Basal Metabolic Rate (BMR)
Basal Metabolic Rate (BMR) is an estimate of the energy your body uses in a 24-hour period while at rest,
in a neutral-temperature environment, after an overnight fast. In practice, BMR represents a “baseline”
energy requirement for essential functions such as breathing, circulation, temperature regulation, and cellular
maintenance. This calculator provides equation-based estimates and is not medical advice.
What this calculator computes
The calculator estimates BMR using one of two widely used predictive equations:
- Mifflin–St Jeor (default) — commonly used in modern nutrition practice.
- Harris–Benedict (revised) — an updated version of the classical Harris–Benedict equation.
Results are reported in kcal/day and also converted to kJ/day.
Units and conversions
Both formulas require:
height in centimeters (cm), weight in kilograms (kg), and age in years.
If inputs are given in imperial units, they are converted first.
Imperial → metric conversions
\[
\begin{aligned}
h_{\text{in}} &= 12\cdot h_{\text{ft}} + h_{\text{in}} \\
h_{\text{cm}} &= h_{\text{in}}\cdot 2.54 \\
m_{\text{kg}} &= m_{\text{lb}}\cdot 0.45359237
\end{aligned}
\]
Mifflin–St Jeor equation
For both sexes, the Mifflin–St Jeor equation has the same core structure and a sex-specific constant.
\[
\mathrm{BMR}=10m + 6.25h - 5a + s
\]
where:
\(m\) is mass in kg,
\(h\) is height in cm,
\(a\) is age in years,
and the constant \(s\) is:
\[
s=
\begin{cases}
5, & \text{male}\\
-161, & \text{female}
\end{cases}
\]
Harris–Benedict equation (revised)
The revised Harris–Benedict equation uses different coefficients for males and females.
Male
\[
\mathrm{BMR}=88.362 + 13.397m + 4.799h - 5.677a
\]
Female
\[
\mathrm{BMR}=447.593 + 9.247m + 3.098h - 4.330a
\]
Energy unit conversion: kcal/day ↔ kJ/day
The calculator converts kilocalories to kilojoules using:
\[
1\ \text{kcal} = 4.184\ \text{kJ}
\]
Therefore:
\[
\mathrm{BMR}_{\text{kJ/day}} = \mathrm{BMR}_{\text{kcal/day}}\cdot 4.184
\]
Interpreting the result
BMR is a resting baseline. It does not include energy used for daily movement, exercise, digestion,
or growth. For many practical applications, BMR is combined with an activity multiplier to estimate daily
energy needs (often called TDEE), but that is a separate calculation.
Why two formulas can give different answers
Predictive equations are built by fitting data from specific populations. Because they use different fitted
coefficients, it is normal for Mifflin–St Jeor and revised Harris–Benedict to produce slightly different BMR
estimates for the same person. The “show both” option helps you compare the two estimates side-by-side.
Important note
BMR equations are screening tools and can be less accurate for some individuals (for example, athletes with
high lean mass, older adults, or people with certain medical conditions). For clinical decision-making,
measured resting metabolic rate (indirect calorimetry) and professional medical guidance are preferred.
