Find your Basal Metabolic Rate — calories burned at complete rest.
Basal metabolic rate is the energy your body uses at complete rest to keep you alive — respiration, circulation, cell repair, brain function and temperature regulation. It is the floor of your daily energy requirement, and for most people it accounts for 60-75% of everything they burn in a day. Exercise, for all the attention it gets, is usually a much smaller slice than people assume.
This calculator uses the Mifflin-St Jeor equation, published in 1990 and now the most widely recommended predictive formula because it validates better against measured metabolic rate in modern populations than the older Harris-Benedict equation.
The constant differs by sex because, at the same height, weight and age, men typically carry more lean mass — and lean tissue is metabolically active in a way fat tissue is not.
A 34-year-old woman, 165 cm, 68 kg:
BMR alone is not what you should eat. Total daily energy expenditure is BMR multiplied by an activity factor:
| Activity level | Multiplier | Description |
|---|---|---|
| Sedentary | 1.2 | Desk job, little or no exercise |
| Lightly active | 1.375 | Light exercise 1-3 days a week |
| Moderately active | 1.55 | Moderate exercise 3-5 days a week |
| Very active | 1.725 | Hard exercise 6-7 days a week |
| Extremely active | 1.9 | Physical job or twice-daily training |
The woman above, moderately active, needs roughly 1,380 x 1.55 = 2,139 kcal to maintain her weight. Our calorie calculator applies these multipliers directly.
Most people overestimate their activity level. Unless you have a physically demanding job or train seriously most days, "lightly active" is closer to the truth than "moderately active" — and picking the wrong tier introduces a larger error than any difference between BMR formulas.
You will see RMR (resting metabolic rate) used almost interchangeably. Strictly, BMR is measured under laboratory conditions after an overnight fast, at complete rest, in a thermally neutral room. RMR is measured under less rigid conditions and comes out around 10% higher. Every online calculator, including this one, is estimating something between the two.
Treat the number as a starting point with a realistic margin of plus or minus 10%, not a measurement. The reliable way to find your actual maintenance intake is empirical: eat a consistent amount for two to three weeks, track weight every morning, and adjust based on the trend. Direct measurement by indirect calorimetry is the only precise alternative, and it requires a lab.
Eating below your BMR for a sustained period is not a shortcut. It risks nutrient deficiency, muscle loss and metabolic adaptation, and it becomes progressively harder to sustain. It is also not appropriate for children, during pregnancy or breastfeeding, or for anyone with a medical condition affecting metabolism. This tool is an estimate, not medical advice — discuss any substantial dietary change with a doctor or a registered dietitian, particularly if you have a history of disordered eating.
BMR is what you burn at complete rest. TDEE is your total daily expenditure including movement, exercise and digestion, and it is BMR multiplied by an activity factor between 1.2 and 1.9. TDEE is the figure to compare your food intake against.
Mifflin-St Jeor, which this calculator uses, validates better than Harris-Benedict for most people. If you know your body fat percentage, the Katch-McArdle formula is more accurate still because it works from lean mass rather than total weight.
No. Eating at BMR ignores everything you burn through daily movement, and sustained intake at that level risks muscle loss and nutritional deficiency. A moderate deficit from your TDEE — typically 15-20% — is more sustainable and preserves lean mass better.
The formula only knows height, weight, age and sex. It cannot see body composition, and lean tissue burns considerably more at rest than fat does. Two people matched on the inputs can genuinely differ by several hundred calories.
Yes, by roughly 1-2% per decade after 20, but most of that is the gradual loss of lean mass rather than an inherent slowdown. Resistance training that preserves muscle substantially blunts the decline.