Basal metabolic rate (BMR) is the energy the body uses at complete rest just to keep vital functions going, and it is most often estimated with the Mifflin-St Jeor equation. For women, BMR = (10 × weight) + (6.25 × height) − (5 × age) − 161; for men, BMR = (10 × weight) + (6.25 × height) − (5 × age) + 5. To reach your daily calorie need, this value is multiplied by a factor that reflects your activity level.
The formulas offer a practical starting point, but the results are estimates. Two people of the same age, height and weight can have different metabolisms depending on muscle mass, hormones, health and daily activity. In this article I show the calculation step by step and explain how you should interpret the result.
- BMR is the largest component of daily energy expenditure; we use energy even at rest.
- Among the commonly used formulas, the Mifflin-St Jeor equation is an estimation method that gives more consistent results in healthy adults.
- Daily calorie need = BMR × activity factor (approximately).
- Formulas can be off by 10 per cent or more for an individual; results should be used as a guiding estimate, not as a prescription.
- In pregnancy, breastfeeding, childhood and some illnesses, these formulas should not be used on their own.
What Is Basal Metabolic Rate?
Basal metabolic rate is the energy the body uses, doing no physical activity, fasted and at complete rest, for basic functions such as breathing, blood circulation, maintaining body temperature and cell renewal. Even if you did not move at all during the day, your body would need this much energy.
Total daily energy expenditure has three parts: basal (resting) metabolism, the energy used to digest food, and physical activity. For most people the largest share belongs to basal metabolism. In clinical practice it is mostly the resting metabolic rate (RMR), whose measurement conditions are more flexible, that is estimated; in everyday use the two terms are often used interchangeably.
You can also look at my article on metabolic rate, in which I discuss the relationship between metabolism and weight more broadly.
How Is Basal Metabolic Rate Calculated?
One of the most common ways to estimate basal metabolism is the Mifflin-St Jeor equation. This equation was developed in 1990 from data on healthy normal-weight and obese adults aged 19–78 whose resting energy expenditure had been measured.
- Women: BMR = (10 × weight [kg]) + (6.25 × height [cm]) − (5 × age [years]) − 161
- Men: BMR = (10 × weight [kg]) + (6.25 × height [cm]) − (5 × age [years]) + 5
Example calculation: woman
For a woman aged 35, 165 cm tall and weighing 70 kg: (10 × 70) + (6.25 × 165) − (5 × 35) − 161 = 700 + 1,031 − 175 − 161 ≈ 1,395 kcal/day.
Example calculation: man
For a man aged 40, 180 cm tall and weighing 85 kg: (10 × 85) + (6.25 × 180) − (5 × 40) + 5 = 850 + 1,125 − 200 + 5 = 1,780 kcal/day.
A 2005 systematic review by the American Dietetic Association reported that, among the four equations most often used in clinics (Harris-Benedict, Mifflin-St Jeor, Owen and WHO/FAO/UNU), the Mifflin-St Jeor equation gave the most consistent result in estimating the measured value within a 10 per cent deviation, in both normal-weight and obese people. The same review stresses that the equation can still make marked errors when applied to individuals, and that older people and some ethnic groups were not well represented in the studies.
How Do You Find Your Daily Calorie Need?
To estimate your daily calorie need, BMR is multiplied by a factor that reflects your activity level. The factors below are approximate values commonly used in practice; choosing your personal activity level correctly is the step that affects the result most.
| Activity level | Description | Approximate factor |
|---|---|---|
| Sedentary | Desk job, no regular exercise | 1.2 |
| Lightly active | Light exercise 1–3 days a week or walking through the day | 1.375 |
| Moderately active | Moderate-intensity exercise 3–5 days a week | 1.55 |
| Very active | Hard exercise 6–7 days a week | 1.725 |
| Extremely active | Heavy physical work or two workouts a day | 1.9 |
If the woman in the example is lightly active, her daily need is estimated at about 1,395 × 1.375 ≈ 1,920 kcal; if the man in the example is moderately active, about 1,780 × 1.55 ≈ 2,760 kcal. For comparison, the NHS states that the average woman needs about 2,000 kcal a day and the average man about 2,500 kcal, and that this amount varies with age, weight, height and exercise.
