Health & Fitness
BMR Calculator
Work out the calories your body burns doing absolutely nothing — and see how three accepted formulas compare.
BMR Calculator
About the BMR Calculator
Your basal metabolic rate is the energy your body spends doing nothing at all — keeping your heart beating, your lungs working, your brain running and your body at temperature. It is the single largest part of what you burn each day, typically 60–70% of the total.
This calculator shows three accepted equations side by side: Mifflin-St Jeor, revised Harris-Benedict, and Katch-McArdle. They disagree by 100 calories or more on the same person, and seeing that spread is more useful than trusting one number to the calorie.
Add an activity level and you also get your total daily energy expenditure. Katch-McArdle needs a body fat percentage — the body fat calculator will estimate one from a tape measure.
How to use the BMR Calculator
- Enter your sex, age, height and weight.
- Choose an activity level to get your total daily expenditure as well as BMR.
- Pick a formula. Mifflin-St Jeor is the usual default.
- For Katch-McArdle, add your body fat percentage — it works from lean mass instead.
- Select Calculate BMR and compare all three results side by side.
The three equations
Mifflin-St Jeor men : 10w + 6.25h − 5a + 5
Mifflin-St Jeor women: 10w + 6.25h − 5a − 161
Harris-Benedict men : 88.362 + 13.397w + 4.799h − 5.677a
Harris-Benedict women: 447.593 + 9.247w + 3.098h − 4.330a
Katch-McArdle : 370 + 21.6 × lean mass (kg)
w = weight in kg, h = height in cm, a = age in years.
Worked example
A 35-year-old man, 80 kg, 180 cm.
Mifflin-St Jeor: (10×80) + (6.25×180) − (5×35) + 5 = 800 + 1125 − 175 + 5 = 1,755
Harris-Benedict: 88.362 + (13.397×80) + (4.799×180) − (5.677×35) = 1,825
A 70-calorie gap between two accepted equations
At a sedentary multiplier of 1.2, that becomes 2,106 versus 2,190 calories a day — an 84-calorie difference before you have measured anything.
Which equation to use
| Equation | Published | Needs | Best for |
|---|---|---|---|
| Mifflin-St Jeor | 1990 | Weight, height, age, sex | Most people — the default |
| Harris-Benedict (revised) | 1984 | Weight, height, age, sex | Comparison; the older standard |
| Katch-McArdle | 1996 | Lean body mass | Lean or very muscular people |
Mifflin-St Jeor is the default here because it has generally performed best against measured metabolic rates in the general population, and it is the equation the Academy of Nutrition and Dietetics has recommended for non-obese and obese adults alike.
Harris-Benedict dates from 1919 and was revised in 1984. It tends to run slightly high, partly because the original population differed from today's.
Katch-McArdle works from lean mass rather than total weight, so it does not care how much fat you carry. That makes it the better choice at the extremes — a very muscular person or a very lean one — where weight-based equations misjudge things. It is only as good as your body fat estimate.
From BMR to daily calories
BMR is what you burn at complete rest. Multiply it by an activity factor to get total daily energy expenditure:
| Activity level | Multiplier | Who it describes |
|---|---|---|
| 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 |
| Extra active | 1.9 | Hard daily exercise or a physical job |
Most people overestimate here. "Moderately active" means genuine structured exercise three to five times a week, not a busy day. If you sit for work and go to the gym twice a week, you are closer to 1.375 than 1.55. Picking a level too high is the most common reason a calorie target does not work.
What BMR is made of
Roughly, your resting energy goes to:
- Liver — about 20%, the single biggest consumer.
- Brain — about 20%, despite being 2% of body weight.
- Muscle — about 20% at rest, and this is the part you can change.
- Kidneys and heart — around 10% between them.
- Everything else — the remainder.
This is why building muscle raises BMR, but modestly. A kilogram of muscle burns roughly 13 calories a day at rest — real, worth having, and nowhere near the "50 calories per pound" figure that circulates online.
BMR, RMR and metabolic age calculator results
RMR (resting metabolic rate) is measured under less strict conditions than true BMR and comes out around 10% higher. In everyday use the two terms are used interchangeably, and these equations sit somewhere between the two definitions.
Metabolic age is a marketing figure, not a clinical one. Scales that report it compare your BMR against an average for different ages — there is no standard definition and no agreed method behind it. This calculator does not produce one.
