Your basal metabolic rate (BMR) is the number of calories your body burns at complete rest to keep you alive: breathing, circulation, temperature and cell repair. It usually accounts for about 60% to 70% of the calories you burn in a day. You calculate it from weight, height, age and sex using a prediction equation, most often Mifflin–St Jeor: for men, 10 × kg + 6.25 × cm − 5 × age + 5; for women, the same with − 161 at the end. A 30-year-old man who is 5′11″ and 180 pounds has an estimated BMR of about 1,799 calories a day.
The three main equations
Mifflin–St Jeor (1990)
W is weight in kilograms, H is height in centimeters and A is age in years. It is the default in most clinical nutrition settings today.
Harris–Benedict (revised by Roza and Shizgal, 1984)
The original 1919 equation was one of the first; it tends to give slightly higher estimates than Mifflin–St Jeor for many modern adults.
Katch–McArdle
LBM is lean body mass in kilograms: weight × (1 − body fat fraction). Because it uses lean mass instead of total weight, it does not need age or sex. Its accuracy depends entirely on how good your body fat estimate is.
Worked example 1: a 30-year-old man
5 ft 11 in (71 in = 180.34 cm), 180 lb (81.65 kg), 18% body fat.
Mifflin–St Jeor: 10 × 81.65 + 6.25 × 180.34 − 5 × 30 + 5 = 816.5 + 1,127.1 − 150 + 5 = 1,799 calories
Harris–Benedict: 88.362 + 13.397 × 81.65 + 4.799 × 180.34 − 5.677 × 30 = 88.4 + 1,093.8 + 865.5 − 170.3 = 1,877 calories
Katch–McArdle: LBM = 81.65 × 0.82 = 66.95 kg; BMR = 370 + 21.6 × 66.95 = 1,816 calories
Worked example 2: a 50-year-old woman
5 ft 4 in (162.56 cm), 140 lb (63.50 kg), 28% body fat.
| Equation | Calculation | BMR |
|---|---|---|
| Mifflin–St Jeor | 635.0 + 1,016.0 − 250 − 161 | 1,240 |
| Harris–Benedict | 447.6 + 587.2 + 503.6 − 216.5 | 1,322 |
| Katch–McArdle | 370 + 21.6 × 45.72 | 1,358 |
The three equations differ by about 120 calories here, which is typical: roughly 5% to 10%. That gap is a reminder that every equation is an estimate of an average person with your measurements. The BMR calculator shows all three side by side.
Which equation should you use?
| Situation | Best choice | Why |
|---|---|---|
| Most adults, body fat unknown | Mifflin–St Jeor | Most accurate in validation studies of healthy and overweight adults |
| Lean or very muscular, with a reliable body fat measurement | Katch–McArdle | Accounts for extra muscle, which other equations underestimate |
| Comparing with older research or tools | Harris–Benedict | Long history, but often overestimates |
| Hospitalized, critically ill, pregnant or under 18 | None of these alone | Clinicians use specialized equations or measure energy expenditure directly |
If you do not know your body fat, estimate it with the body fat calculator and convert to lean mass with the lean body mass calculator, or simply use Mifflin–St Jeor. For the methods behind body fat estimates, see how to measure body fat percentage.
From BMR to daily calories
BMR is only part of your daily burn. Total daily energy expenditure (TDEE) adds the energy of digesting food (about 10% of intake) and all movement. The standard shortcut multiplies BMR by an activity factor:
| Activity level | Multiplier | Example man (BMR 1,799) | Example woman (BMR 1,240) |
|---|---|---|---|
| Sedentary | 1.2 | 2,158 | 1,488 |
| Lightly active | 1.375 | 2,473 | 1,705 |
| Moderately active | 1.55 | 2,788 | 1,922 |
| Very active | 1.725 | 3,103 | 2,139 |
| Extra active | 1.9 | 3,418 | 2,356 |
Use TDEE as a maintenance estimate, then adjust it based on how your weight actually changes over three to four weeks. The calorie calculator does this step, and how to calculate calories for weight loss explains how to set a deficit.
What affects BMR
- Body size and composition: more lean mass means a higher BMR. Muscle burns more energy at rest than fat.
- Age: BMR declines with age, mostly through lost muscle. Mifflin–St Jeor subtracts 5 calories per year, so the example man’s estimate falls by 150 calories between ages 30 and 60 at the same weight.
- Sex: men typically have more lean mass, hence the 166-calorie difference between the Mifflin–St Jeor constants.
- Hormones and health: an overactive thyroid raises BMR; an underactive thyroid lowers it. Fever, illness and recovery from injury raise it.
- Pregnancy and breastfeeding raise energy needs.
- Severe dieting can lower BMR more than weight loss alone predicts, an effect called adaptive thermogenesis.
Measuring instead of estimating
Indirect calorimetry measures oxygen consumption and carbon dioxide output to calculate resting energy use directly. It is available at some clinics, hospitals and fitness centers and is more accurate than any equation for an individual. Results are usually reported as resting metabolic rate (RMR), which is close to BMR.
Common mistakes
- Mixing units. The equations need kilograms and centimeters. Plugging in pounds and inches gives nonsense; convert first (lb × 0.4536, in × 2.54).
- Treating BMR as a calorie target. Eating at or below BMR for long periods is rarely necessary and makes it hard to meet nutrient needs. Weight-loss targets should come from TDEE minus a moderate deficit.
- Counting exercise twice. If you choose a high activity factor, do not also add the calories your watch says you burned in workouts.
- Trusting decimals. An estimate of 1,798.59 calories is not more accurate than “about 1,800.” Round to the nearest 10 or 50.
- Using a guessed body fat in Katch–McArdle. A body fat figure that is off by 5 percentage points shifts the result by roughly 90 calories for an 80-kilogram person.
Units
BMR is given in kilocalories per day (what food labels call Calories). To convert to kilojoules, multiply by 4.184: 1,799 kcal is about 7,525 kJ.
This guide is for general education and is not medical or nutrition advice. Prediction equations estimate averages; your actual needs may differ. Consult a doctor or registered dietitian for personalized guidance.
Frequently asked questions
What is the most accurate BMR formula?
For most adults, the Mifflin–St Jeor equation. A 2005 systematic review in the Journal of the American Dietetic Association found it predicted resting metabolic rate within 10% of measured values more often than other common equations. If you know your body fat percentage accurately, Katch–McArdle can be better for lean or very muscular people.
What is the difference between BMR and RMR?
BMR (basal metabolic rate) is measured under strict conditions: after an overnight fast, fully rested, in a neutral temperature. RMR (resting metabolic rate) is measured under less strict conditions and is usually a few percent higher. Prediction equations are often used for both.
How do I turn BMR into daily calories?
Multiply BMR by an activity factor, from 1.2 for sedentary people to 1.9 for very physical jobs plus training. The result is total daily energy expenditure (TDEE), the calories needed to maintain your weight.
Does BMR go down with age?
Yes, mainly because of loss of muscle mass. In the Mifflin–St Jeor equation, BMR drops by 5 calories per year of age, or 150 calories over 30 years, with weight and height unchanged. Strength training helps slow the decline.
Can I raise my BMR?
Modestly. Building muscle raises resting energy use, and very low calorie intake lowers it. Factors such as age, height, sex and genetics cannot be changed, and thyroid disorders can raise or lower BMR significantly.