Katch-McArdle Formula vs Mifflin-St Jeor vs Harris-Benedict: Which BMR Estimate Should You Trust

Ask the Katch-McArdle formula and two well-regarded rivals to estimate the same person's Basal Metabolic Rate and you will not get the same number three times. That is not a flaw in any one of them; it is a direct consequence of three different research teams, working in three different decades, building three different equations from three different study populations, using three different sets of inputs. The honest question is not "which formula is right and which two are wrong," but "which formula's assumptions actually match this specific person," and that question has a genuinely different answer depending on how lean or muscular that person happens to be.

This article runs all three formulas, Mifflin-St Jeor, the revised Harris-Benedict equation, and the Katch-McArdle formula, against three deliberately different people, and shows exactly where and why they diverge. The Calorie Calculator on this site implements all three, selectable from one dropdown, precisely so this comparison can be run on real, personal numbers rather than taken on faith from an article. It is the same core calculation behind a mifflin st jeor equation calorie calculator, a harris benedict calculator, or any nasm calorie calculator, myplate.gov calorie calculator, or dri calculator that walks through the same BMR-times-activity-multiplier logic, just consolidated into one place with all three formulas available side by side rather than split across three separate tools.

The Katch-McArdle formula and the two it is compared with

Mifflin-St Jeor (1990). Male: 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5. Female: the same, minus 161 instead of plus 5. Derived from a more modern dataset than Harris-Benedict, it is the formula most dietitians and current clinical nutrition guidelines default to for the general population, and it is the default formula on this site's own Calorie Calculator.

Revised Harris-Benedict (Roza & Shizgal, 1984, a statistical revision of the original 1919 equation). Male: 88.362 + 13.397 × weight(kg) + 4.799 × height(cm) − 5.677 × age. Female: 447.593 + 9.247 × weight(kg) + 3.098 × height(cm) − 4.330 × age. Still widely cited and used, though most modern comparisons consider it to run slightly high relative to Mifflin-St Jeor across a broad population.

Katch-McArdle. BMR = 370 + 21.6 × lean body mass (kg), where lean body mass = weight(kg) × (1 − body fat % ÷ 100). This is the odd one out of the three in a specific, important way: it is the only formula of the three that does not use height or age at all, and the only one that requires knowing (or estimating) a body-fat percentage first.

Three people, one calculation each, three very different formulas

Person A: a lean, athletic 28-year-old man, 178cm, 75kg, 10% body fat.

• Mifflin-St Jeor: 10×75 + 6.25×178 − 5×28 + 5 = 1,727.50 kcal/day
• Revised Harris-Benedict: 88.362 + 13.397×75 + 4.799×178 − 5.677×28 = 1,788.40 kcal/day
• Katch-McArdle (lean mass = 75 × 0.90 = 67.5kg): 370 + 21.6×67.5 = 1,828.00 kcal/day

Person B: an average-body-fat 40-year-old woman, 165cm, 70kg, 32% body fat.

• Mifflin-St Jeor: 10×70 + 6.25×165 − 5×40 − 161 = 1,370.25 kcal/day
• Revised Harris-Benedict: 447.593 + 9.247×70 + 3.098×165 − 4.330×40 = 1,432.85 kcal/day
• Katch-McArdle (lean mass = 70 × 0.68 = 47.6kg): 370 + 21.6×47.6 = 1,398.16 kcal/day

Person C: a heavier, muscular 30-year-old man, 183cm, 100kg, 12% body fat.

• Mifflin-St Jeor: 10×100 + 6.25×183 − 5×30 + 5 = 1,998.75 kcal/day
• Revised Harris-Benedict: 88.362 + 13.397×100 + 4.799×183 − 5.677×30 = 2,135.97 kcal/day
• Katch-McArdle (lean mass = 100 × 0.88 = 88kg): 370 + 21.6×88 = 2,270.80 kcal/day

Reading the pattern across all three people

Line the three formulas up person by person and a clear pattern emerges. For Person A, the lean athlete, the spread from lowest to highest is 1,727.50 to 1,828.00, a gap of 100.5 kcal, with Katch-McArdle on top. For Person B, the average-body-fat woman, the spread is 1,370.25 to 1,432.85, a gap of only 62.6 kcal, and this time Harris-Benedict, not Katch-McArdle, sits on top. For Person C, the heavier, muscular man, the spread balloons to 1,998.75 to 2,270.80, a gap of 272.05 kcal, again with Katch-McArdle highest, and by the widest margin of the three comparisons.

