What determines your BMR?
The most important determinant of BMR is fat-free mass (FFM) — muscles, organs and skeleton. Fat-free mass is estimated to explain around 80% of the variation in BMR between individuals. This explains why well-trained people with high muscle mass have a higher BMR than people of the same total weight but more body fat.
In people with low physical activity, BMR typically constitutes about 60% of total energy expenditure, and even more in individuals with very low movement activity such as sedated and mechanically ventilated patients in intensive care units.
Other factors that affect BMR: age (BMR drops approx. 1–2% per decade from age 30 due to decreased muscle mass), gender (men on average have higher BMR per kg body weight thanks to higher proportion of muscle mass) and hormonal balance (primarily thyroid hormones).
How is BMR measured and calculated?
BMR is measured under strictly controlled laboratory conditions: after sleep, in a fasting state and in a thermoneutral environment. The gold standard is indirect calorimetry via Weir's equation (1949), which calculates energy expenditure from oxygen consumption and carbon dioxide production.
In practical contexts, RMR (Resting Metabolic Rate) is more commonly used, measured at any time during the day, which can differ from BMR by up to 10%. Most online calculators — including CalculEat — technically calculate RMR, but use the term BMR in everyday use.
When direct measurement is unavailable, predictive equations are used. The most common are:
- Mifflin-St Jeor (1990) — recommended for healthy adults, best accuracy in modern populations
- Harris-Benedict (revised 1984) — older but still common
- Cunningham (1991) — calculates RMR based on fat-free mass, better for well-trained individuals
All predictive formulas have a margin of error of ±10–15% at the individual level. Actual weight trend over 2–3 weeks is always a more reliable measure than formula-based estimates.
Why should you not eat at your BMR?
BMR is the absolute minimum — not a calorie goal. Eating exactly at BMR means giving the body precisely what it needs to survive at complete rest, but nothing for movement, digestion or normal daily activity.
The consequences of eating at BMR for an extended period include muscle mass loss, fatigue, hormonal disruptions and metabolic adaptation — the body lowers its own BMR to save energy. This makes weight loss increasingly difficult.
The right approach is to calculate your TDEE (BMR × activity factor) and create a moderate deficit from there — typically 300–500 kcal/day for sustainable weight loss.
From BMR to calorie goal — step by step
- Calculate your BMR with the Mifflin-St Jeor formula (age, gender, weight, height).
- Multiply by your activity factor (PAL) — sedentary (×1.2) to extremely active (×1.9).
- The result is your TDEE — your maintenance need.
- Subtract 300–500 kcal/day for weight loss, or add 200–300 kcal/day for muscle building.