Most people tend to overestimate their activity level. If you go to the gym a few days a week but spend the rest of the day sitting, the “lightly active” factor is often a more realistic starting point.
What Factors Affect Basal Metabolism?
The factor that affects basal metabolism most is lean body mass, that is muscle and organ tissue. In the study by Mifflin and colleagues too, lean body mass was found to be the single strongest determinant of resting energy expenditure.
- Muscle mass: Even at rest, muscle tissue uses more energy than fat tissue; that is why strength exercise is important in the long term.
- Age: As age advances, BMR generally falls with the loss of muscle mass.
- Sex: Because average muscle mass is higher in men, BMR is generally higher too.
- Weight and height: A larger body uses more energy to keep its functions going.
- Hormones and health: Some hormonal conditions, above all thyroid hormones, feverish illnesses and some medicines can affect metabolism.
- Long-term low-calorie diets: Very low energy intake and rapid weight loss can lead the body to adapt in a way that reduces its energy expenditure.
Do You Need to Eat Below Your BMR to Lose Weight?
No. To lose weight it is enough for energy intake to be somewhat below total daily expenditure; very low-calorie diets that go below basal metabolism are generally not necessary and are hard to keep up.
Eating very few calories for a long time can lead to problems such as muscle loss, fatigue, hair shedding, vitamin and mineral deficiencies and weight returning quickly when the diet is stopped. I covered the risks in detail in my article on the effects of inadequate calorie intake on the body.
In a healthy weight-loss plan, the quality of the food matters as much as the number of calories: enough protein, plenty of vegetables, wholegrains, healthy fats and regular meals. If you mix up the terms calorie and kilocalorie, the article on the difference between kcal and calorie may be useful.
In Which Situations Can the Formulas Be Misleading?
Because estimation equations were developed for the average adult, they can show a need different from the real one in some groups. In these cases the results must always be assessed with a healthcare professional.
- Pregnancy and breastfeeding: Energy needs change in these periods and standard formulas are not used on their own. To plan your eating in pregnancy in line with your obstetrician's advice, you can look at my page on nutrition during pregnancy.
- Children and teenagers: Extra energy is needed for growth; adult formulas are not suitable.
- Athletes and people with high muscle mass: Because the formulas do not take muscle mass directly into account, they can show a lower need than the real one.
- Older people and people with chronic conditions: In conditions such as thyroid disease and kidney and liver disease, needs should be set together with a doctor.
Using the value you find from the formula as a starting point and watching changes in your weight, waist measurement and energy level over a few weeks is the most practical way to see whether the calculation suits you. To see which range your weight falls into for your height, you can also look at my article on body mass index calculation. For an energy need and sustainable plan specific to you, we can work together within weight management counselling.
Sources
- Mifflin MD, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr, 1990 (PubMed)
- Frankenfield D, et al. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc, 2005 (PubMed)
- NHS – Understanding calories
- WHO – Healthy diet
This article is for general information only; please consult your doctor and dietitian about your own situation.
Frequently Asked Questions
What should basal metabolic rate be?
There is no single ideal BMR value that applies to everyone; the value varies with age, sex, height, weight and especially muscle mass. What matters is to use your own value as a starting estimate and plan your eating accordingly.
How can basal metabolism be sped up?
The most important changeable factor affecting basal metabolism is muscle mass; regular strength exercise and enough protein matter in this respect. Avoiding very low-calorie diets, getting enough sleep and staying active through the day also support total energy expenditure.
Is the BMR value on a body analysis scale accurate?
Body analysis scales usually calculate BMR from the body composition they measure and from estimation equations, so the result is an estimate. It is more meaningful to look at the value not as a one-off figure but as the trend of measurements made under the same conditions.
Do you burn calories while sleeping?
Yes, the body also uses energy while you sleep for functions such as breathing, blood circulation and cell renewal. This expenditure is part of basal metabolism; enough regular sleep also contributes to regulating appetite and energy balance.
Can a BMR calculation be used in pregnancy?
Because energy needs in pregnancy change with the trimester, pre-pregnancy weight and the baby's development, standard BMR formulas should not be used on their own. You need to set your energy and nutrient needs in pregnancy together with your obstetrician and dietitian.
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This content is for information only. For a personalised nutrition programme, please consult a dietitian.