Two real, explainable patterns are hiding in those numbers. First, Harris-Benedict's coefficients, derived from a dataset now more than a century old, are generally considered to run slightly high across a broad population independent of body composition, which is exactly why it edged out Mifflin-St Jeor for every person here regardless of leanness, and even edged out Katch-McArdle for Person B specifically, the one person without an unusually lean or muscular build to give Katch-McArdle's lean-mass logic anything extra to work with. Second, and more importantly, the gap between Katch-McArdle and the other two grows specifically as a person gets both heavier and leaner at the same time. Person C is 100kg at 12% body fat, meaning 88kg of that weight is lean mass; Mifflin-St Jeor and Harris-Benedict both effectively treat that full 100kg the same way they would treat a 100kg person carrying, say, 30% body fat, since neither formula has any way to distinguish the two. Katch-McArdle, built on lean mass directly, does distinguish them, and credits Person C's mostly-muscle 100kg with a meaningfully higher metabolic rate than either weight-based formula would infer on its own.

Why lean mass matters biologically, not just statistically

The reason this is not just a quirk of the equations is that muscle tissue is metabolically more active at rest than fat tissue: it consumes more energy per unit of mass simply to maintain itself, even without any exercise happening at all. Two people at the identical total body weight, one carrying substantially more muscle and less fat than the other, genuinely do burn calories at rest at somewhat different rates, and a formula that only ever looks at total weight has no way to capture that real physiological difference. That is the entire reason the Katch-McArdle formula exists as a distinct approach rather than just another set of coefficients on the same weight-height-age-sex inputs: it deliberately trades those familiar, easy-to-enter inputs for a harder-to-obtain one, body-fat percentage, specifically to capture a real biological distinction the easier inputs cannot see.

Where each formula actually came from

None of these three equations were built in a vacuum, and their origins explain a lot about why they disagree. The original Harris-Benedict equation dates to 1919, derived from direct calorimetry measurements on a study population that, by modern standards, was small and not especially diverse; the 1984 Roza and Shizgal revision used here recalculated the coefficients against a larger dataset but kept the same basic weight-height-age-sex structure. Mifflin-St Jeor, published in 1990, was built specifically to improve on Harris-Benedict using a more contemporary, more representative sample, and it has since been validated favorably against indirect calorimetry (a lab measurement of actual resting energy expenditure from oxygen consumption) in enough follow-up studies that it displaced Harris-Benedict as the default recommendation in many clinical and dietetic guidelines. The Katch-McArdle formula comes from a different research tradition entirely, body composition science rather than general population nutrition surveys, which is exactly why it swaps height and age for a direct lean-mass calculation instead. None of the three is "newer, therefore automatically better" in a simple sense; each represents a different modeling choice about which inputs matter most.

How this BMR gap turns into a bigger TDEE gap

A BMR disagreement does not stay the same size once it feeds into a full daily calorie target. Total Daily Energy Expenditure, what is tdee in plain terms, multiplies BMR by an activity-level factor, and that multiplication scales up any existing gap between formulas right along with it. Take Person C's 272.05 kcal gap between Katch-McArdle and Mifflin-St Jeor and run it through each activity level's multiplier: at sedentary (×1.2), the gap grows to 326.46 kcal; at light activity (×1.375), 374.07 kcal; at moderate (×1.55), 421.68 kcal; at very active (×1.725), 469.29 kcal; and at extra active (×1.9), 516.90 kcal.

That last figure is worth sitting with: for a very active or extra-active version of Person C, choosing Katch-McArdle over Mifflin-St Jeor, or vice versa, shifts the resulting maintenance calorie target by nearly 500 kcalories a day, comfortably larger than this site's own Calorie Calculator's entire "mild pace" 250 kcal loss or gain adjustment. In other words, for a heavier, muscular, highly active person specifically, which BMR formula gets selected can matter more to the final daily number than the difference between the mild and standard loss or gain targets built on top of it. How to calculate TDEE accurately, for someone in that specific situation, genuinely depends on picking the right BMR formula first, not just on getting the activity multiplier right afterward, and the same logic applies whether the tool doing that calculation is this site's Calorie Calculator, a myplate calorie calculator, or a dedicated tdee weight loss calculator built around only one of the three formulas compared here.

The real cost of a bad body-fat estimate

That trade only pays off if the body-fat percentage entered is reasonably accurate, and this is the Katch-McArdle formula's most important practical limitation. The relationship is direct: lean mass = weight × (1 − body fat % ÷ 100), and BMR = 370 + 21.6 × lean mass, so every percentage point of error in the body-fat input shifts lean mass by 1% of total body weight, and shifts the final BMR estimate by 21.6 kcal for every kilogram that 1% represents.

Concretely, for Person C at 100kg: a body-fat estimate that is off by just 5 percentage points, say a home scale or a rough visual estimate reads 17% instead of the true 12%, shifts lean mass from 88kg down to 83kg, a 5kg swing, which moves the calculated BMR by 21.6 × 5 = 108 kcal, larger than the entire 100.5 kcal spread between the highest and lowest formula for Person A above. A rough, guessed body-fat percentage can introduce more error into the Katch-McArdle formula than the entire disagreement between the other two formulas it was meant to improve upon. This is exactly why the Katch-McArdle formula is only as good as its body-fat input, and why anyone without a reasonably reliable measurement method (DXA, BIA, skinfold calipers taken correctly, or a comparably validated method) may get a more trustworthy result from Mifflin-St Jeor instead, despite Mifflin-St Jeor's own, different set of simplifying assumptions.

