The Mifflin-St Jeor equation is the single most widely used formula in modern nutrition for estimating how many calories your body burns at complete rest — your basal metabolic rate, or BMR. Published by Mifflin, St Jeor and colleagues in the American Journal of Clinical Nutrition in 1990, it outperformed the older Harris-Benedict equation in head-to-head validation studies and is now the default choice of registered dietitians and weight-management programs.
This calculator walks you through the two-step process that every nutrition coach uses: first it converts your pounds and inches into metric, then it plugs your age, sex, height and weight into the Mifflin-St Jeor equation to get your resting burn. Finally, it multiplies that resting figure by an activity multiplier to give you your total daily energy expenditure (TDEE) — the number of calories you actually burn on a normal day. From that TDEE we also pull out a weight-loss target (a 500 kcal/day deficit, which targets about one pound lost per week) and a weight-gain target (a 500 kcal/day surplus).
How to Use
- Enter your Age in completed years. The equation assumes adult physiology; BMR drops roughly 1–2% per decade after you finish growing.
- Pick your Sex — male or female. The equation adds +5 for men and subtracts 161 for women, reflecting the higher lean-muscle-to-fat ratio on average.
- Type your Weight in pounds. The engine converts internally to kilograms at 1 lb = 0.4536 kg.
- Type your Height in total inches. For reference, 5 ft 10 in is 70 inches; 5 ft 5 in is 65 inches. The engine converts to centimeters at 1 in = 2.54 cm.
- Choose your Daily Activity Level. Pick the band that matches a typical average week, not your best or worst week:
- Sedentary (×1.2) — desk job, little intentional exercise.
- Lightly active (×1.375) — light walking or 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 daily training, or a physically demanding job.
- Read the result card. The highlighted BMR line is your resting burn; the TDEE row for your chosen activity is the number of calories you need per day to hold your weight steady.
The result card also shows every activity multiplier side by side, so you can immediately see what happens if you start training more regularly — or if a busy week at work drops you from "moderately active" back to "sedentary."
The Mifflin-St Jeor Equation
The formula is deliberately simple:
Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
A worked example: a 30-year-old man who weighs 180 lb and stands 70 in tall. Converting first: 180 lb = 81.6 kg and 70 in = 177.8 cm. Plugging in: (10 × 81.6) + (6.25 × 177.8) − (5 × 30) + 5 = 816 + 1,111.25 − 150 + 5 = about 1,783 kcal/day at rest. Multiply by the moderate activity factor 1.55 and his TDEE is roughly 2,764 kcal/day. Eating 2,264 kcal/day instead creates a 500 kcal/day deficit — the classic 3,500 kcal-per-week gap that targets about one pound of fat lost per week.
Why two sex-specific constants? Lean muscle tissue burns more calories at rest than fat tissue does, and on average men carry more lean mass per pound of body weight. The +5/−161 split is the equation's empirical correction for that difference; it is not a perfect fit for every individual, which is why validation studies still report roughly a ±10% accuracy band.
Why Mifflin-St Jeor Beat Harris-Benedict
The original Harris-Benedict equation was published in 1919 on a very small, homogeneous sample of young, lean, white men. When researchers re-validated it against modern indirect-calorimetry measurements, it overestimated resting burn by as much as 5–15%. The Mifflin-St Jeor study measured more than 400 men and women across a wide range of ages and body compositions, then derived the coefficients by linear regression. In the 2005 Frankenfield comparison, Mifflin-St Jeor was accurate within 10% for about 82% of individuals — the best track record of any resting-burn formula that uses only height, weight, age and sex. That is why the Academy of Nutrition and Dietetics and most clinical weight-management protocols default to it.
The honest caveat: any formula that ignores body fat percentage, menstrual cycle, thyroid status, and recent weight change will be an estimate. If you are losing weight on a plan the calculator recommends but the scale does not move for two weeks, your true TDEE is lower than the estimate — drop another 100–200 kcal per day and re-weigh.
Examples
Example 1 — The reference case. A 30-year-old man, 180 lb, 70 in tall, moderately active. His BMR is about 1,783 kcal/day. With the ×1.55 activity multiplier his TDEE is about 2,764 kcal/day, his weight-loss target is 2,264 kcal/day, and his weight-gain target is 3,264 kcal/day.
