. The input variables also differ: some formulas weight age more heavily, others prioritize height or weight differently, and the Katch-McArdle formula ignores sex entirely once lean mass is known. Because each equation was trained on a specific sample, it will produce slightly different predictions when applied to any individual outside that sample. The variance between formulas on the same person typically ranges from 50 to 200 calories per day, which is exactly what this tool is designed to reveal.
This tool provides estimates for informational purposes only. BMR predictions are not a substitute for clinical metabolic testing or personalized medical nutrition advice. Individual results may vary.