What BMR is
Basal metabolic rate is the energy your body uses to stay alive while doing nothing: breathing, pumping blood, keeping your temperature steady, running your brain and repairing tissue. For most people it is the largest part of daily calorie burn, typically 60 to 70% of the total.
BMR is not your calorie target. It is the floor that everything else sits on top of. To get your real daily burn you add the energy of movement, exercise and digestion, which is what the TDEE calculator does.
How to use the calculator
Enter your sex, age, height and weight. Those four inputs are enough for the Mifflin-St Jeor and Harris-Benedict equations, and the calculator shows both.
The body fat field is optional. If you fill it in, the tool also runs Katch-McArdle, which works from lean mass instead of total weight. If you do not know your body fat, the body fat calculator can estimate it from a few tape measurements.
All three results appear together so you can see how much they agree. For most people they land within about 100 kcal of each other.
The three formulas
Mifflin-St Jeor (1990) is the default. In studies comparing predictive equations against measured metabolic rate, it has been the most accurate for the general adult population, including people with overweight.
Men: BMR = 10 x weight (kg) + 6.25 x height (cm) - 5 x age + 5
Women: BMR = 10 x weight (kg) + 6.25 x height (cm) - 5 x age - 161
Revised Harris-Benedict (Roza and Shizgal, 1984) updates the original 1919 equation. It tends to give slightly higher numbers than Mifflin-St Jeor.
Men: BMR = 88.362 + 13.397 x kg + 4.799 x cm - 5.677 x age
Women: BMR = 447.593 + 9.247 x kg + 3.098 x cm - 4.330 x age
Katch-McArdle ignores sex and age and uses only lean body mass, which is why it needs your body fat percentage.
BMR = 370 + 21.6 x lean body mass (kg)
Lean body mass = weight x (1 - body fat % / 100)
Which result should you trust?
| Situation | Best formula | Why |
|---|---|---|
| Most adults, body fat unknown | Mifflin-St Jeor | Best validated for the general population |
| Lean, muscular lifter with a reliable body fat figure | Katch-McArdle | Accounts for extra muscle that weight-only formulas miss |
| Higher body fat | Mifflin-St Jeor | Katch-McArdle can underestimate if body fat is overstated |
| Comparing with older apps or charts | Harris-Benedict | Many older tools still use it |
Katch-McArdle is only as good as the body fat number you feed it. A tape or smart-scale estimate can be off by 3 to 5 percentage points, which moves the result by roughly 50 to 100 kcal. If you are unsure, stick with Mifflin-St Jeor.
Why your BMR changes
Muscle burns more energy at rest than fat, so two people of the same weight can have different BMRs. Building muscle with something like the upper/lower split nudges BMR up, though by less than fitness marketing suggests: roughly 13 kcal per kg of muscle per day.
BMR also falls slowly with age, mostly because people lose muscle, and drops when you lose weight simply because there is less of you to run. That is why a calorie target that worked at the start of a diet needs revisiting every 4 to 5 kg (about 10 lb) lost.
Limitations
Every equation here predicts an average. Individuals can sit 10% or more above or below it because of genetics, thyroid function, medication and other factors. Lab tests such as indirect calorimetry measure BMR directly, but few people need that level of precision.
The equations were developed in adults and are not meant for children, pregnancy or people with conditions that affect metabolism. If you are managing a medical condition, check targets with your doctor or a registered dietitian.
Never eat below your BMR for long stretches without medical supervision. It makes the diet harder to sustain and costs you muscle.
Worked example
Take a 30-year-old woman who is 165 cm (5 ft 5 in) tall, weighs 62 kg (137 lb) and estimates her body fat at 26%.
- Mifflin-St Jeor: 10 x 62 + 6.25 x 165 - 5 x 30 - 161 = 1,340 kcal
- Revised Harris-Benedict: 447.593 + 9.247 x 62 + 3.098 x 165 - 4.330 x 30 = 1,402 kcal
- Katch-McArdle: lean mass 62 x 0.74 = 45.9 kg, so 370 + 21.6 x 45.9 = 1,361 kcal
The three answers sit within about 60 kcal of each other, which is typical. That gap is smaller than the error in most people's food tracking, so there is no point agonising over which one is right. Pick Mifflin-St Jeor unless you have a solid reason not to, and let your weigh-ins tell you whether the final calorie target needs nudging.
Notice how small the age term is: five extra years lowers the Mifflin-St Jeor result by only 25 kcal. Changes in muscle and body weight move BMR far more than birthdays do.
Next steps
Take your BMR to the TDEE calculator to add your activity level, then use the macro calculator to split those calories into protein, carbs and fat.
If your goal is fat loss, our guide on calculating calories for weight loss walks through setting a deficit you can actually stick to.
Frequently asked questions
What is the difference between BMR and TDEE?
BMR is what you burn at complete rest. TDEE adds everything else: walking, exercise, work and digesting food. TDEE is the number to base your eating on.
Which BMR formula is most accurate?
For most adults, Mifflin-St Jeor has the best track record. Katch-McArdle can be better for lean, muscular people, but only if your body fat estimate is reliable.
Why do the three results differ?
Each equation was built from a different group of people using different methods. Differences of 50 to 150 kcal are normal and smaller than the day to day error in tracking food.
Does lifting weights increase BMR?
Yes, a little. Each kilogram of muscle adds roughly 13 kcal a day at rest. The bigger benefit of lifting during a diet is keeping the muscle you already have.
Should I eat my BMR to lose weight?
No. Eating at BMR ignores all the energy you use moving around, so it is usually a steep and hard to sustain deficit. Base your target on TDEE minus a moderate deficit instead.
References
- Mifflin MD, St Jeor ST, Hill LA et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247
- Roza AM, Shizgal HM. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. Am J Clin Nutr. 1984;40(1):168-182
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc. 2005;105(5):775-789
- McArdle WD, Katch FI, Katch VL. Exercise Physiology: Nutrition, Energy, and Human Performance. Lippincott Williams & Wilkins
Results are estimates for healthy adults and are not medical advice. See our medical disclaimer.