BMR Calculator
Your BMR is the energy your body spends just staying alive: breathing, circulating blood, maintaining temperature and repairing cells. It is the floor everything else is built on.
What BMR actually measures
Basal metabolic rate is the energy your body uses in a strictly defined state: awake but completely at rest, lying down, in a thermally neutral room, at least twelve hours after your last meal. In that state the calories go to keeping cells alive — the brain alone accounts for around 20 % of it, the liver a similar share, and the heart and kidneys about 10 % between them.
BMR is the largest single component of how much you burn in a day, typically 60–75 % of the total for a sedentary adult. That is why it is the starting point for any calorie target: activity is added on top of it, not the other way round.
You will also see the term RMR (resting metabolic rate). It is measured under less strict conditions and usually lands about 10 % higher. In practice the two are used interchangeably.
The three equations
Mifflin-St Jeor (1990) is the default here and the one most dietitians now use. It was validated on a more contemporary population than its predecessor and is accurate to within about 10 % for most non-obese adults.
Harris-Benedict, originally from 1919 and revised by Roza and Shizgal in 1984, is still widely quoted. It tends to read slightly higher than Mifflin-St Jeor.
Katch-McArdle takes a different route: it works from lean body mass and so needs a body fat measurement, but for lean or very muscular people it is usually the most accurate of the three. If you know your body fat percentage, our lean body mass calculator gives you the input it needs.
A worked example
A 35-year-old woman, 165 cm tall, weighing 62 kg, using Mifflin-St Jeor:
- 10 × 62 = 620
- 6.25 × 165 = 1,031.25
- 5 × 35 = 175
- 620 + 1,031.25 − 175 − 161 = 1,315 kcal/day
That is what she would burn lying still for 24 hours. With a desk job and light walking — an activity factor of about 1.4 — her actual daily expenditure is closer to 1,840 kcal. The TDEE calculator handles that second step.
How much to trust the number
These are regression equations fitted to measured populations. For any individual they carry an error of roughly ±10 %, and occasionally more. Things they cannot account for include:
- Body composition. Muscle is metabolically active tissue; fat is much less so. Two people of identical height, weight, age and sex can differ by 200 kcal a day.
- Thyroid function. Hypothyroidism and hyperthyroidism shift BMR substantially.
- Adaptive thermogenesis. Extended calorie restriction lowers BMR by more than the weight loss alone predicts, which is a large part of why plateaus happen.
- Genetics. Measured BMR varies between individuals of the same profile by up to about 25 %.
Use the number as a starting hypothesis. Track your intake and weight for two to three weeks, and let the real-world trend correct the estimate.
Frequently asked questions
Which equation should I use?
Mifflin-St Jeor unless you have a reliable body fat measurement, in which case Katch-McArdle is likely to be closer. Harris-Benedict is included mainly because it is still so widely cited; it is not the better choice for most people.
Can I raise my BMR?
Modestly and slowly. Adding lean muscle raises it, although the effect is often overstated: a kilogram of muscle burns roughly 13 kcal a day at rest, not the 50 sometimes claimed. Avoiding prolonged aggressive calorie restriction matters more, since that actively suppresses metabolic rate.
Why does BMR fall as I age?
Chiefly because lean mass declines. From roughly the fourth decade most people lose muscle steadily unless they resistance-train, and less metabolically active tissue means fewer calories burned at rest. Resistance training and adequate protein slow this considerably.
Is BMR the same as the calories I should eat?
No, and eating at your BMR is usually a mistake. BMR excludes all movement, all digestion and all exercise. Multiply it by an activity factor to get your total daily expenditure first, then set a deficit or surplus from that figure.