The Mifflin-St Jeor Equation, Explained (With a Worked Example)
TL;DR: The Mifflin-St Jeor equation estimates your BMR — the calories you'd burn at complete rest — from weight, height, age, and sex. Multiply that by an activity factor (1.2 to 1.9) to get your TDEE, the calories you actually burn in a day. Men add 5 at the end; women subtract 161. It's the most accurate general-purpose formula in common use (within ~10% of measured values for most people), which is why nearly every calorie app — including ours — starts here. It's still an estimate, so treat the number as a starting point and adjust from real results.
Every calorie app hands you a daily number and rarely explains where it came from. For most of them — MyFitnessPal, YAZIO, Cal AI, CalTracker — the answer is the same 35-year-old formula: the Mifflin-St Jeor equation. Here's exactly what it does, worked through with real numbers, plus the honest limits of what a formula can tell you.
What it actually calculates
Two terms do all the work here:
- BMR (Basal Metabolic Rate) — the energy your body burns doing absolutely nothing: breathing, pumping blood, keeping your brain and organs running. If you stayed in bed all day, this is roughly what you'd burn. It's the biggest chunk of your daily calories, usually 60–70%.
- TDEE (Total Daily Energy Expenditure) — BMR plus everything else: walking to work, typing, exercising, fidgeting, and even the energy used to digest your food. This is the number your calorie goal is built on.
Mifflin-St Jeor gives you BMR. You get to TDEE by multiplying by an activity factor. Then you subtract to lose weight, or add to gain.
The formula
Both versions use weight in kilograms, height in centimetres, and age in years. The only thing that changes between them is the number on the end.
Men: BMR = (10 × weight) + (6.25 × height) − (5 × age) + 5
Women: BMR = (10 × weight) + (6.25 × height) − (5 × age) − 161
Using pounds and inches? Convert first: weight in kg = pounds ÷ 2.205; height in cm = inches × 2.54. Or skip the arithmetic entirely and use our calorie & TDEE calculator, which handles metric and imperial.
A worked example
Let's run it for a 30-year-old woman, 70 kg, 168 cm, moderately active.
Step 1 — BMR. Plug into the women's formula:
BMR = (10 × 70) + (6.25 × 168) − (5 × 30) − 161
BMR = 700 + 1050 − 150 − 161
BMR = 1,439 calories/day
Step 2 — TDEE. "Moderately active" means an activity factor of 1.55:
TDEE = 1,439 × 1.55
TDEE ≈ 2,230 calories/day
Step 3 — a goal. To lose weight at a steady, sustainable pace, subtract a deficit. A 500-calorie daily deficit works out to roughly 0.45 kg (1 lb) per week of fat loss:
Target = 2,230 − 500 = ~1,730 calories/day to lose weight
To maintain, she'd eat around 2,230; to gain, she'd add a surplus of 250–500. (For a 30-year-old man with the identical stats, Step 1 ends in +5 instead of −161, giving a BMR of 1,605 — the 166-calorie gap between the two constants reflects average differences in body composition.)
Activity multipliers
The multiplier is where most people go wrong — it's tempting to overrate yourself. Be honest; "I go to the gym" doesn't make you "very active" if you sit the other 15 hours.
| Activity level | Multiplier | Roughly means |
|---|---|---|
| Sedentary | × 1.2 | Desk job, little or no exercise |
| Lightly active | × 1.375 | Light exercise 1–3 days/week |
| Moderately active | × 1.55 | Moderate exercise 3–5 days/week |
| Active | × 1.725 | Hard exercise 6–7 days/week |
| Very active | × 1.9 | Physical job or twice-daily training |
Why this formula won
Mifflin-St Jeor was published in 1990 (Mifflin, St Jeor et al., American Journal of Clinical Nutrition) from a study of 498 healthy adults, measured with indirect calorimetry. It replaced the older Harris-Benedict equation from 1919, which was built on a smaller, less representative sample and tends to overestimate for modern populations.
In head-to-head reviews, Mifflin-St Jeor predicts resting metabolic rate within about 10% of the measured value more often than any other equation of its kind, with an average error around 5%. That's why the Academy of Nutrition and Dietetics has recommended it and why it's the default in most reputable calorie apps and calculators today.
Where it's wrong (and why that's OK)
A formula built on four inputs can't capture everything:
- It ignores body composition. Muscle burns more at rest than fat, but the equation only sees total weight — so it tends to under-estimate for very muscular people and over-estimate for very high-body-fat individuals. (Formulas like Katch-McArdle use body-fat percentage instead, if you know yours accurately.)
- The activity multiplier is a guess on top of a guess. Two people who both call themselves "moderately active" can differ by hundreds of calories a day.
- Your real metabolism varies with sleep, stress, hormones, and dieting history in ways no static formula tracks.
None of this makes it useless — it makes it a starting point. The right workflow is: get your estimate, eat to it consistently for 2–3 weeks, watch the scale trend, and adjust the target based on what actually happened to your weight. Reality beats any formula. That's also the honest limit of AI food scanning, which we cover in how accurate AI calorie counters really are.
From equation to daily habit
The math gives you a target; a tracker tells you whether you're hitting it. CalTracker computes your goal with this exact Mifflin-St Jeor equation (weight, height, age, sex, five activity levels, and your chosen deficit or surplus), shows an estimated weekly weight change, and lets you override the number if your own results say otherwise. Want the number without installing anything? The free calorie & TDEE calculator runs the same formula in your browser.
FAQ
What is the Mifflin-St Jeor equation?
A 1990 formula that estimates your Basal Metabolic Rate — the calories your body burns at rest — from weight, height, age, and sex. Multiply by an activity factor to get TDEE, the calories you burn in a normal day. It's the equation most modern calorie apps and dietitians use.
What is the formula for men and women?
Both use weight (kg), height (cm), and age (years). Men: (10 × weight) + (6.25 × height) − (5 × age) + 5. Women: the same, but − 161 instead of + 5.
Is Mifflin-St Jeor accurate?
It's the most accurate general-population BMR formula in common use — within ~10% of measured values for most people, average error around 5%. But it doesn't account for body-fat percentage, so it's less reliable for very muscular or very high-body-fat individuals. Use it as a starting point and adjust from real results.
What's the difference between BMR and TDEE?
BMR is what you burn at complete rest. TDEE is BMR plus all your daily movement, exercise, and digestion — found by multiplying BMR by an activity factor (1.2 sedentary to 1.9 very active). Your calorie goal is based on TDEE.
This article is general information, not medical or nutrition advice. Calorie needs are individual — consult a doctor or registered dietitian before starting a diet, especially with any medical condition or history of disordered eating. Equation outputs are estimates and shouldn't replace professional guidance.
Your number, then the habit
CalTracker sets your goal with the Mifflin-St Jeor formula, then tracks it. Free forever for core tracking; logs offline too.