Mifflin St. Jeor Calculator

What the Mifflin-St Jeor equation is

You have probably landed here because someone, a coach, a textbook, a fitness app, told you to use the Mifflin-St Jeor equation. Good news: that is exactly what this calculator runs, and this page explains what it does with your numbers and how far to trust the answer.

Mifflin-St Jeor is a formula for estimating how many calories your body burns at rest. Two researchers, Mifflin and St Jeor, published it in 1990 after measuring the resting energy of nearly 500 healthy people. It arrived to fix a problem. The older equation everyone had used since 1919, Harris-Benedict, was built on early twentieth century bodies and tended to read a little high for modern ones. Mifflin-St Jeor fit today's population better, and it has been the equation most dietitians reach for ever since.

This tool takes that resting number one step further. It multiplies it by how active you are, to estimate the calories you actually burn across a normal day, which is the figure most people are really after.

The formula this calculator uses

No mystery about what is under the hood. The resting figure comes from the Mifflin-St Jeor equation in its sex-separated form:

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

Weight in kilograms, height in centimetres, age in years. Enter pounds or inches and the tool converts them first, so the answer is the same whichever units you pick. The two lines differ only at the end, +5 for men and −161 for women, which is the equation's shorthand for men carrying, on average, a bit more muscle.

A worked example

Take a 40 year old man, 178 cm tall, 82 kg. Into the men's line:

BMR = (10 × 82) + (6.25 × 178) − (5 × 40) + 5
BMR = 820 + 1112.5 − 200 + 5
BMR ≈ 1738 Calories/day

That is what his body would burn lying still all day. To turn it into a daily total, the calculator multiplies by an activity factor. If he trains a few times a week, a moderate factor of 1.55, his estimated daily burn is about 1738 × 1.55, or roughly 2,690 calories. That larger number, not the resting one, is what holds his weight steady.

From BMR to daily calories

The resting figure on its own is not your calorie need, because you are almost never lying perfectly still. So the calculator applies an activity factor and shows the result as your Total Daily Energy Expenditure, or TDEE, along with a table of what every activity level would give. Here is what those factors mean:

This step moves the number more than people expect. On the same body, the gap between the sedentary factor and the very active one is often 700 calories a day, which is the difference between losing weight and gaining it. The catch is that the factor is a rough bracket, not a measurement, and most of us overrate how active we are. Pick honestly, and treat the result as a starting point you refine after a few weeks of watching the scale. (If you only want the resting figure without this step, the BMR Calculator gives you that.)

Which of our metabolism tools to use

We run a few calculators in this corner of the site, and they overlap enough to be worth a plain map, because more than one leans on this same equation.

• This page and the BMR Calculator use the identical Mifflin-St Jeor equation. The difference is presentation: the BMR tool centres on the resting number, this one centres on the daily total after activity. Use whichever matches the number you want.
• The Harris-Benedict Calculator uses the older equation that Mifflin-St Jeor was designed to improve on. Handy if a coach or a form specifically asks for Harris-Benedict.
• The RMR Calculator estimates Resting Metabolic Rate, the close cousin of BMR, using a different equation again.
• The TDEE Calculator is the dedicated tool if the daily total is all you care about.

Different pages, because people search for different names and want different things. The maths under each stays honest to the name on the label.

Where the estimate can be off

Accurate as it is for a formula, Mifflin-St Jeor is still an estimate from population averages, and a few things sit outside its reach:

• It does not know your body composition. Two people at the same age, height, weight and sex get the same answer, even though muscle burns more at rest than fat. The formula reads your weight, not what that weight is made of.
• It was built on healthy adults. For children, for the very elderly, and for people far outside a typical weight range, it drifts, and other methods may fit better.
• Your body is not a fixed number. Thyroid conditions, medications, pregnancy, breastfeeding and illness all shift real energy needs in ways height and weight cannot capture.
• TDEE is a starting estimate, not a prescription. The activity step in particular carries a wide margin, so use the total as a place to begin, not a rule.

For everyday planning it is a genuinely good tool. For medical decisions, or if you have a health condition that affects your metabolism, treat the number as background and check it with a doctor or a registered dietitian.

Questions people ask

Is Mifflin-St Jeor the most accurate equation?

For the general population, it is generally the most accurate of the common formulas. When researchers compared them against lab measurements, Mifflin-St Jeor landed within about 10 percent of the measured value more often than the older equations. The one exception is very lean, muscular people, where a formula that uses body fat percentage, like Katch-McArdle, can do better because it accounts for muscle directly.

Is this different from a BMR calculator?

Same equation, different focus. A BMR calculator stops at the resting number. This one carries it through the activity step to give your estimated daily calories. If you saw a slightly different resting figure on another BMR tool, it is almost always because that tool used a different equation, not because the maths here is off.

Should I eat my TDEE to lose weight?

No. Your TDEE is roughly the calories that hold your weight where it is. To lose, you eat somewhat below it; to gain, somewhat above. How far in either direction depends on your goals and health, which is worth working out with a professional rather than guessing.

References

Where the numbers come from. The equation this tool runs is Mifflin-St Jeor, published in the American Journal of Clinical Nutrition in 1990, and the two lines above are quoted from that paper. That it tends to beat the older equations on accuracy comes from a systematic review by the Academy of Nutrition and Dietetics, and the older equation it improved on is the Harris-Benedict revision by Roza and Shizgal.

1. Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition. 1990;51(2):241-247. https://doi.org/10.1093/ajcn/51.2.241
2. Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic Association. 2005;105(5):775-789. https://doi.org/10.1016/j.jada.2005.02.005
3. Roza AM, Shizgal HM. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. American Journal of Clinical Nutrition. 1984;40(1):168-182. https://doi.org/10.1093/ajcn/40.1.168