Basal Metabolic Rate is the energy your body burns at complete rest simply staying alive — running your heart, brain, lungs, kidneys and cellular repair. It is the single largest component of daily energy use for most people, and the foundation every calorie target is built on.
BMR, RMR and TDEE
Three related numbers get used interchangeably but mean different things:
- BMR — energy burned at total rest, measured after fasting in a thermally neutral room on waking. The strict laboratory definition.
- RMR — resting metabolic rate, measured under less rigid conditions. Typically runs 5–10% above BMR and is what most equations really estimate.
- TDEE — total daily energy expenditure: BMR plus movement, exercise and the energy cost of digesting food. This is your true maintenance intake.
The three equations
Mifflin-St Jeor, published in 1990, is the current default. It was validated against modern populations and is generally the most accurate of the three for people of typical body composition.
Harris-Benedict is the original from 1919, revised in 1984. It tends to read slightly high because it was derived from a leaner, more active population than today's.
Katch-McArdle works from lean body mass rather than total weight, so it is the only one that accounts for body composition. If you know your body fat percentage reasonably accurately, it is the most reliable of the three — particularly for very muscular or very heavy people.
Mifflin-St Jeor men: BMR = 10·kg + 6.25·cm − 5·age + 5 women: BMR = 10·kg + 6.25·cm − 5·age − 161 Katch-McArdle BMR = 370 + 21.6 × lean body mass (kg)
Activity multipliers
Multiply BMR by the factor matching your week to estimate maintenance calories. Most people overestimate their own activity — when in doubt, drop one level.
| Level | Multiplier | Description |
|---|---|---|
| Sedentary | 1.20 | Desk job, little deliberate exercise |
| Light | 1.375 | Light exercise 1–3 days per week |
| Moderate | 1.55 | Moderate exercise 3–5 days per week |
| Active | 1.725 | Hard exercise 6–7 days per week |
| Very active | 1.90 | Physical job, or twice-daily training |
What changes your BMR
- Lean mass — muscle is metabolically active tissue, so more of it raises BMR. This is the main lever you actually control.
- Age — BMR declines roughly 1–2% per decade after 20, largely through muscle loss rather than age itself.
- Sex — men average higher BMR at the same height and weight because they carry more lean mass.
- Body size — larger bodies need more energy, which is why both height and weight appear in every equation.
- Thyroid function — hypothyroidism and hyperthyroidism move BMR substantially in either direction.
- Prolonged severe dieting — sustained large deficits reduce BMR beyond what the weight loss alone predicts.
Worked example
Using the values pre-loaded in the calculator above:
| Input | Value |
|---|---|
| Units | Metric (cm / kg) |
| Age (yrs) | 30 |
| Sex | Male |
| Height (cm or in) | 175 |
| Weight (kg or lb) | 75 |
| Output | Value |
|---|---|
| BMR (Mifflin-St Jeor) | 1,699 kcal/day |
| Harris-Benedict (revised) | 1,763 kcal/day |
| Katch-McArdle | enter body fat % |
| Calories per hour at rest | 70.8 |
Frequently asked questions
Which BMR formula should I use?
Mifflin-St Jeor for most people. If you have a reliable body fat measurement, Katch-McArdle is better, especially at either extreme of muscularity.
How accurate are BMR equations?
They land within about 10% of measured values for most people. Individual metabolism varies, so treat the result as a starting point and adjust based on how your weight actually responds over two to three weeks.
Should I eat below my BMR to lose weight?
Generally no. Create the deficit against TDEE, not BMR. Sustained intakes below BMR make it hard to meet nutrient needs and tend to cost you muscle alongside fat.
Can I increase my BMR?
Yes, mainly by building and keeping muscle through resistance training and adequate protein. The effect is real but modest — roughly 10–15 kcal per day per kilogram of muscle gained.