August 28, 2026

The Mifflin-St Jeor Equation: The Most Accurate BMR Formula

When it comes to calculating how many calories your body needs, the starting point is always your Basal Metabolic Rate (BMR) — the number of calories your body burns at complete rest just to keep you alive. And when it comes to estimating BMR, the Mifflin-St Jeor equation is widely regarded as the most accurate formula available for the general population. It's the formula used by most dietitians, fitness professionals, and calorie calculators — including ours.

In this article, we'll explain where the Mifflin-St Jeor equation came from, break down exactly how it works with step-by-step examples, compare it to older formulas to show why it's more accurate, and explain how our weight loss calculator uses it as the foundation for all its calorie recommendations.

What Is BMR and Why Does It Matter?

Your Basal Metabolic Rate represents the minimum number of calories your body needs to sustain life while at complete rest. This includes the energy required for breathing, circulating blood, maintaining body temperature, producing hormones, cell repair and regeneration, brain function, and all the other involuntary processes that keep you alive. BMR typically accounts for 60–70% of your total daily calorie expenditure — it's by far the largest component of how many calories you burn each day.

Understanding your BMR is essential for weight management because it tells you the baseline number of calories your body requires before any activity is factored in. Once you know your BMR, you can multiply it by an activity factor to calculate your Total Daily Energy Expenditure (TDEE) — the total number of calories you burn in a typical day including all movement and exercise. Your TDEE is the number you need to know to create a calorie deficit for weight loss or a surplus for muscle gain.

Without an accurate BMR estimate, every calorie calculation that follows is built on shaky ground. If your BMR is overestimated, you'll eat too much and won't lose weight. If it's underestimated, you'll eat too little, potentially triggering metabolic adaptation, fatigue, and muscle loss. This is why choosing the right BMR formula matters.

The History Behind Mifflin-St Jeor

The equation was published in 1990 by Dr. M. Daniel Mifflin and Dr. Steven T. St Jeor in the American Journal of Clinical Nutrition. At the time, the most commonly used BMR formula was the Harris-Benedict equation, which had been developed in 1919. While the Harris-Benedict equation was a significant achievement for its era, it was based on a relatively small sample size and tended to overestimate BMR by 5–15% in many individuals, particularly those who were overweight or obese.

Mifflin and St Jeor set out to create a more accurate equation by studying a larger, more diverse group of 498 healthy adults (251 men and 247 women) aged 19 to 78. They measured resting metabolic rate using indirect calorimetry — the gold standard for metabolic measurement — and used regression analysis to develop formulas that predicted BMR from weight, height, age, and sex.

The resulting equations proved to be significantly more accurate than the Harris-Benedict formula. Subsequent validation studies, including a major analysis by Frankenfield and colleagues published in the Journal of the American Dietetic Association in 2005, confirmed that Mifflin-St Jeor was the most reliable equation for predicting resting metabolic rate in both normal-weight and overweight individuals, with an accuracy within 10% of measured values in most cases.

The Mifflin-St Jeor Formulas

The equation comes in two versions — one for men and one for women — reflecting the physiological differences in body composition between the sexes. Men typically have more lean muscle mass and less body fat than women of the same weight, which means they burn more calories at rest.

For men:

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

For women:

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

The coefficients in the formula represent the following:

• 10 × weight: Each kilogram of body weight contributes approximately 10 calories to your BMR. This reflects the metabolic cost of maintaining all your body tissues — muscle, organs, bone, and fat.

• 6.25 × height: Taller people have more body surface area and more lean tissue, which increases metabolic rate. Each centimeter of height adds about 6.25 calories.

• 5 × age: Metabolic rate decreases with age — approximately 5 calories per year. This reflects the natural loss of muscle mass and slowing of cellular processes that occur as we age.

• +5 (men) / −161 (women): This constant accounts for the average difference in body composition between sexes. The 166-calorie gap between the two formulas reflects the fact that men typically have more metabolically active lean tissue.

Step-by-Step Examples

Let's work through two complete examples to see how the equation works in practice.

Example 1: A 28-year-old woman

• Weight: 65 kg (143 lbs)

• Height: 163 cm (5'4")

• Age: 28 years

BMR = (10 × 65) + (6.25 × 163) − (5 × 28) − 161

BMR = 650 + 1,018.75 − 140 − 161

BMR = 1,368 kcal/day

This means her body burns approximately 1,368 calories per day at complete rest. To find her total daily expenditure, we'd multiply by an activity factor. If she's moderately active (exercises 3–5 days per week):

TDEE = 1,368 × 1.55 = 2,120 kcal/day

For a moderate weight loss deficit of 500 calories: her daily target would be 1,620 kcal/day.

Example 2: A 42-year-old man

• Weight: 88 kg (194 lbs)

• Height: 178 cm (5'10")

• Age: 42 years

BMR = (10 × 88) + (6.25 × 178) − (5 × 42) + 5

BMR = 880 + 1,112.5 − 210 + 5

BMR = 1,788 kcal/day

If he has a sedentary desk job (activity factor 1.2):

TDEE = 1,788 × 1.2 = 2,145 kcal/day

His maintenance calories are about 2,145 per day. To lose weight at a moderate pace, he'd aim for about 1,645 kcal/day (a 500-calorie deficit).

