Basal Metabolic Rate (BMR) Calculator
Estimate your Basal Metabolic Rate using the Mifflin-St Jeor equation. BMR is the calories your body needs at complete rest.
Basal metabolic rate and how it is estimated
BMR is the energy your body uses at complete rest to keep organs running. It accounts for around 60 to 70% of daily energy expenditure in a sedentary adult, and every equation used to estimate it is a population regression that carries real individual error.
Formula
Mifflin-St Jeor - Men: (10 x kg) + (6.25 x cm) - (5 x age) + 5 | Women: (10 x kg) + (6.25 x cm) - (5 x age) - 161
This equation is accurate to within 10% for roughly 82% of people. For any given individual the error can be 200 kcal or more in either direction.
Why the same body needs different amounts of energy
Organs do most of the work. The brain, liver, heart, and kidneys together make up about 6% of body mass but consume roughly 60 to 70% of resting energy. Skeletal muscle is around 40% of mass but only about 20% of resting expenditure.
This explains why adding muscle raises BMR less than commonly claimed. A kilogram of muscle burns roughly 13 kcal a day at rest, so 5 kg of hard-won muscle adds about 65 kcal - real, but not the several hundred often quoted.
It also explains the age decline. BMR falls roughly 1 to 2% per decade after 20, driven mainly by loss of lean mass and reduced organ metabolic activity rather than by age itself.
Which equation to use
Several equations are in common use and they disagree, sometimes substantially.
- Mifflin-St Jeor (1990): the current default in clinical practice. Derived from a more contemporary population than its predecessors and validated as the most accurate of the weight-based equations.
- Harris-Benedict (1919, revised 1984): the historical standard. Tends to overestimate by around 5%, since it was derived from a leaner population.
- Katch-McArdle: uses lean body mass rather than total weight, so it needs a body fat measurement. More accurate for lean or very muscular people, and the only one that does not need a sex term.
- Cunningham: similar to Katch-McArdle and often preferred for athletes, producing slightly higher values.
BMR is not RMR, and neither is TDEE
True BMR requires a strict laboratory protocol: 12 hours fasted, fully rested, thermoneutral room, measured on waking after undisturbed sleep. Almost nothing labelled BMR outside a metabolic ward meets that standard.
Resting metabolic rate is measured under looser conditions and runs about 10% higher. Most equations, including Mifflin-St Jeor, are in practice predicting RMR.
Total daily energy expenditure adds physical activity, the thermic effect of food (roughly 10% of intake), and non-exercise activity thermogenesis, which varies enormously between individuals - fidgeting and spontaneous movement can differ by several hundred kilocalories a day.
Worked example
A 34-year-old woman, 68 kg, 165 cm. Mifflin-St Jeor: (10 x 68) + (6.25 x 165) - (5 x 34) - 161 = 680 + 1031.25 - 170 - 161 = 1,380 kcal/day.
Harris-Benedict on the same inputs gives 1,447 kcal, about 5% higher. If her body fat were measured at 28%, lean mass would be 48.96 kg and Katch-McArdle would give 370 + (21.6 x 48.96) = 1,428 kcal.
Three defensible methods spanning nearly 70 kcal. This spread is normal and is the reason any calculated figure should be treated as a starting point to be adjusted against real-world results over two to four weeks.
Comparison of BMR equations
| Equation | Inputs needed | Best suited to |
|---|---|---|
| Mifflin-St Jeor | weight, height, age, sex | General adult population |
| Harris-Benedict (revised) | weight, height, age, sex | Historical comparison |
| Katch-McArdle | lean body mass | Lean or muscular individuals |
| Cunningham | lean body mass | Trained athletes |
| Schofield | weight, age, sex | WHO and public health use |
Frequently Asked Questions
Related calculators
References
- Mifflin MD et al. - A new predictive equation for resting energy expenditure (1990)
- Academy of Nutrition and Dietetics - evidence analysis on RMR equations
Last reviewed: 2026-08-07. This page is informational. For legal, medical, tax, or financial decisions, confirm the result with a qualified professional.