BMR Calculator

Last updated: 2026-05-22

Basal metabolic rate

Basal metabolic rate (BMR) is the energy the body uses at complete rest to sustain its core functions — circulation, breathing, body-temperature regulation, and cellular maintenance — over a 24-hour period. It excludes the energy cost of exercise, incidental movement, and the digestion of food. BMR is the lower bound of daily energy expenditure: the amount the body would consume doing nothing at all.

For most people, BMR accounts for 60–75% of total daily calorie burn. The remainder comes from physical activity and the thermic effect of food (the energy spent digesting and absorbing meals).

What determines BMR

BMR scales with the amount of metabolically active tissue a body carries. The standard equations estimate it from four readily measured quantities:

• Weight is the strongest single driver. A heavier body has more active tissue and a larger surface area for heat loss, so it burns more at rest.
• Height adds independent predictive power: at the same weight, a taller body has higher resting energy needs.
• Age lowers BMR gradually — roughly 1–2% per decade in adults — mainly through age-related loss of muscle mass.
• Sex enters as an offset because, on average, men carry more lean mass than women at the same height and weight, raising resting burn by roughly 100–200 kcal/day.

Lean (muscle) tissue is more metabolically active than fat tissue, which is why body composition matters and why one of the equations below works from lean body mass directly.

Equations

This tool computes BMR from one of three published equations, selected by the user.

Mifflin-St Jeor (1990) is the modern default:

Harris-Benedict, revised (Roza & Shizgal, 1984) is the classic equation, with separate constants for each sex:

$\text{BMR}\_{\text{male}} = 88.362 + 13.397\,w + 4.799\,h - 5.677 \times \text{age}$

$\text{BMR}\_{\text{female}} = 447.593 + 9.247\,w + 3.098\,h - 4.330 \times \text{age}$

Katch-McArdle ignores sex, age, and height, working instead from lean body mass:

$\text{BMR} = 370 + 21.6 \times \text{LBM}, \quad \text{LBM} = w \times (1 - \text{body fat})$

Worked example

A 30-year-old man weighing 80 kg at 180 cm tall, using Mifflin-St Jeor:

$\text{BMR} = 10 \times 80 + 6.25 \times 180 - 5 \times 30 + 5 = 800 + 1125 - 150 + 5 = 1{,}780 \text{ kcal/day}$

The same person at age 55 substitutes 55 for the age term, lowering the result by $5 \times 25 = 125$ kcal/day to about 1,655 kcal/day — the mechanism behind the age-related decline described above.

Choosing an equation

• Mifflin-St Jeor — the recommended default. Validation studies (Frankenfield et al., 2005) found it the most accurate predictor of measured BMR for non-obese and obese adults alike, typically within ±10% of indirect calorimetry.
• Harris-Benedict — the original 1919 equation, revised in 1984. It tends to overestimate BMR slightly for modern populations, since lifestyle and body composition have shifted over a century. Useful mainly for comparison with older references.
• Katch-McArdle — the best choice when body-fat percentage is known. Because it scales with lean mass rather than total weight, it does not misjudge lean or very muscular people the way the weight-based formulas do. Its weakness is the opposite: it is only as accurate as the body-fat estimate.

Where the estimate falls short

The equations are population-average models and can be off by 20% or more in specific cases:

• Atypical body composition. Bodybuilders, with more muscle per kg of weight, have measured BMRs higher than the weight-based formulas predict; sedentary older adults, with less muscle, tend to be lower. The Katch-McArdle equation is often more accurate when body-fat percentage is known.
• Thyroid disorders. Hypothyroidism lowers metabolic rate and hyperthyroidism raises it. Anyone in active diagnosis or treatment should rely on guidance from a physician rather than a formula.
• Genetic variation. Even after controlling for the other factors, individual BMRs can vary by about ±200 kcal. The predicted number is the center of a distribution, not a fixed value.
• Indirect calorimetry — a clinical breath-gas analysis that measures oxygen consumption directly — is accurate to within about ±5% and outperforms all three equations, but requires lab equipment.

Applications

BMR is the starting point for estimating total energy needs. Multiplying it by an activity factor (1.2 for sedentary up to 1.9 for extremely active) gives total daily energy expenditure (TDEE), the figure used to set calorie targets for weight loss or muscle gain. The TDEE Calculator performs this step.

BMR also sets a practical lower bound for calorie intake. Eating below it for extended periods can trigger metabolic adaptation, in which the body becomes more efficient at running on less and weight loss stalls. For this reason, healthy fat-loss diets generally keep intake above BMR and create the deficit by raising activity rather than by cutting food further.

In all cases the estimate is best treated as a starting hypothesis to calibrate against measured intake and weight trend over one or two weeks, then adjusted.