BMR Calculator Formula Explained: Mifflin-St Jeor vs Harris-Benedict (2026)

Every online BMR calculator runs one of three equations under the hood. The number it gives you can vary by 200 kcal or more for the same person — not because metabolism is changing, but because the formulas were derived from different populations, in different decades, with different goals in mind. This article walks through the math behind each one, shows worked examples, and explains which formula to trust for your situation.

The short answer: use Mifflin-St Jeor for everyday energy planning. Use Katch-McArdle only if you have a measured body fat percentage and a lot of muscle. Ignore Harris-Benedict unless a coach or clinician specifically asks for it. Plug your numbers into the CalcFit BMR calculator to skip the arithmetic, but read on to understand what the formulas actually mean.

What BMR Actually Is

Basal metabolic rate is the energy your body burns at complete rest, fully fasted, in a thermoneutral environment, to keep your basic biology running — heart beating, brain processing, lungs ventilating, organs metabolizing. It is the floor of your daily energy budget, typically 60–75% of your total daily energy expenditure (TDEE). Everything else — moving, eating, fidgeting, exercising — sits on top.

The technical definition requires a strict laboratory protocol: overnight fast, 8+ hours of sleep, fully supine on waking, 20–25°C ambient temperature, no caffeine, no stress. In practice, almost no online calculator and very few clinics measure true BMR. What is reported as “BMR” in calculators is functionally resting metabolic rate (RMR), which runs about 10% higher than strict BMR because the conditions are looser. The Mifflin-St Jeor and Harris-Benedict equations technically predict RMR despite their naming, but the convention of calling the output “BMR” is universal and harmless for daily planning.

Mifflin-St Jeor: The 1990 Standard

Published by M. D. Mifflin and S. T. St Jeor in 1990 (American Journal of Clinical Nutrition, n=498), this is the formula the Academy of Nutrition and Dietetics officially recommends for non-athletic adult populations. It is the default in most modern calculators, including the CalcFit BMR tool.

The only sex difference is the final constant: +5 for men, −161 for women. That 166-kcal gap reflects average body composition differences (men carry more lean mass on average at the same weight) — it is a population mean, not a rule that holds for every individual. A muscular woman and an undermuscled man can have BMRs that contradict the constants.

Worked Example: Mifflin-St Jeor

30-year-old man, 80 kg, 180 cm. BMR = (10 × 80) + (6.25 × 180) − (5 × 30) + 5 = 800 + 1,125 − 150 + 5 = 1,780 kcal/day.

40-year-old woman, 65 kg, 165 cm. BMR = (10 × 65) + (6.25 × 165) − (5 × 40) − 161 = 650 + 1,031 − 200 − 161 = 1,320 kcal/day.

These numbers are what your body would burn lying still for 24 hours. To get the calories you actually eat at maintenance, multiply BMR by an activity factor to get your TDEE. The man at moderate activity (×1.55) lands at 2,759 kcal; the woman at the same level lands at 2,046 kcal.

Harris-Benedict: The 1919 Original

The Harris-Benedict equation predates Mifflin-St Jeor by 71 years. It was derived from a sample of 239 people (136 men, 103 women) at the Carnegie Institution in Washington. The formula is more complex than Mifflin-St Jeor and produces slightly higher BMR estimates in most modern subjects.

Roza and Shizgal revised the equation in 1984 using a more representative sample, tightening the coefficients. The revised version is closer to Mifflin-St Jeor in accuracy but is still typically 1–3% higher.

Worked Example: Harris-Benedict

Same 30-year-old man, 80 kg, 180 cm. Original Harris-Benedict: 66.473 + (13.7516 × 80) + (5.0033 × 180) − (6.755 × 30) = 66.5 + 1,100.1 + 900.6 − 202.7 = 1,864 kcal/day. Revised: 88.362 + (13.397 × 80) + (4.799 × 180) − (5.677 × 30) = 88.4 + 1,071.8 + 863.8 − 170.3 = 1,854 kcal/day.

That is 74–84 kcal higher than the Mifflin-St Jeor result of 1,780 for the same person. A 4–5% discrepancy. Sustained over a year of food planning, that gap is roughly 27,000–30,000 kcal — about 3–4 kg (8 lb) of fat at the body fat conversion factor of 7,700 kcal/kg.

Katch-McArdle: For Body Composition Outliers

The Katch-McArdle formula sidesteps weight and height entirely and uses lean body mass directly. It is the most accurate option for very muscular or very lean individuals — provided you have a real body fat percentage measurement, not a guess.

The same 30-year-old, 80 kg man with 12% body fat has 70.4 kg of lean body mass. Katch-McArdle: 370 + (21.6 × 70.4) = 370 + 1,520.6 = 1,891 kcal/day. That is 111 kcal higher than the Mifflin-St Jeor estimate — because Mifflin-St Jeor assumes an average body composition for an 80 kg male, and an actively trained 12% body fat male is leaner than average.

