What is BMR?
Your **Basal Metabolic Rate (BMR)** represents the absolute minimum amount of energy in calories your body needs to function while at complete rest. This energy is used to power vital life-sustaining processes such as breathing, blood circulation, cell production, and nutrient processing.
Think of BMR as your body's "idling speed." Even if you spent 24 hours in bed without moving a muscle, your body would still consume this specific number of calories just to keep your organs alive and your brain signaling.
Where does the energy go?
Many people assume that physical exercise is their primary calorie burner. In reality, for most adults, BMR accounts for **60% to 75%** of total daily energy expenditure. Typical organ-level energy distribution looks as follows:
Liver
27%
Brain
20%
Heart
7%
Kidneys
10%
Mifflin-St Jeor: The Industry Standard
This calculator utilizes the **Mifflin-St Jeor equation**, which was introduced in 1990 as a more accurate alternative to the older Harris-Benedict formula. Extensive clinical research has shown that it remains remarkably accurate for modern sedentary and active populations alike, with an error margin of less than 10% for most adults.
Metabolic Efficiency Tip
While you cannot significantly change the calorie demand of your vital organs, you can increase your BMR by increasing lean muscle mass. Muscle is five times more metabolically active than fat, meaning the more muscle you have, the higher your "idling speed" becomes.
Metabolic Limitations
The Mifflin-St Jeor formula is based on Total Body Weight. However, it does not distinguish between Muscle and Fat. An athlete with high muscle mass will have a significantly higher actual BMR than a sedentary individual of the same weight, as muscle requires more energy to maintain.
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Common Questions
Q: Should I eat exactly my BMR?
A: No. BMR is the energy required for survival at rest. Even minor daily activity (walking, talking) increases your needs. You should almost always eat above your BMR but below your TDEE to lose weight safely.
Q: Why does BMR drop with age?
A: As we age, we naturally lose lean muscle mass (sarcopenia) and our cells become slightly less efficient at energy production. Strength training helps mitigate this decline.
Q: How do hormones affect BMR?
A: Thyroid hormones (T3 and T4) are the primary regulators of BMR. Conditions like hypothyroidism can lower BMR significantly, while hyperthyroidism can raise it.
Q: Can sleep deprivation lower my BMR?
A: Yes. Lack of sleep disrupts glucose metabolism and hormonal balance (Cortisol/Ghrelin), which can lead to a lower metabolic rate and increased cravings.
Scientific Sources
Clinical Reference: Mifflin-St Jeor (1990) | Metabolism Journal
Editorial Status
Last Updated: January 2026 | Reviewed by Metabolic Specialists