Mifflin-St Jeor Equation Calculator

What is the Mifflin-St Jeor equation?

The Mifflin-St Jeor equation was published in 1990 by Mark Mifflin, Sachiko St Jeor and colleagues in the American Journal of Clinical Nutrition. They measured the resting energy expenditure of 498 healthy adults aged 19 to 78 – both normal-weight and obese – and built a simple prediction formula from weight, height, age and sex. Because it was based on a more modern population than older equations, it has become the default BMR formula in many dietetics guidelines and apps.

Formula for men

BMR (kcal/day) = 10 × weight in kg + 6.25 × height in cm − 5 × age in years + 5

Formula for women

BMR (kcal/day) = 10 × weight in kg + 6.25 × height in cm − 5 × age in years − 161

The 166-calorie difference between the constants reflects the average difference in lean body mass between men and women of the same size. In imperial units, multiply pounds by 4.536 and inches by 15.88 instead.

How to calculate BMR with the Mifflin-St Jeor equation

Example: man, 30, 80 kg, 180 cm

1. 10 × 80 = 800
2. 6.25 × 180 = 1,125
3. 5 × 30 = 150
4. 800 + 1,125 − 150 + 5 = 1,780 kcal per day

Example: woman, 35, 65 kg, 165 cm

1. 10 × 65 = 650
2. 6.25 × 165 = 1,031.25
3. 5 × 35 = 175
4. 650 + 1,031.25 − 175 − 161 = 1,345 kcal per day

From BMR to TDEE

BMR is the energy you would use lying still all day. To estimate the calories you actually burn, multiply BMR by an activity factor. These factors are widely used estimates rather than precise measurements, so treat the result as a starting point.

Using TDEE for weight loss or gain

Eating below TDEE leads to weight loss over time; eating above it leads to gain. A deficit of about 500 calories a day is a common starting point for gradual loss – roughly 0.5 kg (1 lb) a week at first, though weight change slows as you get lighter and the body adapts. Avoid going below your BMR for long periods without professional supervision, and re-calculate after every 4–5 kg of change.

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

Harris-Benedict tends to give slightly higher results than Mifflin-St Jeor for most people. Katch-McArdle (BMR = 370 + 21.6 × lean mass in kg) can be more accurate for very lean or very muscular people, but only if the body-fat estimate is good. Our BMR calculator lets you compare all three side by side.

How accurate is the Mifflin-St Jeor equation?

A 2005 systematic review by Frankenfield and colleagues for the American Dietetic Association compared common equations against resting metabolic rate measured by indirect calorimetry. Mifflin-St Jeor predicted resting metabolic rate within 10% of the measured value for more non-obese and obese adults than the other equations, which is why the review recommended it. But no equation is exact: for a noticeable minority of people the error was larger than 10%, and errors tended to be larger in some ethnic groups and older adults.

• Less accurate for very muscular people (underestimates) and some people with high body fat.
• Not designed for children, pregnancy or breastfeeding, or people who are ill or recovering from surgery.
• Medical conditions and medicines that affect metabolism – such as thyroid disorders – change real needs.

The practical test is your own data: track food intake and weight for 2–3 weeks. If weight is stable, your intake is close to your true TDEE, whatever the formula says.

Mifflin-St Jeor in pounds and inches

If you work in imperial units, convert first (1 kg = 2.2046 lb; 1 inch = 2.54 cm) or use the imperial version: BMR = 4.536 × weight (lb) + 15.88 × height (in) − 5 × age + 5 for men, or − 161 for women. A 30-year-old man weighing 176.4 lb and 70.9 in tall: 4.536 × 176.4 + 15.88 × 70.9 − 150 + 5 ≈ 1,781 kcal – the same as the metric example.

How BMR changes with age

The equation subtracts 5 kcal for every year of age, so BMR falls by about 50 kcal per decade at the same weight and height. Much of the real-world decline comes from losing muscle; strength training and enough protein help limit it.

Common mistakes

• Using −5 for women – the correct constant is −161.
• Entering pounds or inches into the metric formula.
• Choosing an activity factor that is too high – most people with desk jobs are sedentary or lightly active even if they exercise a few times a week.
• Adding exercise calories from a watch on top of an activity factor that already includes exercise.
• Treating the result as exact rather than a starting estimate to adjust.

Resting vs basal metabolic rate

Strictly, basal metabolic rate is measured after an overnight fast, fully rested, in a warm room; resting metabolic rate (RMR) is measured under slightly less strict conditions and is typically a little higher. Mifflin and St Jeor measured resting energy expenditure, so the equation technically predicts RMR – but the two terms are used interchangeably in everyday nutrition, and the difference is small for planning.

Using your BMR in a nutrition plan

Once you have a TDEE estimate, split it into protein, carbohydrate and fat. A common approach is to set protein first (for example 1.2–1.6 g per kg of body weight for active adults), fat at 20–35% of calories and carbohydrate for the rest. Our fat intake and DRI calculators help with the details. Then weigh yourself under the same conditions a few times a week and compare the weekly average: if it does not move the way you expect after two or three weeks, adjust calories by 100–200 a day rather than re-running formulas.

Sources

1. Mifflin M.D., St Jeor S.T. et al. (1990) A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr 51:241–247 — doi.org/10.1093/ajcn/51.2.241
2. Frankenfield D. et al. (2005) Comparison of predictive equations for resting metabolic rate. J Am Diet Assoc 105:775–789 — doi.org/10.1016/j.jada.2005.02.005
3. Roza A.M., Shizgal H.M. (1984) The Harris Benedict equation reevaluated. Am J Clin Nutr 40:168–182 — doi.org/10.1093/ajcn/40.1.168