The same arithmetic at 22% body fat (less muscular, more average physique) gives 62.4 kg lean mass, BMR = 370 + (21.6 × 62.4) = 1,718 kcal. Now Katch-McArdle is lower than Mifflin-St Jeor — because the person is below the average lean-mass assumption built into Mifflin-St Jeor. Use the CalcFit body fat calculator first if you intend to use Katch-McArdle.

Side-by-Side Comparison

The same 30-year-old man, 80 kg, 180 cm, depending on which assumptions you make:

All five numbers are in the same ballpark, which is reassuring. The disagreement is meaningful at the margin — 5–6% — but no formula is going to land within 100 kcal of indirect calorimetry every time. Treat every BMR number as an estimate accurate to ±10%.

Accuracy: What the Validation Studies Show

The strongest single comparison is Frankenfield, Roth-Yousey, and Compher (2005, Journal of the American Dietetic Association), which validated four BMR equations against indirect calorimetry across normal-weight, overweight, and obese adults. Their findings, paraphrased:

• Mifflin-St Jeor predicted within 10% of measured BMR in 82% of normal-weight adults, 70% of overweight adults, and 67% of adults with obesity.
• Revised Harris-Benedict hit within 10% in 69% of normal-weight, 64% of overweight, and 59% of adults with obesity.
• WHO/FAO/UNU equations performed similarly to Harris-Benedict, with a tendency to overestimate in older adults.
• Original Harris-Benedict (1919) overestimated BMR by 5–8% on average across the modern sample.

For athletes specifically, Tinsley et al. (2019, European Journal of Applied Physiology) found that Mifflin-St Jeor underestimates BMR in resistance-trained men by 5–7%, while Katch-McArdle (using DEXA-measured lean mass) sits within 3% of measured values. If you train seriously, Katch-McArdle with a real body fat input wins.

Practical Decision Rule

A short decision tree for which formula to use:

Sedentary or moderately active adult, no body fat measurement? Use Mifflin-St Jeor. It is the most validated option for the general population and is what the CalcFit BMR calculator defaults to.

Strength athlete or very lean person with a real DEXA / Navy method body fat measurement? Use Katch-McArdle. The lean-mass-based input outperforms weight-and-height proxies in this population.

Older adult (65+) with sarcopenia concerns? Mifflin-St Jeor still works, but pair it with a body composition check. The age coefficient in any BMR formula is a population average — losing 4 kg of muscle to sarcopenia drops your true BMR more than the −5 kcal-per-year term captures.

Clinical or research context? Use whichever formula the protocol specifies. Many older clinical pathways still call for original or revised Harris-Benedict; that is a convention, not a statement that it is more accurate.

Common Pitfalls

Mistaking BMR for TDEE. BMR is what you burn at complete rest. Eating at BMR while moving through a normal day puts you in a substantial deficit. Always multiply BMR by an activity factor (1.2 to 1.9) to get the calories you eat at maintenance. See the TDEE calculator for the conversion.

Forgetting that BMR is a moving target. Lose 10 kg of fat, and your BMR drops by roughly 100 kcal because there is less mass to maintain. Gain 5 kg of muscle and your BMR rises by 30–60 kcal. Re-run the formula every time your body composition shifts meaningfully — quarterly is a reasonable cadence.

Trusting Katch-McArdle with a guessed body fat percentage. The whole accuracy advantage of Katch-McArdle vanishes when the body fat input is itself an estimate. Guessing 12% when the true value is 18% inflates the BMR by roughly 100 kcal. If you do not have a real measurement, stick with Mifflin-St Jeor.

Comparing BMR across calculators. A 200 kcal gap between two BMR calculators almost always means they are running different formulas. Check the methodology page; if one says “based on Mifflin-St Jeor” and the other does not specify, the unspecified one is likely Harris-Benedict and will read higher.

The Bottom Line

Mifflin-St Jeor is the right default formula for almost everyone using a BMR calculator in 2026. It is the most validated, the most accurate across normal-weight adults, and the one most modern tools (including the CalcFit BMR calculator) run by default. Harris-Benedict is a historical artifact that overestimates in modern populations; the 1984 revision narrows the gap but still trails Mifflin-St Jeor. Katch-McArdle wins for muscular and very lean individuals, but only if you have a real body fat measurement to feed it.

Whichever formula you use, treat the output as a ±10% estimate, not a target. Use it as the starting point for your TDEE, build your eating plan from there, and adjust by 100–200 kcal per fortnight based on the actual scale and body-composition trend — not based on which formula you happened to plug into.