Example 2 — A lightly active woman. A 25-year-old woman, 140 lb, 65 in tall (5 ft 5 in), light activity. Weight = 63.5 kg, height = 165.1 cm. BMR = (10 × 63.5) + (6.25 × 165.1) − (5 × 25) − 161 = 635 + 1,031.9 − 125 − 161 ≈ 1,381 kcal/day. At the ×1.375 light multiplier, TDEE ≈ 1,900 kcal/day, and a 1 lb/week weight-loss target lands near 1,400 kcal/day.
Example 3 — A very active man. A 40-year-old man, 200 lb, 72 in, who trains 6 days a week. Weight = 90.7 kg, height = 182.9 cm. BMR ≈ (10 × 90.7) + (6.25 × 182.9) − (5 × 40) + 5 ≈ 1,769 kcal/day. At the ×1.725 very-active multiplier his TDEE is roughly 3,050 kcal/day — eating less than that to "cut" would underfuel the training and risk muscle loss.
FAQ
What is the Mifflin-St Jeor equation?
It is a formula that estimates your basal metabolic rate (BMR) — the calories your body burns at complete rest — from four inputs: weight in kilograms, height in centimeters, age in years, and sex. For men it is BMR = 10W + 6.25H − 5A + 5; for women it is BMR = 10W + 6.25H − 5A − 161. It was published in 1990 and is currently the most accurate general-population BMR formula validated in independent studies.
How is BMR different from TDEE?
BMR is the number of calories you would burn if you stayed in bed all day. TDEE (total daily energy expenditure) multiplies BMR by an activity factor to account for walking, working, exercise and daily movement. The multipliers run from ×1.2 (sedentary) up to ×1.9 (extra active). Your TDEE is the number of calories you need to eat to hold your weight steady.
What activity multiplier should I choose?
Pick the band that matches a typical average week rather than your most intense week. If you work a desk job and exercise 2–3 times a week, choose lightly active (×1.375). If you have a standing job or walk heavily and also train 3–5 days a week, choose moderately active (×1.55). When in doubt, start with the lower multiplier — underestimating TDEE is the most common reason people stall on a weight-loss plan.
How accurate is the Mifflin-St Jeor equation?
In independent validation, it predicts measured resting burn within about 10% for roughly 80% of adults. It is the most accurate four-input formula available, but it cannot account for body fat percentage, muscle mass, thyroid function, medications, or recent weight loss. If you have dieted hard in the last few months, your real BMR is likely 10–15% lower than the formula says, because metabolic adaptation (adaptive thermogenesis) slows your burn.
How many calories should I eat to lose weight?
Subtract about 500 kcal per day from your TDEE. That creates a 3,500 kcal-per-week deficit, which the classic energy-balance rule estimates at roughly one pound of fat lost per week. This calculator surfaces that number as the "Weight-Loss Target." Avoid dropping below 1,500 kcal/day for women or 1,800 kcal/day for men without medical supervision, because deeper deficits risk muscle loss, nutrient gaps and rebound overeating.
How many calories should I eat to gain weight or build muscle?
Add about 250–500 kcal per day to your TDEE. The 500 kcal surplus targets roughly one pound of weight gained per week; a 250 kcal surplus adds leaner, slower weight gain. Pair the surplus with progressive resistance training — without it, most of the added weight will be fat rather than muscle.
Is Mifflin-St Jeor better than Harris-Benedict?
Yes, for most adults. The 1919 Harris-Benedict equation was derived from a small, homogeneous sample and tends to overestimate resting burn by 5–15% on modern bodies. Mifflin-St Jeor was derived from a much larger, more diverse sample and is the current clinical standard. Some advanced coaches still use Katch-McArdle (which requires body-fat percentage) for athletes, but for everyday adults Mifflin-St Jeor is the best single equation.
Disclaimer
This Mifflin-St Jeor calculator is provided for informational purposes only and is not medical advice. It is an estimate derived from a population-level formula and does not replace individualized guidance from a registered dietitian, physician, or licensed clinician. BMR and TDEE figures do not account for pregnancy, breastfeeding, thyroid disorders, eating disorders, medications, competitive athletic conditioning, or recent weight loss. Always consult a qualified health professional before making major changes to your diet, calorie intake, or exercise program, especially if you have a chronic health condition.