BMR Calculator (Mifflin-St Jeor)

What is a normal BMR?

For adult men, BMR typically ranges from 1,400 to 1,800 kcal/day. For adult women, from 1,200 to 1,500 kcal/day. These are population averages — your individual BMR may be higher or lower depending on your muscle mass, thyroid function, genetics, and other factors. BMR decreases with age (approximately 1–2% per decade after 20) primarily due to loss of muscle mass. Comparing your BMR to population averages is less useful than tracking your own BMR over time as your weight and body composition change.

How does muscle mass affect BMR?

Muscle is metabolically expensive tissue — it burns approximately 7–10 kcal per pound per day at rest, while fat burns only 2–3 kcal per pound. This means a person with more muscle mass burns significantly more calories at rest than a person of the same weight with more fat mass. During weight loss, preserving muscle mass through resistance training and adequate protein intake (1.6–2.2 g/kg body weight) is the most effective way to prevent BMR from dropping — a phenomenon called "metabolic adaptation." This is why someone who loses weight without exercise often ends up with a lower BMR than someone of the same weight who maintained muscle through strength training.

Does eating very few calories reduce BMR?

Yes — and this is called metabolic adaptation or "starvation mode." When caloric intake drops significantly below what your body needs, several compensatory mechanisms activate: thyroid hormone output decreases, muscle is broken down for energy (reducing BMR), and non-exercise activity thermogenesis (NEAT — unconscious movements like fidgeting) drops. These mechanisms can reduce total daily energy expenditure by 10–25% below what your weight and activity level would normally predict. The degree of metabolic adaptation varies significantly between individuals. This is why very-low-calorie diets (below 1,200 kcal/day for women, 1,500 for men) without medical supervision tend to produce initial results that plateau quickly and reverse when normal eating resumes.

How accurate is the Mifflin-St Jeor formula?

The Mifflin-St Jeor equation has a mean absolute percentage error of approximately 10% compared to indirect calorimetry (the gold-standard lab measurement of BMR through oxygen consumption). This means for any individual, the formula could be off by ±10% in either direction — a 1,600 kcal/day BMR could be predicted as anywhere from 1,440 to 1,760 kcal/day. For reference, the older Harris-Benedict equation has an error rate of approximately 15%. The Mifflin-St Jeor formula performs well across different body weights but tends to underestimate BMR in very muscular individuals (because muscle mass is not directly measured) and in older adults (because it does not account for the specific metabolic slowdown of aging beyond the age coefficient).

What is the difference between BMR and RMR?

BMR (Basal Metabolic Rate) and RMR (Resting Metabolic Rate) are closely related but not identical. BMR is measured under strict clinical conditions: 12 hours of fasting, 8 hours of sleep, in a thermally neutral environment, after lying quietly for 30 minutes. RMR (sometimes called Resting Energy Expenditure or REE) is measured under less strict conditions — no fasting required, no sleep requirement. RMR is typically 5–10% higher than BMR because the body is not in a fully fasted state. For practical purposes, the terms are used interchangeably in most diet and fitness contexts, and the same formulas are applied to both.

Should I eat below my BMR to lose weight?

No — eating below BMR is not necessary for weight loss and can be counterproductive. Eating below BMR for extended periods triggers stronger metabolic adaptation and signals the body that food is scarce, which increases fat storage efficiency and reduces metabolic rate. It is also nearly impossible to meet your nutritional needs (protein, vitamins, minerals) when eating below 1,200 kcal/day for women or 1,500 kcal/day for men. A sustainable deficit of 300–500 kcal/day below your TDEE (not BMR) is the standard approach. If your calculated BMR is 1,500 and your TDEE with activity is 2,200, eating 1,500–1,700 kcal/day creates a healthy deficit without triggering significant metabolic adaptation.

How does age affect BMR?

BMR declines approximately 1–2% per decade after age 20, primarily due to loss of muscle mass (sarcopenia) rather than aging itself. From age 30 to 70, a typical person loses approximately 10–15% of their muscle mass if they are sedentary. This means that without strength training, a 50-year-old of the same weight and height as a 30-year-old will have a measurably lower BMR. The good news is that resistance training can prevent and even reverse age-related muscle loss at any age. Studies in people in their 70s and 80s show that strength training increases muscle mass and metabolic rate even in very elderly individuals.