Skip to main content
Almost every calorie calculator on the internet — whether it promises weight loss, muscle gain, or maintenance numbers — runs on the same piece of mathematics under the hood: the Mifflin-St Jeor equation. Published in 1990, it has survived more than three decades of scientific scrutiny and remains the most widely recommended formula for estimating basal metabolic rate. This guide explains what the equation is, how each part of it works, how accurate it really is, and where its limits lie.
What Is the Mifflin-St Jeor Equation?
The Mifflin-St Jeor equation is a predictive formula for basal metabolic rate (BMR) — the number of calories the body burns at complete rest to keep its basic systems running: breathing, circulation, cell repair, and temperature regulation.
Developed by Mifflin and colleagues and published in 1990, the equation was created by measuring the resting metabolic rates of hundreds of adults with indirect calorimetry (the laboratory gold standard) and then finding the simple formula that best predicted those measurements from easy-to-know inputs: weight, height, age, and sex.
Its importance is best understood in context. Before it arrived, the standard was the Harris-Benedict equation from 1919, built from data on a much smaller and less diverse group. As validation studies accumulated through the 1990s and 2000s, Mifflin-St Jeor consistently predicted measured metabolic rates more accurately across broader populations — and the Academy of Nutrition and Dietetics now recommends it as the preferred equation for healthy adults when body fat percentage is unknown.
The Two Formulas: Men and Women
The equation comes in two versions. The only difference is the constant at the end.
For men:
BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
For women:
BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
The result is in kilocalories per day (kcal/day) — what most people simply call “calories.”
The 166-calorie gap between the male constant (+5) and the female constant (−161) is not arbitrary. At the same height, weight, and age, men carry more lean muscle mass on average than women, and muscle tissue burns more energy at rest than fat tissue. The constants encode that average body-composition difference.
Breaking Down Each Variable
Each term in the equation represents a real physiological effect:
- 10 × weight (kg) — Body mass is the single biggest driver of BMR. A larger body has more tissue to maintain, so every additional kilogram adds roughly 10 kcal/day to resting expenditure. This is why BMR varies so much between individuals.
- 6.25 × height (cm) — Taller people have larger frames, more skin surface, and longer organs to maintain. Height contributes less per unit than weight, but across the normal range it still shifts the result by hundreds of calories.
- 5 × age (years) — Metabolism slows with age, mostly because muscle mass declines and hormonal profiles shift. The equation subtracts 5 kcal/day per year of age — a 50-year-old otherwise identical to a 30-year-old burns about 100 fewer calories at rest.
- The sex constant (+5 / −161) — Adjusts for average differences in lean mass between men and women, as described above.
Notice what is not in the equation: body fat percentage, activity level, muscle mass, genetics, or medical conditions. Those omissions are both the formula’s strength (it needs only four easy inputs) and its main limitation (real bodies vary in ways it cannot see).
A Brief History: Why This Equation Exists
The story starts in 1919 with Harris and Benedict, who derived their equation from measurements of 239 subjects — mostly young, lean men. For seventy years it was the clinical standard, and calculators everywhere used it.
By the late 1980s, researchers noticed a problem: the Harris-Benedict equation systematically overestimated metabolic rate in many modern populations, particularly in people carrying extra weight. Mifflin and colleagues set out to build a better formula using a larger, more representative sample measured with modern indirect calorimetry.
Their 1990 paper produced the equation above. Subsequent validation studies — comparing predicted BMR against laboratory-measured values in thousands of subjects — found that Mifflin-St Jeor landed within 10% of the measured value for roughly 8 out of 10 people, noticeably better than its 1919 predecessor. That accuracy record, reproduced across many independent studies, is why nearly every reputable online calculator adopted it as the default.
Worked Examples: Calculating BMR by Hand
The arithmetic is simple enough to do on a phone calculator. Two examples show the full process.
Example 1: A 30-year-old man, 80 kg, 180 cm
BMR = (10 × 80) + (6.25 × 180) − (5 × 30) + 5 = 800 + 1125 − 150 + 5 = 1,780 kcal/day
Example 2: A 30-year-old woman, 60 kg, 165 cm
BMR = (10 × 60) + (6.25 × 165) − (5 × 30) − 161 = 600 + 1031.25 − 150 − 161 = 1,320.25 ≈ 1,320 kcal/day
Using imperial units
The formula requires metric inputs, so convert first:
- Weight: pounds ÷ 2.205 = kilograms (e.g., 176 lb ÷ 2.205 ≈ 79.8 kg)
- Height: inches × 2.54 = centimeters (e.g., 70 in × 2.54 = 177.8 cm)
A common mistake is plugging pounds and inches directly into the formula — the result will be wildly wrong. Always convert first, or use a calculator that handles the conversion automatically, like a dedicated TDEE calculator that applies Mifflin-St Jeor behind the scenes.
How Accurate Is the Mifflin-St Jeor Equation Really?
“Most accurate available” is not the same as “exact.” Here is what the research actually shows:
- In healthy, normal-weight adults, the equation predicts measured resting metabolic rate within ±10% for roughly 80–82% of people.
- For a person with a true BMR of 1,600 kcal/day, that means the formula usually lands between 1,440 and 1,760.
- It outperforms the original Harris-Benedict equation, which hits the ±10% window for only about 38% of subjects and tends to overestimate by around 5% on average.
- Equations that use body fat percentage (like Katch-McArdle) can match or beat it — but only when the body fat figure is accurate, which requires DEXA scans or skilled caliper measurements most people do not have.
The practical takeaway: Mifflin-St Jeor gives a solid starting estimate for calorie planning, but anyone fine-tuning a diet should treat the number as a baseline and adjust based on real-world results over two to three weeks — actual weight change is the final arbiter.
