Mifflin-St Jeor Calculator — BMR and Calories

Mifflin-St Jeor is the equation registered dietitians reach for when a metabolic cart is not available. It was fitted in 1990 on 498 adults measured by indirect calorimetry, and it has held its place for thirty-five years because nothing simple has beaten it. Enter sex, age, weight and height:

The equation, twice

Almost every site prints the rounded version. The 1990 paper printed something slightly different, and the difference is worth seeing.

As published, a single regression covering both sexes (R² = 0.71):

REE = 9.99 × weight + 6.25 × height − 4.92 × age + 166 × sex − 161, where sex is 1 for males and 0 for females.

As everyone uses it, with the coefficients rounded and the sex term split into two constants:

• Men: 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + 5
• Women: 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) − 161

Both appear in the original paper, so the rounded form is not a folk simplification — the authors published it. On a typical adult the two land within a few kcal of each other. The detail worth keeping is that single 166 kcal sex term: the entire male-female gap in this equation is one flat constant, standing in for an average difference in lean mass rather than for anything else.

Worked out in pounds and inches

The equation is metric. Convert before you calculate.

A 35-year-old man at 180 lb and 5 ft 10 in → 81.6 kg, 177.8 cm:

10 × 81.6 + 6.25 × 177.8 − 5 × 35 + 5 = 816 + 1,111 − 175 + 5 = 1,757 kcal/day

A 35-year-old woman at 150 lb and 5 ft 5 in → 68.0 kg, 165.1 cm:

10 × 68 + 6.25 × 165 − 5 × 35 − 161 = 680 + 1,031 − 175 − 161 = 1,375 kcal/day

Each term does a distinct job: weight and height push the number up, age pulls it down at a flat 5 kcal per year, and the constant handles sex. If you want the conversion done separately, the pounds to kg and kg to pounds pages handle that step.

The accuracy number to remember

When the Academy of Nutrition and Dietetics graded the competing equations, Mifflin-St Jeor won, and the winning margin was this: it predicts resting metabolic rate within 10% of the measured value in about 82% of non-obese adults and 70% of adults with obesity. In the obese group the misses ran up to 9% high and up to 21% low. The recommendation that follows is rated strong but conditional: measure resting metabolic rate if you can, and only fall back on the equation — with actual, not adjusted, body weight — when you cannot.

Turn that around and the practical rule falls out. One adult in five gets a number more than 10% off. On the 1,757 kcal example that is a genuine range of roughly 1,580 to 1,930. So do not treat a 40 kcal difference between two calculators as meaningful when the equation’s own error bar is four times that wide.

Using it for weight loss without over-trusting it

The output is a resting floor, not a food budget. The sequence that works:

1. Get BMR from the calculator above.
2. Multiply by an activity factor — 1.2 sedentary through 1.9 for daily hard training — to reach total daily expenditure. The BMR vs TDEE page walks through that step.
3. Subtract a moderate deficit from the total, never from the basal figure.
4. Weigh yourself over three to four weeks and adjust from what the scale actually does.

Step 4 is what makes step 1’s error tolerable. The equation gives you a defensible place to start; your own trend line is the only thing that can tell you where the estimate landed for you.

What it cannot see

It has no input for body composition, so an athlete and a sedentary person of identical weight, height, age and sex receive identical numbers. It was fitted on adults aged 19 to 78, so teenagers and children sit outside its sample. And it says nothing about thyroid function, medication or a long history of dieting. If your result feels far from what your body does, read slow metabolism test before concluding anything, or compare against the older Harris-Benedict calculator to see how much two reasonable equations can disagree.

This content is informational and does not replace evaluation by a physician or registered dietitian.