BMR Calculator: How Many Calories Your Body Burns at Rest

Your basal metabolic rate, or BMR, is the energy your body needs just to stay alive: heartbeat, breathing, brain activity, body temperature, cell repair. You burn it lying on the sofa, and it is the foundation every calorie target is built on. This calculator estimates your BMR with four validated equations at once, because no single formula knows your metabolism, and seeing where they agree tells you more than any one number.

Key takeaways

1. Of the commonly used equations, Mifflin-St Jeor is the most reliable: it lands within 10% of measured values in more people than any other

2. Fat-free mass was the best single predictor of resting energy expenditure, better than weight, height or age alone

3. Adjusted for fat-free mass, metabolism stays stable from 20 to 60 and only then starts to decline

Below you will find what BMR actually measures, how the four equations differ, how accurate they are, what drives your number, and what really changes with age.

What Is BMR?

BMR is the energy you spend at complete rest: measured in the morning, fasted, lying down, in a comfortable room temperature. Researchers also use the term resting energy expenditure (REE) or resting metabolic rate (RMR) for measurements taken under slightly less strict conditions. For everyday use the difference is small, and the equations in this calculator were built from both kinds of measurements.

BMR is not the same as your daily calorie need. On top of it come the energy you use to digest food and everything you do while awake, from walking to the office to a hard training session. Add those and you get your total daily energy expenditure, or TDEE. This page is about the resting part only. For your total, use the TDEE calculator.

The BMR Equations Compared

Every BMR calculator uses a prediction equation: a formula fitted to people whose metabolism was measured in a lab. This calculator shows four of them side by side:

Run them for a 40-year-old man who is 180 cm tall and weighs 80 kg, and you get 1,730 kcal (Mifflin-St Jeor), 1,797 kcal (Harris-Benedict), 1,729 kcal (Oxford) and 1,768 kcal (Müller). If he also knows his body fat is 20%, the fat-free mass equation gives 1,674 kcal. That is a spread of about 120 kcal for one body. None of the equations is wrong. Each describes the average person of the population it was built from.

How Accurate Is a BMR Calculator?

Accurate for groups, approximate for individuals. A systematic review by Frankenfield and colleagues compared the equations most used in clinical practice. Mifflin-St Jeor predicted resting metabolic rate within 10% of the measured value in more nonobese and obese people than any other equation, and it had the narrowest error range. The same review warns that noteworthy individual errors remain, and that older adults and ethnic minorities were underrepresented in the studies.

The older formulas run higher. In the Mifflin data, the original Harris-Benedict equations overestimated measured values by 5%, and Roza and Shizgal put the precision of Harris-Benedict at about 14% for a healthy person. The Müller team found that the older WHO equations systematically overestimated low and underestimated high values in a modern German population.

That is why this calculator leads with Mifflin-St Jeor and shows the others as a range. If the equations cluster tightly for you, you can trust the estimate more. If they spread widely, your body is further from the populations they describe. The only way to know your true BMR is indirect calorimetry, a breath-gas measurement offered by some clinics and sports labs.

What Drives Your BMR: Fat-Free Mass

Weight is a crude input, because a kilogram of muscle and a kilogram of fat are not the same to your metabolism. The Mifflin team found that fat-free mass was the best single predictor of resting energy expenditure. Their fat-free mass equation is simple: 19.7 × fat-free mass (kg) + 413. Enter your body fat percentage above and the calculator adds this estimate. If you do not know it, the body fat calculator gives you a quick estimate.

The fat-free mass view also explains two things people often misread. Men have a higher BMR in every weight-based equation, but in a database of 6,421 measurements, sex had no effect on adults' total energy expenditure once fat-free mass and fat mass were accounted for. The difference is body composition, not a different engine. And two people with the same weight can have quite different BMRs if one carries more muscle. The standard equations cannot see that, the fat-free mass equation can.

Does Metabolism Slow With Age?

Less than most people think, and later. In a large pooled analysis, Pontzer and colleagues pooled doubly labelled water measurements from people aged 8 days to 95 years. Adjusted for fat-free mass, energy expenditure was stable from about 20 to 60 years. Only from around 60 did it decline, by about 0.7% per year, with basal expenditure falling at a similar rate.

