Mifflin-St Jeor Calculator - BMR & TDEE Formula

Formula	BMR (kcal/day)
Mifflin-St Jeor (1990)	1,669
Harris-Benedict revised (1984)	1,722
Katch-McArdle (lean mass)	1,583

Published	1990, Am J Clin Nutr
Accuracy	Within about 10% for most adults
Men	+5 constant
Women	−161 constant
Inputs	Weight, height, age, sex
Default	First-line equation in dietetics

What the Mifflin-St Jeor equation is

The Mifflin-St Jeor equation is a predictive formula for resting energy expenditure published in 1990 by Mifflin, St Jeor and colleagues in the American Journal of Clinical Nutrition. It was built to improve on the older Harris-Benedict equation and has repeatedly been found to be the most accurate of the common equations for healthy adults. It needs only weight, height, age and sex, which is why it became the first-line formula in dietetic practice. The equation estimates basal metabolic rate (BMR) — the calories burned at complete rest — and you multiply that by an activity factor to get total daily energy expenditure (TDEE). Its main limitation is that it treats all body weight alike: it can read high in people with obesity and low in very muscular people, where Katch-McArdle, which uses lean mass, does better.

Common Uses for Mifflin-St Jeor Calculator

Calculate BMR with the equation used as the clinical default
Convert BMR to TDEE with the standard activity factors
Compare Mifflin-St Jeor with Harris-Benedict and Katch-McArdle
Build a calorie target from a defensible baseline
Check whether another calculator's number is plausible

The Mifflin-St Jeor equation in full

The equation uses metric units and returns kilocalories per day. 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 only difference between the sexes is the final constant, +5 versus −161 — a 166 kcal gap at identical size and age. If you work in pounds and inches, convert first: 1 lb = 0.4536 kg and 1 inch = 2.54 cm.

How to use this calculator

Enter sex, age, height and weight in metric or imperial units. The calculator returns BMR from the Mifflin-St Jeor equation and shows the same inputs through Harris-Benedict and Katch-McArdle so you can see the estimation range. To turn BMR into a daily calorie target, multiply by the activity factor that matches your real week, not your best week. The optional body-fat field is only used by Katch-McArdle; Mifflin-St Jeor does not require it.

The standard activity factors (1.2 to 1.9)

TDEE = BMR × activity factor. The multipliers below are the long-standing convention used by the American Council on Exercise and most dietetic guidance. Pick the lower level when in doubt: overestimating activity is the most common reason a calorie target fails.

1.2 — Sedentary: desk job, little or no deliberate exercise.
1.375 — Lightly active: light exercise or sport 1-3 days a week.
1.55 — Moderately active: moderate exercise 3-5 days a week.
1.725 — Very active: hard exercise 6-7 days a week.
1.9 — Extra active: physical job, or training twice a day.

Worked example: the same body, two sexes

Take a 30-year-old who is 175 cm and 72 kg. For a man: (10 × 72) + (6.25 × 175) − (5 × 30) + 5 = 720 + 1,093.75 − 150 + 5 = 1,668.75, about 1,669 kcal/day. For a woman with identical measurements: 720 + 1,093.75 − 150 − 161 = 1,502.75, about 1,503 kcal/day. At a moderate activity factor of 1.55, the man's TDEE is about 2,587 kcal/day and the woman's about 2,329 kcal/day.

How accurate is Mifflin-St Jeor?

For healthy non-obese adults the equation typically lands within about 10% of measured resting energy expenditure; for a 1,600 kcal BMR that is a range of roughly 1,440-1,760. The 2005 systematic review by Frankenfield, Roth-Yousey and Compher found Mifflin-St Jeor the most reliable of the common predictive equations in both non-obese and obese adults, which is why it replaced Harris-Benedict as the default. Accuracy is not uniform: it tends to overestimate in people with high body fat and underestimate in very muscular people, and it was not derived for children, pregnancy, or illness.

Mifflin-St Jeor vs Harris-Benedict vs Katch-McArdle

The three equations answer the same question with different inputs. Use the comparison to understand why calculators disagree, not to cherry-pick the highest number.

