Mifflin-St Jeor Calculator: The Most Accurate BMR Formula, Explained

What Is the Mifflin-St Jeor Equation?

The Mifflin-St Jeor equation estimates your Basal Metabolic Rate (BMR) — the number of calories your body burns at complete rest just to keep you alive: breathing, circulating blood, regulating temperature, and running basic cell repair. It says nothing about exercise or daily movement on its own; it's the floor your body needs before you do anything at all.

Mifflin, St Jeor, and colleagues published the equation in 1990 in the American Journal of Clinical Nutrition, built from a larger and more metabolically diverse study sample than the formulas that came before it. It has since become the formula most clinicians and dietitians reach for first, and it's the equation behind our own BMR Calculator.

The Formula

For men:

BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(y) + 5

For women:

BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(y) − 161

The only difference between the two is the constant at the end — a +5 for men, a −161 for women — which corrects for the average difference in body composition between the sexes at the same height, weight, and age.

If you're working in pounds and inches rather than kilograms and centimeters, convert first: weight(kg) = weight(lb) ÷ 2.205, and height(cm) = height(in) × 2.54. Or skip the conversion entirely and let the BMR Calculator do it for you.

Step-by-Step Example

A 35-year-old man, 80 kg, 180 cm:

BMR = 10(80) + 6.25(180) − 5(35) + 5
    = 800 + 1,125 − 175 + 5
    = 1,755 calories/day

At complete rest — asleep, unmoving, digesting nothing — his body burns roughly 1,755 calories in 24 hours.

A 28-year-old woman, 65 kg, 165 cm:

BMR = 10(65) + 6.25(165) − 5(28) − 161
    = 650 + 1,031.25 − 140 − 161
    = 1,380.25 calories/day

Her resting energy expenditure comes out to about 1,380 calories/day.

Notice both examples land in a similar range even though the inputs differ — that's the formula working as intended. Small changes in weight or height shift the result by tens of calories, not hundreds; age and sex shift it more.

Why the Mifflin-St Jeor Equation Is Considered the Most Accurate

BMR formulas get validated by comparing their predictions against indirect calorimetry — a lab measurement of the oxygen you consume and carbon dioxide you produce at rest, which is about as close to "ground truth" as this gets outside a research hospital. A 2005 systematic review by Frankenfield and colleagues in the Journal of the American Dietetic Association compared the major predictive equations against measured resting metabolic rate and found Mifflin-St Jeor came out on top for accuracy in both normal-weight and obese adults.

| Formula | Accuracy (within ±10% of measured BMR) | | --- | --- | | Mifflin-St Jeor | ~80% | | Harris-Benedict (revised, 1984) | ~70% | | Owen | ~65–70% | | Katch-McArdle | requires body-fat % input |

That ±10% band matters more than it sounds. It means for roughly 4 out of 5 adults, the equation's estimate lands within about 150–250 calories of what a lab would measure directly — tight enough to build a real nutrition plan around, loose enough that you shouldn't treat the output as a precise medical number.

The Academy of Nutrition and Dietetics recommends Mifflin-St Jeor as the preferred predictive equation for both normal-weight and obese adults, which is a meaningful distinction — the older Harris-Benedict formula was built from a much smaller, less diverse sample and tends to run hot (overestimating) as body weight rises.

From BMR to TDEE: Turning Your Result Into a Daily Calorie Number

BMR on its own isn't something you eat to — it's a resting baseline. To get a number you can actually plan meals around, you need Total Daily Energy Expenditure (TDEE), which multiplies your BMR by an activity factor that accounts for movement, exercise, and the calories your body burns digesting food.

| Activity Level | Description | Multiplier | | --- | --- | --- | | Sedentary | Little or no structured exercise | BMR × 1.2 | | Lightly active | Light exercise 1–3 days/week | BMR × 1.375 | | Moderately active | Moderate exercise 3–5 days/week | BMR × 1.55 | | Very active | Hard exercise 6–7 days/week | BMR × 1.725 | | Extra active | Physical job plus daily training | BMR × 1.9 |

Continuing the first example: a 35-year-old man with a BMR of 1,755 who works a desk job and lifts weights three days a week (moderately active) has a TDEE of roughly 1,755 × 1.55 ≈ 2,720 calories/day. That's the number he'd eat at to hold his current weight steady.

You can run this calculation directly with the TDEE Calculator, which chains the Mifflin-St Jeor BMR into the activity multiplier for you, or use the Calorie Calculator to go one step further and get a goal-adjusted target.

Setting a Calorie Target for Your Goal

Once you have TDEE, the adjustment for weight loss, gain, or maintenance is a matter of simple arithmetic layered on top:

Weight Loss

• TDEE − 500 calories/day → roughly 0.5 kg (1 lb) of loss per week
• TDEE − 1,000 calories/day → roughly 1 kg (2 lb) per week — aggressive, and generally not recommended without medical supervision

For our 2,720-calorie example, a 500-calorie deficit puts him at about 2,220 calories/day for a sustainable ~0.5 kg/week loss.

Weight Gain

• TDEE + 300–500 calories/day for steady, mostly-lean gain when paired with resistance training
• Larger surpluses tend to accelerate fat gain more than muscle gain

Weight Maintenance

• Eat at TDEE and adjust in small increments (100–150 calories) based on your actual weekly weight trend, not the calculator's output alone

Use a Macro Calculator once you have a calorie target to split it into protein, carbohydrate, and fat — the calorie number from Mifflin-St Jeor is the foundation the rest of a nutrition plan gets built on.