The equations tell a different story. Mifflin-St Jeor subtracts 5 kcal for every year of age, so the same 180 cm, 80 kg man drops from 1,805 kcal at 25 to 1,580 kcal at 70. A fixed age term cannot tell a slower engine from a smaller amount of lean tissue, and the Pontzer data point to the second: per kilogram of fat-free mass, midlife metabolism holds steady. The practical reading: the lever you control is your lean mass.

This is where strength training earns its place. Keeping and building muscle protects the lean tissue that drives your resting metabolism. The article on building muscle after 50 shows how to train for it.

How to Use Your Result

Your BMR is a reference point, not a calorie target. Here is how to put it to work:

  • To find your daily calories: BMR covers rest only. The TDEE calculator adds your activity level and gives you a maintenance number.
  • To lose fat: plan your deficit from your total expenditure, not from your BMR. The calorie deficit calculator turns it into a daily target, and the protein calculator helps you keep muscle while you lose.
  • To track change over time: recalculate after meaningful changes in weight or body fat. If you enter body fat, compare the fat-free mass estimate over months: it moves with your lean mass.

If your measured weight trend does not match what the numbers predict, trust the trend. The huuman app keeps your weight, meals and training in one place, so your huuman Coach can adjust your plan to how your body actually responds. For the rest of your picture, explore all huuman tools.

Frequently Asked Questions

What is a normal BMR?

It depends on size, sex and age, so there is no single normal value. As a guide, the Mifflin-St Jeor equation gives about 1,730 kcal per day for a 40-year-old man who is 180 cm tall and weighs 80 kg, and about 1,320 kcal for a 40-year-old woman who is 165 cm tall and weighs 65 kg. Enter your own data above for your estimate.

Which BMR formula is the most accurate?

For healthy adults, Mifflin-St Jeor. A systematic review found it within 10% of measured values in more people than Harris-Benedict and the other common equations. If you know your body fat percentage, the fat-free mass equation adds a useful second view, because lean mass is the strongest single predictor of BMR.

What is the difference between BMR and TDEE?

BMR is what you burn at complete rest. TDEE, your total daily energy expenditure, adds digestion and all your daily activity on top. For most people TDEE is clearly higher than BMR. Use BMR to understand your baseline and the TDEE calculator to set your daily calories.

Does more muscle increase your BMR?

Yes. Fat-free mass is the best single predictor of resting energy expenditure. In the Mifflin fat-free mass equation, every extra kilogram of lean mass adds about 20 kcal per day. That is modest per kilogram, but lean mass also shapes how your metabolism ages, which makes strength training worth it for more than the calories.

Why do men have a higher BMR than women?

Mainly because of body composition. At the same weight, men usually carry more fat-free mass. In a large pooled analysis of adults, sex had no effect on total energy expenditure once fat-free mass and fat mass were taken into account. The equations use sex as a stand-in for that difference in lean mass.

Further reading

References

  1. Mifflin MD et al. - A new predictive equation for resting energy expenditure in healthy individuals (1990)
  2. Frankenfield D et al. - Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review (2005)
  3. Roza AM & Shizgal HM - The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass (1984)
  4. Harris JA & Benedict FG - A biometric study of human basal metabolism (1918)
  5. Henry CJ - Basal metabolic rate studies in humans: measurement and development of new equations (2005)
  6. Müller MJ et al. - World Health Organization equations have shortcomings for predicting resting energy expenditure in persons from a modern, affluent population (2004)
  7. Pontzer H et al. - Daily energy expenditure through the human life course (2021)
  8. Kruizenga HM et al. - Predicting resting energy expenditure in underweight, normal weight, overweight, and obese adult hospital patients (2016)

About this article · Written by the huuman Team. Our content is based on peer-reviewed research and clinical guidelines. We follow editorial standards grounded in scientific evidence.

This article is for educational purposes only and does not constitute medical advice. Health and training decisions should be discussed with qualified professionals.