The Mifflin-St Jeor equation: how scientists calculate how many calories you need

Every calorie calculator you have ever used — from NHS tools to gym apps — almost certainly uses the same formula underneath. It was published in 1990 by two American researchers, Mark Mifflin and Sachiko St Jeor, and it remains the most accurate widely-used method for estimating how many calories a person burns each day.

Understanding how the equation works helps you make sense of the number it produces — and, more importantly, helps you decide what to do with it.

What is BMR — and why does it matter?

Before getting to the equation itself, it helps to understand what it is actually measuring: your Basal Metabolic Rate, or BMR.

BMR is the number of calories your body would burn if you did absolutely nothing — no movement, no digestion, just lying still and keeping your organs running. Breathing, pumping blood, regulating temperature, repairing cells: all of this costs energy, even at rest.

For most people, BMR accounts for somewhere between 60% and 75% of all the calories they burn in a day. This is why diet matters so much more than exercise for weight management — your body is burning the majority of its calories before you even get out of bed.

The Mifflin-St Jeor equation

The formula calculates BMR differently for men and women, because body composition typically differs between the sexes — men generally carry more muscle mass relative to body weight, and muscle burns more calories than fat even at rest.

The only difference between the two formulas is the constant at the end: +5 for men, −161 for women. Everything else — the weight, height, and age components — is identical.

What does each part of the formula mean?

Each variable in the equation reflects something real about how your body works:

• Weight (10 × kg) — heavier bodies require more energy to maintain. More mass means more cells to keep alive.
• Height (6.25 × cm) — taller people generally have larger organs and more surface area, which increases their resting energy needs.
• Age (−5 × years) — BMR naturally declines with age. Muscle mass tends to decrease and metabolic processes slow down over time, which is why the age term subtracts from the total.
• Sex constant (+5 / −161) — accounts for average differences in body composition between men and women.

From BMR to TDEE — adding activity into the equation

BMR only tells you half the story. To find out how many calories you actually need each day, you need to account for how active you are. This is where TDEE — Total Daily Energy Expenditure — comes in.

TDEE is calculated by multiplying your BMR by an activity multiplier:

Activity level	Multiplier	What it looks like
Sedentary	× 1.2	Desk job, little or no exercise
Lightly active	× 1.375	Light exercise 1–3 days a week
Moderately active	× 1.55	Exercise 3–5 days a week
Very active	× 1.725	Hard exercise 6–7 days a week
Extra active	× 1.9	Physical job or twice-daily training

So if your BMR is 1,600 kcal and you exercise moderately, your TDEE would be roughly 1,600 × 1.55 = 2,480 kcal per day. That is the number of calories you need to maintain your current weight.

How accurate is it?

The Mifflin-St Jeor equation was validated in a 1990 study of 498 adults and has since been tested against more precise laboratory methods — including indirect calorimetry, which measures oxygen consumption to calculate energy expenditure directly.

For most healthy adults, the equation is accurate to within around 10% of actual measured BMR. A 2005 review published in the Journal of the American Dietetic Association concluded it was the most accurate of the commonly-used predictive equations, outperforming older formulas like the Harris-Benedict equation.

Why do two people weigh the same but need different calories?

This is one of the most common sources of confusion around calorie calculators. The answer comes down to the variables the equation cannot measure directly.

Two people who weigh the same, are the same height and age, and have the same biological sex could still have very different calorie needs if their body composition differs. Someone with a higher proportion of muscle mass will burn significantly more calories at rest than someone of the same weight who carries more body fat — because muscle tissue is metabolically active in a way that fat tissue is not.

The Mifflin-St Jeor equation cannot distinguish between muscle and fat. This is a known limitation. More precise methods, such as measuring lean body mass and using the Katch-McArdle formula, can account for this — but they require knowing your body fat percentage, which most people do not have easy access to.

For practical purposes, the Mifflin-St Jeor result is an excellent starting point. Most people find it accurate enough to build a sensible nutrition plan around.

Using your TDEE to hit your goal

Once you know your TDEE, adjusting for your goal is straightforward:

• To lose weight: eat less than your TDEE. A deficit of 500 kcal per day leads to roughly 0.5 kg of fat loss per week. A deficit of 250 kcal per day is more gradual and often more sustainable.
• To maintain weight: eat at your TDEE.
• To gain weight or build muscle: eat more than your TDEE. A surplus of 250–500 kcal per day supports steady muscle gain when combined with resistance training.

It is worth noting that these are starting estimates, not precise prescriptions. Real-world results depend on many factors — food quality, hormonal health, sleep, stress, and gut health all influence how your body uses the energy you give it. The TDEE figure is a baseline to work from and adjust over time based on your actual results.

A worked example

Let us walk through a real calculation. Take a 35-year-old woman who is 165 cm tall and weighs 70 kg, with a moderately active lifestyle.

Our calorie calculator does all of this working automatically — including the macro split — once you enter your details.

The bottom line

The Mifflin-St Jeor equation is over 30 years old, but it remains the gold standard for estimating calorie needs in everyday settings. It accounts for the four factors that matter most — weight, height, age, and sex — and when combined with an honest assessment of your activity level, it gives you a reliable starting point for any nutrition goal.

No formula is perfect. But understanding what goes into the calculation helps you interpret the output sensibly, adjust based on real-world results, and avoid the trap of treating a single number as an absolute truth.

Use it as a starting point. Track your results over two to three weeks. Then adjust up or down by 100–200 kcal based on what you observe. That is how the equation works best in practice.