Mifflin-St Jeor (1990): weight, height, age, sex. Best general-purpose choice and the current clinical default.
Harris-Benedict, revised 1984: same inputs, larger coefficients. Reads a few percent higher and is kept mainly for continuity with older clinical material.
Katch-McArdle: uses lean body mass only, so it needs body-fat percentage. Best for athletes, very muscular people, and some people with obesity when body composition is known.

BMR vs RMR: what the number actually measures

Basal metabolic rate is measured under strict conditions: after an overnight fast, lying still, in a thermoneutral room. Resting metabolic rate is measured under looser conditions and is usually a few percent higher. In practice the two are close enough that consumer calculators use BMR as the label for both. If you are comparing against a lab test, check which protocol was used before deciding the calculator is wrong.

When the equation is less reliable

Predictive equations are population averages, not measurements. They are weaker for children and adolescents, pregnancy and lactation, adults with very high body fat, highly muscular athletes, people with fever, trauma, burns or thyroid disease, and people with limb loss. In those cases use the result as a starting hypothesis, then adjust from observed weight change over two to three weeks. For clinical nutrition support, indirect calorimetry remains the reference method.

From BMR to a calorie target

Multiply BMR by your activity factor to get TDEE, then adjust for your goal. For fat loss, a deficit of 10-20% of TDEE is a common starting range; a 500 kcal daily deficit corresponds to roughly 0.45 kg (1 lb) of fat per week, though real-world loss is rarely linear. For muscle gain, a surplus of about 5-10% limits unnecessary fat gain. Recalculate after roughly 4-5 kg of weight change, because BMR falls as body mass falls.

Sources and further reading

The formulas and accuracy claims on this page follow the original equation and the standard comparative review. The activity multipliers are the conventional ACE/dietetic table, not part of the 1990 paper itself.

Mifflin MD, St Jeor ST, Hill LA, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247. doi:10.1093/ajcn/51.2.241
Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc. 2005;105(5):775-789. doi:10.1016/j.jada.2005.02.005
Roza AM, Shizgal HM. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. Am J Clin Nutr. 1984;40(1):168-182.
Harris JA, Benedict FG. A Biometric Study of Basal Metabolism in Man. Carnegie Institution of Washington; 1919.
American Council on Exercise. Activity factor and calorie-expenditure guidance.

Frequently Asked Questions

What is the Mifflin-St Jeor formula?

Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5. Women: the same equation with −161 instead of +5. The result is kilocalories per day at complete rest.

What is the Mifflin-St Jeor equation for women?

BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161. For a 30-year-old woman who is 175 cm and 72 kg, that is 720 + 1,093.75 − 150 − 161 = about 1,503 kcal/day.

Which activity factor should I use?

Start at 1.2 if you have a desk job and do not exercise, 1.375 for light exercise 1-3 days a week, 1.55 for moderate exercise 3-5 days, 1.725 for hard exercise 6-7 days, and 1.9 only for a physical job or twice-daily training. Most people overestimate and belong one level lower than they think.

Is Mifflin-St Jeor more accurate than Harris-Benedict?

Yes for most adults. Comparative studies, including the 2005 systematic review by Frankenfield and colleagues, found Mifflin-St Jeor closer to measured resting energy expenditure. Harris-Benedict, especially the 1919 original, tends to read higher.

What is the difference between BMR and RMR?

BMR is measured under strict lab conditions: supine, fasted, thermoneutral. RMR (resting metabolic rate) is measured more practically and usually reads a few percent higher. Most online calculators, including this one, use the terms interchangeably and report a BMR-style number.

Should I eat my BMR to lose weight?

No. BMR covers only resting functions; you also burn calories through movement and digestion. Eating at BMR would put most people in a large deficit. Set your intake from TDEE, then apply a moderate deficit of roughly 10-20%.

Why does this calculator disagree with another one?

Differences come from the formula, the activity factor, rounding, and whether the other tool reports BMR or TDEE. A 5-10% spread between equations is normal. Compare like with like: BMR against BMR, TDEE against TDEE.

Does Mifflin-St Jeor work for obesity or very muscular people?

It is less reliable at both extremes. In obesity it often overestimates because fat mass is metabolically quieter than the equation assumes; in very lean, muscular people it can underestimate. When body fat percentage is known, Katch-McArdle is usually better.