Can you eat below your BMR?
Briefly and under supervision, sometimes. As a self-directed plan, it is a poor idea. Sustained intake below BMR tends to produce muscle loss alongside fat loss, nutrient shortfalls, fatigue and poor adherence — and the muscle loss lowers your BMR further, making the next stage harder.
The usual guidance is not to drop below roughly 1,200 calories a day for women or 1,500 for men without medical supervision. Our calorie calculator flags targets that fall below those floors rather than quietly presenting them.
This is an estimate, not a prescription. Individual energy needs vary by roughly ±10% even between people with identical height, weight, age and sex. Nothing here is medical or dietary advice. If you are pregnant, breastfeeding, managing a health condition, taking medication that affects appetite or metabolism, or have a history of disordered eating, talk to a doctor or registered dietitian before changing what you eat.
Average BMR for women and men by age
BMR falls slowly with age — about 50 calories a day per decade on the Mifflin-St Jeor equation. It rises with weight and height. These tables show typical values.
Women, 5'5" (165 cm)
| Age | 60 kg (132 lb) | 75 kg (165 lb) | 90 kg (198 lb) |
|---|---|---|---|
| 25 | 1,345 | 1,495 | 1,645 |
| 45 | 1,245 | 1,395 | 1,545 |
| 65 | 1,145 | 1,295 | 1,445 |
Men, 5'10" (178 cm)
| Age | 75 kg (165 lb) | 90 kg (198 lb) | 105 kg (231 lb) |
|---|---|---|---|
| 25 | 1,742 | 1,892 | 2,042 |
| 45 | 1,642 | 1,792 | 1,942 |
| 65 | 1,542 | 1,692 | 1,842 |
So an average adult woman burns roughly 1,300–1,500 calories a day at rest, and an average man roughly 1,600–1,900. Your own number can differ by 10% or more.
BMI and BMR: not the same thing
The names sound alike, but they answer different questions.
- BMI (body mass index) compares weight with height. It is a screening number. Try the BMI calculator.
- BMR (basal metabolic rate) is the calories you burn at complete rest. It is an energy number.
To turn BMR into a daily calorie target, multiply by your activity factor — the calorie calculator does this for you.
Related guides
Other ways to use this calculator
- Basal metabolic rate calculator — BMR and basal metabolic rate are the same thing.
- BMR chart and BMR and age — the table above shows how BMR falls with age, including the average male BMR.
Frequently asked questions
Basal metabolic rate is the energy your body uses at complete rest to stay alive — heartbeat, breathing, brain function, body temperature and cell repair. It is typically 60–70% of everything you burn in a day. A 35-year-old man of 80 kg and 180 cm has a BMR of about 1,755 calories by the Mifflin-St Jeor equation, before any activity is added.
Mifflin-St Jeor for most people — it has generally matched measured metabolic rates best in the general population and is the equation dietetic bodies recommend. Harris-Benedict runs slightly high and is included for comparison. Katch-McArdle is better if you are very lean or very muscular, because it works from lean mass rather than total weight, but it needs a body fat estimate.
When your body composition is unusual for your weight — very muscular or very lean. Weight-based equations assume an average amount of body fat, so they misjudge people at either extreme. Katch-McArdle uses lean mass instead, sidestepping the problem. It is only as accurate as your body fat figure, so estimate that carefully first with the body fat calculator.
Not as a self-directed plan. Sustained intake below BMR tends to cost you muscle as well as fat, brings nutrient shortfalls and fatigue, and is hard to stick to — and losing muscle lowers your BMR further. Standard guidance is to stay above roughly 1,200 calories a day for women and 1,500 for men unless a doctor is supervising you.
Roughly 1,300 to 1,500 calories a day. A 45-year-old woman who is 165 cm tall and weighs 75 kg has a BMR of about 1,395 calories on the Mifflin-St Jeor equation. Age, height and weight all change it.
Sources and further reading
- Mifflin MD et al. (1990) — A new predictive equation for resting energy expenditure — the default equation used by this calculator
- Frankenfield D et al. — Comparison of predictive equations for resting metabolic rate — why Mifflin-St Jeor is preferred over Harris-Benedict
- NIH — Body Weight Planner — an evidence-based model of energy expenditure
- NHS — Calorie counting and energy needs — practical guidance and safe intake floors