So which one should you actually trust?

For most people, most of the time, Mifflin-St Jeor is the sensible default, and it is this site's own Calorie Calculator's default for exactly that reason: it requires no body-fat input, it is validated across a broad general population, and current clinical guidelines lean on it more than the older Harris-Benedict equation. Harris-Benedict remains a legitimate, widely cited alternative, useful mainly as a second reference point rather than a clearly superior option, given its tendency to run slightly high relative to more modern equations.

The Katch-McArdle formula earns its place specifically for people who know they sit meaningfully outside the "average body composition for this weight" assumption baked into the other two: visibly lean or visibly muscular people, in either direction, where Person A and Person C above are illustrative examples, including anyone specifically bulking (in the gym sense, deliberately eating in a surplus to build muscle) or already carrying an above-average amount of muscle mass, both cases where lean-mass-based logic tends to track reality more closely than a total-weight-based formula. Its accuracy advantage is real, but it is conditional, not automatic: it depends entirely on the body-fat percentage entered being close to the truth, and a bad guess there can introduce more error than the formula was chosen to eliminate. Someone with a genuinely reliable body-fat measurement, ideally from a validated method rather than a rough visual guess, and an obviously atypical build should reach for Katch-McArdle. Everyone else is generally better served by Mifflin-St Jeor's simpler, weight-based logic.

The formula gap carries straight through to loss and gain targets

Every calorie target built on top of a BMR estimate inherits that formula's own gap. This site's Calorie Calculator, and most others like it, build a loss or gain target by adding or subtracting a fixed amount from maintenance, using the widely cited estimate that roughly 3,500 kcal of stored energy corresponds to about one pound (0.45kg) of body weight, the same 3,500 figure behind how many calories to gain a pound, how many calories in a lb, calories to pounds, and 1 lb fat how many calories all being different phrasings of the same underlying constant. Because that adjustment is a fixed number of calories added to whichever maintenance figure a chosen BMR formula produced, the entire formula-choice gap discussed throughout this article passes straight through to the final loss or gain target unchanged, rather than being averaged out or corrected anywhere along the way.

This matters most for anyone chasing an aggressive goal. Someone attempting to lose 20 pounds in 3 months, for instance, is targeting roughly 70,000 kcal of total deficit (20 × 3,500), or about 778 kcal/day over 90 days, a genuinely demanding pace under any formula. Layer a 250-to-270 kcal BMR disagreement on top of a deficit that size, of the kind the Katch-McArdle formula and Mifflin-St Jeor produced for Person C above, and the practical daily target could reasonably land anywhere across roughly a 250 to 500 kcal range depending purely on which formula was used to define "maintenance" in the first place, on top of whatever the goal itself already required. A bulking calculator or bulking calorie calculator, aimed at the opposite goal, a deliberate surplus to build muscle, inherits the identical formula-choice sensitivity, just in the gaining direction instead of the losing one.

None of this is an argument for abandoning the 3,500 kcal per pound estimate, grams to calories macronutrient math, or any of the standard tools built on them; it is an argument for being honest that "how many calories should I eat" always carries two separate layers of estimation stacked on top of each other, which BMR formula, and then which deficit or surplus pace, each with its own margin of uncertainty, rather than a single, precise number handed down from one equation.

A closing sanity check, not a final verdict

It is worth being direct about what all of this can and cannot settle. None of these three formulas can tell anyone how many calories does an average person consume a day in some universal sense, since that number varies enormously by age, sex, activity level, and body composition, which is exactly the point of running a personalised calculation instead of quoting a single national average. Daily calories for men and daily calories for women both come out differently for the same reason the three formulas disagree with each other: different underlying assumptions about who counts as "average," applied to a specific person who may or may not actually match that average.

The National Institutes of Health's more advanced Body Weight Planner tool goes a step further than any static formula discussed here, modeling how metabolism itself gradually adapts over the course of a long diet rather than assuming one fixed BMR number for the entire duration, a level of dynamic detail beyond what Mifflin-St Jeor, Harris-Benedict, or Katch-McArdle, each a single-point-in-time estimate, are built to capture. That is a genuinely useful next step for anyone who wants more precision than a static formula can offer, but it does not make the comparison in this article obsolete: even a dynamic model has to start from some initial BMR estimate, and choosing the right formula for that starting point, informed by exactly the kind of body-composition reasoning worked through above, still matters.

Treat whatever gap remains between the three formulas, 60 to 270 kcal at the BMR level, and up to roughly 500 kcal once carried through to a full TDEE figure in the examples above, not as a sign that one number is definitively "the truth" and the others are wrong, but as the honest range of uncertainty that any single formula, however well validated, still carries for a specific individual it has never actually measured. The real fix for that uncertainty is not agonising over which formula to trust in the abstract; it is picking the one whose assumptions best match your actual body, then adjusting the resulting target based on a few weeks of real, observed results, which will always outrank any formula's first guess.