Basal metabolic rate and resting energy needs
Basal metabolic rate, or BMR, estimates the energy the body uses at rest to maintain essential physiological functions such as breathing, circulation, ion gradients, temperature regulation, tissue maintenance, and cellular metabolism. A BMR calculator helps explain how sex, age, height, body mass, and lean body mass influence baseline resting energy needs.
Core formulas
The Mifflin-St Jeor equation estimates BMR from weight, height, age, and sex-specific constants:
\[
\begin{aligned}
BMR_{\text{male}} &= 10W + 6.25H - 5A + 5 \\
BMR_{\text{female}} &= 10W + 6.25H - 5A - 161
\end{aligned}
\]
The Harris-Benedict equation also uses weight, height, age, and sex-specific constants:
\[
\begin{aligned}
BMR_{\text{male}} &= 88.362 + 13.397W + 4.799H - 5.677A \\
BMR_{\text{female}} &= 447.593 + 9.247W + 3.098H - 4.330A
\end{aligned}
\]
The Katch-McArdle equation uses lean body mass instead of sex, height, and age:
\[
\begin{aligned}
BMR &= 370 + 21.6 \cdot LBM
\end{aligned}
\]
Here, \(W\) is body weight in kilograms, \(H\) is height in centimeters, \(A\) is age in years, and \(LBM\) is lean body mass in kilograms. The calculator also converts the result from kcal/day to kJ/day:
\[
\begin{aligned}
BMR_{\text{kJ/day}} &= BMR_{\text{kcal/day}} \cdot 4.184
\end{aligned}
\]
How to interpret results
A higher BMR usually reflects greater body size, more lean mass, younger age, or formula constants associated with higher resting energy cost. A lower BMR may reflect smaller body size, older age, lower lean mass, or formula constants associated with lower resting energy cost. BMR is a baseline estimate, not a complete daily calorie need.
- Use metric units directly or imperial units through the unit selector.
- Use Katch-McArdle only when lean body mass is available.
- Compare formulas to see how different equations estimate resting energy differently.
- Do not treat BMR as total daily energy expenditure.
Example: for an adult male with weight 78 kg, height 178 cm, and age 35 years, the Mifflin-St Jeor equation is:
\[
\begin{aligned}
BMR &= 10 \cdot 78 + 6.25 \cdot 178 - 5 \cdot 35 + 5 \\
&= 1722.5\ \text{kcal/day}
\end{aligned}
\]
This calculator is useful for physiology learning, nutrition education, energy expenditure comparisons, and understanding resting metabolism. It should not be used as a medical prescription because true energy needs depend on physical activity, thermic effect of food, hormones, illness, training status, body composition, and adaptive metabolism.
