Guide to BMR
What is BMR?
Basal Metabolic Rate (BMR) is the minimum amount of energy your body needs to maintain basic vital functions at rest. This includes heart work, breathing, body temperature maintenance, cell functioning, and all other processes essential for survival. BMR typically accounts for 60-75% of total daily energy expenditure (TDEE).
Factors affecting BMR
- Sex - men typically have higher BMR due to greater muscle mass
- Weight - larger body mass requires more energy to maintain
- Height - taller people have larger body surface area and higher needs
- Age - after 20, BMR decreases by about 2% per decade
- Muscle tissue - muscle burns more calories than fat even at rest
- Genetics - individual differences in metabolic efficiency
Using BMR
BMR is the starting point for calculating total calorie needs. Multiplying BMR by activity factor (1.2-1.9) gives you TDEE. Knowing TDEE allows planning weight loss (300-500 calorie deficit) or muscle gain (200-300 calorie surplus). BMR is especially useful for people who want precise diet control.
Increasing BMR
While BMR is largely genetically determined, it can be increased through building muscle mass (strength training), maintaining adequate physical activity, consuming enough protein (1.6-2.2 g/kg body weight), proper sleep (7-9 hours), and avoiding extreme diets and starvation that slow metabolism.
The three formulas agree within 73 kcal. Your body-fat guess moves it by 345
People agonise over which BMR equation to use. For an 80 kg, 180 cm, 35-year-old man the three standard formulas return 1,755, 1,825 and 1,752 kcal — a spread of 73, or 4.2%. Change the body-fat estimate that one of them depends on from 10% to 30% and its answer moves 345 kcal. The input you guess matters nearly five times more than the formula you pick.
How it works
- Calculates basal metabolic rate with Mifflin-St Jeor, Harris-Benedict and Katch-McArdle side by side.
- Shows the disagreement between them, which is smaller than most people expect.
- Makes the sensitivity to lean-mass input visible, since that is where the real uncertainty lives.
Mifflin-St Jeor (men) BMR = 10W + 6.25H − 5A + 5 Harris-Benedict (men) BMR = 88.362 + 13.397W + 4.799H − 5.677A Katch-McArdle BMR = 370 + 21.6 × lean body mass W in kg, H in cm, A in years lean body mass = weight × (1 − body-fat fraction)
Worked example
The same man through all three formulas, then Katch-McArdle across plausible body-fat values.
- Mifflin-St Jeor → 1,755 kcal
- Harris-Benedict → 1,825 kcal
- Katch-McArdle at 20% body fat → 1,752 kcal
- at 10% body fat → 1,925 kcal
- at 30% body fat → 1,580 kcal
The formulas differ by 73 kcal. The body-fat assumption inside one of them shifts the answer by 345. Over a year at a fixed intake those two uncertainties predict 3.5 kg and 8.2 kg of change respectively — the guess dominates the method.
Reading the result
- Use Mifflin-St Jeor unless you have a measured body-fat percentage. It needs no estimate you cannot verify, and it is the equation most commonly recommended for general use. Katch-McArdle is more precise in principle and less reliable in practice, because it inherits every error in your body-fat figure.
- A ten-point error in body fat moves the Katch-McArdle result by 173 kcal — more than twice the disagreement between the formulas. Bathroom scales that report body fat by impedance routinely err by that much, so feeding one into the equation adds noise rather than precision.
- All three are population regressions with roughly ±10% individual variation at the same height, weight and age. Two people identical on paper can genuinely differ by 200 kcal, which no equation can resolve — treat any output as the centre of a range.
- BMR is not the same as what you burn in a day. It is the resting cost of staying alive, typically 60–75% of total expenditure once ordinary activity is included, so it is an input to a daily target rather than the target itself.
Common questions
- Which BMR formula should I use?
- Mifflin-St Jeor, unless a clinic has measured your body composition. The three agree within 4.2%, so the choice barely matters — and Katch-McArdle only outperforms the others when the body-fat figure you feed it is genuinely accurate.
- Why does my calculator give a different number from another one?
- Usually a different equation, and occasionally rounding. The gap between Mifflin-St Jeor and Harris-Benedict here is 70 kcal, which is real but small compared with the ±10% individual variation that neither equation can capture.