Why Mifflin-St Jeor Is More Accurate Than Alternatives

Several BMR formulas exist, and understanding why Mifflin-St Jeor outperforms them helps build confidence in the numbers:

Harris-Benedict (1919): The oldest widely-used equation. It tends to overestimate BMR by 5–15%, particularly in overweight individuals. A revised version was published in 1984, but even the revision doesn't match Mifflin-St Jeor's accuracy. Using Harris-Benedict might lead you to believe you burn more calories than you actually do, resulting in a calorie target that's too high for effective weight loss.

Katch-McArdle: This formula uses lean body mass instead of total body weight, which in theory should be more accurate since muscle is more metabolically active than fat. However, it requires knowing your body fat percentage, which most people don't have an accurate measurement of. Without precise body composition data, Mifflin-St Jeor provides a more reliable estimate for the average person.

WHO/FAO/UNU equations: These international formulas are used in public health settings but are less accurate at the individual level because they use broader population averages and don't account for age-related metabolic decline as precisely.

A 2005 study comparing multiple equations against measured resting metabolic rate found that Mifflin-St Jeor predicted BMR within 10% of measured values in 82% of normal-weight individuals and 70% of overweight individuals — better performance than any other commonly used equation.

Limitations of Mifflin-St Jeor

While Mifflin-St Jeor is the best widely-available equation, it's important to understand its limitations:

It estimates average BMR, not individual BMR. Even with the best equation, individual metabolic rates can vary by 10–15% from the predicted value due to genetics, hormonal factors, body composition differences, and other variables. The formula gives you a strong starting point, but you may need to adjust based on your actual results.

It uses total body weight, not body composition. Two people with the same weight but different muscle-to-fat ratios will have different BMRs. A muscular person will burn more calories at rest than someone of the same weight with more body fat. Mifflin-St Jeor doesn't account for this, though it's still more accurate than formulas that ignore body composition entirely.

It may be less accurate at extreme body weights. For very lean individuals (body fat below 10% for men, below 18% for women) or very obese individuals (BMI above 40), the formula may be less precise. Specialized formulas or direct metabolic testing may be more appropriate in these cases.

It doesn't account for metabolic adaptation. When you've been in a calorie deficit for an extended period, your body may reduce its metabolic rate beyond what the formula predicts — a phenomenon known as adaptive thermogenesis. This is why periodic recalculation and adjustment of calorie targets is important during a weight loss journey.

How Our Calculator Uses Mifflin-St Jeor

Our weight loss calculator uses the Mifflin-St Jeor equation as the foundation for all its calculations. When you enter your weight, height, age, and gender, it first computes your BMR using the formulas above. Then it multiplies your BMR by the appropriate activity factor to determine your TDEE. From there, it applies a 500-calorie deficit to give you a daily calorie target for weight loss.

If you enter a goal weight and target date, the calculator works backward from your TDEE to determine the exact daily deficit needed to reach your goal by that date. It also checks whether the resulting calorie target falls below safe minimums (1,200 kcal/day for women, 1,500 kcal/day for men) and warns you if the pace would be too aggressive.

The result is a personalized calorie plan built on the most validated BMR formula in nutrition science. For a complete understanding of how calorie deficits work, see our guide on how to calculate your calorie deficit. To understand what your BMI number means alongside your BMR, read our article on BMI explained.

Practical Tips for Using Your BMR

Recalculate every 5–10 kg of weight change. As you lose weight, your BMR decreases because a lighter body requires fewer calories. If you started at 90 kg and are now 80 kg, your BMR will be noticeably lower than when you began. Recalculate to keep your calorie targets accurate.

Use it as a starting point, not gospel. If you're consistently losing weight faster or slower than predicted, adjust your calorie target accordingly. The formula gives you a mathematical estimate — your body's actual response may differ slightly.

Don't eat below your BMR for extended periods. Your BMR represents the minimum energy your body needs for basic functioning. Consistently eating below BMR can signal to your body that it's in a crisis state, potentially triggering metabolic slowdown, hormonal disruption, and muscle loss.

Pair BMR knowledge with actual tracking. The best way to validate your BMR estimate is to track your food intake and weight over 2–3 weeks. If you're eating at your calculated TDEE and your weight is stable, your BMR estimate is likely accurate. If you're gaining or losing, adjust accordingly.

Key Takeaways

The Mifflin-St Jeor equation is the most accurate widely-available formula for estimating BMR, validated in numerous studies as superior to older alternatives like Harris-Benedict. It uses weight, height, age, and sex to estimate the calories your body burns at rest. While no formula is perfect — individual variation, body composition, and metabolic adaptation all play roles — Mifflin-St Jeor provides the most reliable starting point for building a personalized calorie plan. Our weight loss calculator uses this equation to give you accurate BMR, TDEE, and calorie deficit numbers tailored to your body.

Ready to see your personalized numbers? Use our free weight loss calculator to get your BMR, TDEE, and daily calorie target based on the Mifflin-St Jeor equation.