Mifflin-St Jeor vs Other BMR Equations
| Equation | Year | Key Inputs | Strengths | Weaknesses |
|---|---|---|---|---|
| Mifflin-St Jeor | 1990 | Weight, height, age, sex | Best-validated for general population; simple inputs | Less accurate for athletes, very obese, elderly |
| Harris-Benedict (original) | 1919 | Weight, height, age, sex | Historic standard; widely cited | Overestimates ~5%; built on narrow 1919 sample |
| Harris-Benedict (revised) | 1984 | Weight, height, age, sex | Improved on the original | Still generally trails Mifflin-St Jeor in validation |
| Katch-McArdle | — | Lean body mass | Excellent if lean mass is known | Useless without an accurate body-fat measurement |
| Cunningham | 1980 | Lean body mass | Good for athletic populations | Same body-fat-measurement problem |
| Oxford/Henry | 2005 | Weight, age, sex (by age band) | Built on a large modern database | Less commonly implemented in calculators |
For the average person without a body-composition scan, Mifflin-St Jeor remains the sensible default — which is exactly why health authorities and most evidence-based fitness resources, including explainers on healthline.com, treat it as the reference equation.
From BMR to TDEE: The Activity Multipliers
BMR is only the resting baseline — typically 60–70% of total daily burn. To estimate TDEE (Total Daily Energy Expenditure), multiply BMR by an activity factor:
| Activity Level | Multiplier | Description |
|---|---|---|
| Sedentary | 1.2 | Little or no exercise, desk job |
| Lightly active | 1.375 | Light exercise 1–3 days per week |
| Moderately active | 1.55 | Moderate exercise 3–5 days per week |
| Very active | 1.725 | Hard exercise 6–7 days per week |
| Extra active | 1.9 | Very hard exercise plus a physical job |
Using Example 1 above (BMR 1,780) at a moderately active lifestyle:
TDEE = 1,780 × 1.55 = 2,759 kcal/day
This is the number that actually guides eating: a deficit below it for fat loss, a surplus above it for muscle gain. Note that activity level is self-reported and people routinely overestimate it — when in doubt, pick the lower category.
When the Equation Doesn’t Apply: Limitations
Mifflin-St Jeor was validated on typical adults, and its accuracy degrades outside that population:
- Athletes and very muscular individuals — High muscle mass raises true BMR above the prediction. Lean-mass-based formulas serve them better.
- People with obesity — The equation performs better here than Harris-Benedict, but individual variation is still wide; clinical settings often prefer measured values.
- Older adults — Body-composition shifts with aging in ways the simple age term only approximates.
- Children and teenagers — Growth requires extra energy the adult equation does not account for. Pediatric formulas exist for this reason.
- Pregnancy, illness, and thyroid disorders — All alter metabolic rate beyond what the four inputs can capture.
- Very low-calorie dieting — Prolonged deficits trigger metabolic adaptation, lowering actual expenditure below the predicted baseline.
None of this invalidates the equation for its intended audience — healthy adults estimating a starting point. It just means the number is an estimate, not a measurement.
Common Mistakes When Using the Formula
- Mixing up units — Pounds and inches must be converted to kilograms and centimeters first.
- Confusing BMR with TDEE — BMR is the at-rest floor, not daily calorie needs. Eating at BMR with an active lifestyle creates an unintended deficit.
- Using it as a medical diagnosis — The equation estimates energy needs; it cannot diagnose metabolic disorders.
- Ignoring the activity multiplier’s subjectivity — Most people pick one category too high.
- Expecting decimal-place precision — Rounding to the nearest 10 kcal is honest; the formula’s error margin is far larger than single calories.
Students juggling coursework with fitness goals often discover that the same discipline that keeps their class attendance above the required threshold — consistent routines, honest tracking — is what makes calorie estimates actually work. And for anyone planning early-morning training sessions around a class timetable, knowing how many classes can safely be missed helps protect both the workout schedule and exam eligibility.
Frequently Asked Questions
What is the Mifflin-St Jeor equation?
It is the most widely validated formula for estimating basal metabolic rate, published in 1990. Using weight, height, age, and sex, it predicts the calories the body burns at complete rest, and it is the default equation in most online BMR and TDEE calculators.
What is the Mifflin-St Jeor formula for men and women?
For men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5. For women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161. The result is in kcal per day.
Is Mifflin-St Jeor more accurate than Harris-Benedict?
Yes, according to the bulk of validation research. Mifflin-St Jeor predicts measured BMR within 10% for roughly 8 in 10 people, while the original Harris-Benedict equation manages that for fewer than 4 in 10 and tends to overestimate.
What is the difference between BMR and TDEE?
BMR is the energy burned at complete rest — the baseline. TDEE (Total Daily Energy Expenditure) is BMR plus everything else: movement, exercise, and digestion. TDEE is calculated by multiplying BMR by an activity factor between 1.2 and 1.9.
Can I use Mifflin-St Jeor for weight loss?
Yes, as a starting point. Calculate BMR, multiply by an honest activity level to get TDEE, then eat moderately below TDEE (never consistently below BMR) and adjust based on actual weight trends over two to three weeks.
Why are there separate formulas for men and women?
Because at the same height, weight, and age, men carry more lean muscle mass on average, and muscle burns more energy at rest than fat. The different constants (+5 for men, −161 for women) encode that average body-composition difference.
Does the equation work for athletes?
Not very well. High muscle mass pushes true BMR above the prediction. Athletes get better estimates from formulas based on lean body mass, such as Katch-McArdle — provided they have an accurate body-fat measurement.
