Metabolic Rate Concepts (BMR basics)
Basal Metabolic Rate (BMR) is an estimate of how many calories the body uses in a day at rest
to support essential functions (breathing, circulation, temperature regulation, cellular maintenance).
In practice, BMR is used as a starting point to estimate maintenance calories.
Total Daily Energy Expenditure (TDEE) is an estimate of total calories burned per day after accounting
for activity. A simple educational model is:
\[
\begin{aligned}
\mathrm{TDEE} &\approx \mathrm{BMR}\cdot m
\end{aligned}
\]
where m is an activity multiplier (sedentary, lightly active, moderately active, very active, extra active).
Important: This calculator provides estimates for learning and planning.
Individual needs vary (body composition, training status, health, sleep, stress, measurement error).
It is not medical advice.
Units used by the formulas
The BMR equations below are defined using:
weight in kg, height in cm, and age in years.
If inputs are entered in pounds (lb) or inches (in), they must be converted first.
Unit conversions
\[
\begin{aligned}
H_{\mathrm{cm}} &= H_{\mathrm{in}}\cdot 2.54 \\
W_{\mathrm{kg}} &= W_{\mathrm{lb}}\cdot 0.45359237
\end{aligned}
\]
Mifflin–St Jeor equation (default)
The Mifflin–St Jeor equation is widely used for modern estimates of resting energy expenditure.
It depends on sex, weight, height, and age.
Male
\[
\begin{aligned}
\mathrm{BMR} &= 10W + 6.25H - 5A + 5
\end{aligned}
\]
Female
\[
\begin{aligned}
\mathrm{BMR} &= 10W + 6.25H - 5A - 161
\end{aligned}
\]
where:
W = weight (kg), H = height (cm), A = age (years).
The output is in kcal/day.
Harris–Benedict equation (revised option)
The Harris–Benedict family of equations is older but still commonly seen in educational settings.
The revised form used in this calculator is:
Male
\[
\begin{aligned}
\mathrm{BMR} &= 88.362 + 13.397W + 4.799H - 5.677A
\end{aligned}
\]
Female
\[
\begin{aligned}
\mathrm{BMR} &= 447.593 + 9.247W + 3.098H - 4.330A
\end{aligned}
\]
From BMR to TDEE (maintenance calories)
To estimate maintenance calories, the calculator multiplies BMR by an activity multiplier:
\[
\begin{aligned}
\mathrm{TDEE} &\approx \mathrm{BMR}\cdot m
\end{aligned}
\]
Typical multipliers (approximate) include:
The difference between TDEE and BMR is sometimes called an activity “add-on”:
\[
\begin{aligned}
\text{Activity add-on} &= \mathrm{TDEE} - \mathrm{BMR}
\end{aligned}
\]
Optional conversion to kJ/day
Nutrition labels and scientific contexts sometimes use kilojoules (kJ). The conversion is:
\[
\begin{aligned}
1\ \text{kcal} &= 4.184\ \text{kJ}
\end{aligned}
\]
Therefore
\[
\begin{aligned}
\text{kJ/day} &= \text{kcal/day}\cdot 4.184
\end{aligned}
\]
How to interpret the graphs
-
Gauge / meter: places BMR (and TDEE if selected) on a shared kcal/day scale.
Hover markers to see values and the chosen assumptions.
-
Daily energy breakdown card: shows BMR plus the activity add-on to reach TDEE.
This helps visualize how activity changes maintenance calories.
-
Comparison strip: shows estimated TDEE across activity categories from sedentary to extra active,
holding BMR constant.
Limitations and practical notes
-
These equations are population-based approximations. Real energy needs can differ substantially.
-
Changes in body composition (fat mass vs lean mass) can shift true resting energy expenditure.
-
For nutrition planning, many people treat TDEE as a starting estimate and adjust based on measured weight trends over time.
