What BMR measures
Basal Metabolic Rate is the energy your body uses over 24 hours doing nothing but staying alive — breathing, circulating blood, maintaining temperature, repairing cells and running your brain. It is measured under strict conditions: awake but at complete rest, in a neutrally temperate room, having fasted for 12 hours.
For most adults BMR accounts for 60% to 70% of total daily energy expenditure. Your brain alone uses about 20% of it, roughly 300 to 400 calories a day, regardless of what you are thinking about.
BMR is not the same as RMR
Resting Metabolic Rate is measured under less strict conditions and typically comes out 5% to 10% higher. The two terms are often used interchangeably, and every predictive equation — including the ones here — actually estimates something closer to RMR.
The three equations
Mifflin-St Jeor (1990) — the default
Women: (10 × kg) + (6.25 × cm) − (5 × age) − 161
The most accurate predictive equation for the general population, and the one the Academy of Nutrition and Dietetics recommends. Accurate within 10% for about 82% of healthy adults.
Harris-Benedict (revised 1984)
Women: 447.593 + (9.247 × kg) + (3.098 × cm) − (4.330 × age)
The original from 1919, revised in 1984. Still widely cited but tends to overestimate by around 5% for modern populations, since body composition has shifted since the original study group.
Katch-McArdle
Uses lean mass rather than total weight, so it makes no assumption about sex — muscle burns the same energy in anyone. This is the most accurate option if you know your body fat percentage. Enter it in the advanced options above.
What actually changes your BMR
| Factor | Effect |
|---|---|
| Lean muscle mass | The largest modifiable factor. Each kg of muscle burns 10–15 kcal a day at rest. |
| Age | BMR falls roughly 1–2% per decade after 20, mostly through muscle loss rather than age itself. |
| Body size | Larger bodies need more energy. This is why BMR scales with both height and weight. |
| Sex | Men average 5–10% higher at the same size, driven by greater lean mass. |
| Thyroid function | Hypothyroidism can lower BMR by 15–30%; hyperthyroidism raises it. |
| Prolonged dieting | Adaptive thermogenesis can suppress BMR by 10–15% below predicted. |
| Genetics | Accounts for a 5–10% spread between individuals of identical measurements. |
Frequently asked questions
What is a normal BMR?
For adult men, typically 1,600 to 1,800 kcal a day; for adult women, 1,300 to 1,500. But these are averages across a wide range — a tall, muscular man may be above 2,000 and a small, older woman below 1,100.
Your own figure above is more useful than any range.
Can I increase my BMR?
Yes, mainly by adding muscle. Resistance training two to three times a week is the most reliable route, though the effect is modest — each kilogram of muscle adds only 10 to 15 kcal a day at rest.
Eating enough matters too. Prolonged severe restriction suppresses BMR, so recovering from a long diet by eating at maintenance for a period can raise it back.
Which equation should I use?
Mifflin-St Jeor unless you know your body fat percentage, in which case Katch-McArdle is more accurate because it works from lean mass directly.
Harris-Benedict is included for comparison but tends to overestimate slightly.
How is BMR different from TDEE?
BMR is what you burn at complete rest. TDEE is BMR plus everything else — exercise, walking, fidgeting and digesting food. TDEE is what you actually need to eat to maintain weight.
See the TDEE calculator for the full figure.