Mifflin-St Jeor vs. Other BMR Formulas

Harris-Benedict (1919) and the 1984 Revision

The original Harris-Benedict equation is over a century old, built from a small, non-diverse study population by the standards of modern nutrition science. It tends to overestimate BMR by roughly 5%, and more than that in people carrying more body fat, because the original sample skewed lean. A 1984 revision improved on the original coefficients but still trails Mifflin-St Jeor in head-to-head validation studies.

Katch-McArdle

Katch-McArdle calculates BMR from lean body mass rather than total weight, which makes it more accurate for people with unusually low or high body-fat percentages — competitive athletes and bodybuilders, in particular. The tradeoff is that it requires a reasonably accurate body-fat percentage as an input, and most people don't have one measured precisely enough (skinfold calipers and consumer bioelectrical-impedance scales can both be off by several percentage points) to make Katch-McArdle more accurate in practice than Mifflin-St Jeor's simpler weight-and-height inputs.

Cunningham

Cunningham is a close relative of Katch-McArdle, also built on lean body mass, and shares the same practical limitation: it's only as good as the body-composition number you feed it.

Bottom line on formula choice: for the large majority of adults who don't have a precise, recent body-fat measurement, Mifflin-St Jeor is the better default — it needs only age, sex, height, and weight, all of which you can measure accurately at home, and it's validated as the most accurate of the equations that use those four inputs alone.

What Affects Your BMR Beyond the Formula's Inputs

The equation only takes four inputs, but BMR itself responds to more than that:

• Muscle mass. Muscle tissue is more metabolically active than fat tissue, so two people with identical age, height, and weight can still have meaningfully different real BMRs if one carries more muscle. This is the gap Katch-McArdle tries to close and Mifflin-St Jeor doesn't directly capture.
• Age. BMR declines gradually with age — roughly 1–2% per decade after 20 — largely driven by the slow loss of muscle mass (sarcopenia) that comes with aging unless it's actively countered with resistance training.
• Genetics. Resting metabolic rate varies between individuals of otherwise identical stats for reasons that aren't fully explained by measurable factors — some people simply run a genuinely faster or slower baseline metabolism.
• Hormones. Thyroid conditions (hyper- or hypothyroidism), menopause, and other hormonal shifts can move BMR meaningfully in either direction, sometimes enough to make a formula-based estimate unreliable until the condition is managed.
• Body temperature. Fever raises metabolic rate — roughly 10–13% per degree Celsius of elevation — because your body is working harder to maintain the higher temperature.
• Severe calorie restriction. Prolonged, aggressive dieting can trigger metabolic adaptation, where the body reduces energy expenditure below what the formula would predict as a response to the deficit. This is one reason very low-calorie diets often stall out even when the math "should" produce continued loss.

Common Mistakes When Using a BMR or TDEE Calculator

• Overestimating activity level. Most people round up — "moderately active" often actually describes someone who trains three days a week but sits the rest of the time. When in doubt, pick the tier below where you'd instinctively place yourself; overestimating activity is the single most common reason a calorie target ends up too high.
• Treating the output as exact. The ±10% validation band means the honest way to use this number is as a starting point, not a precise medical figure. Track your actual weight over 2–3 weeks and adjust from there.
• Not recalculating after a change. Recalculate whenever your weight shifts by about 5 kg (11 lb) or more, or if your activity level changes significantly — a number calculated at your starting weight drifts out of date as your weight moves.
• Ignoring medical conditions and medications. Thyroid disorders, PCOS, Cushing syndrome, and certain medications (corticosteroids, some antidepressants) can shift metabolism in ways no general-population formula accounts for. If you have one of these conditions, treat the calculator's output as a rougher starting estimate and lean more heavily on real-world weight trends — or a conversation with your healthcare provider.

Frequently Asked Questions

Is the Mifflin-St Jeor equation accurate for people with obesity? Yes. Unlike the original 1919 Harris-Benedict formula, Mifflin-St Jeor was validated in both normal-weight and obese populations, and the Academy of Nutrition and Dietetics recommends it for both groups specifically because of that.

How is this different from a TDEE calculator? BMR is the resting number this equation produces on its own. TDEE takes that BMR and multiplies it by an activity factor to estimate total calories burned across a full day, including exercise and movement. Our TDEE Calculator runs both steps together.

How often should I recalculate my BMR? Whenever your weight changes by 5 kg (11 lb) or more, or your activity level changes significantly enough that the old multiplier no longer fits.

Does the formula work for athletes? For most athletes, yes. Very lean athletes — under roughly 10% body fat for men or 20% for women — may get a more precise estimate from the Katch-McArdle formula, since it accounts for lean body mass directly rather than total weight.

Why do different calculators give me different numbers? Different tools use different underlying equations (Harris-Benedict, Katch-McArdle, Mifflin-St Jeor) and sometimes different activity multipliers. A 100–200 calorie spread between calculators for the same person is normal and not clinically meaningful — it's the ±10% validation range showing up in practice, not an error in either tool.

Bottom Line

Mifflin-St Jeor is the most validated, most widely recommended formula for estimating resting metabolic rate from measurements you can take at home — no body-fat percentage, no lab equipment, just age, sex, height, and weight. Use it as your starting point, layer an honest activity multiplier on top to get TDEE, and adjust based on how your actual weight moves over a few weeks rather than treating the first number you get as final.

Run your own numbers with the BMR Calculator, then take it further with TDEE and Calorie calculators to build a full daily target.

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