# A BMR calculator with three equations side by side.

This BMR calculator estimates your basal metabolic rate, the energy your body burns at complete rest, with three published equations at once: Mifflin-St Jeor, the revised Harris-Benedict and, when you know your body fat, Katch-McArdle. Below are the formulas, what changes your BMR and why the equations disagree.

Use the calculator on the page: https://trainaq.com/tools/bmr-calculator. For healthy adults 18 and over. An estimate, not a measurement or medical advice. Pregnancy, breastfeeding, thyroid conditions, illness and some medicines change resting energy; ask your doctor or a registered dietitian.

## What is BMR?

BMR, basal metabolic rate, is the energy your body burns at complete rest to stay alive: breathing, circulation, keeping warm and running your brain. It is measured lying down, awake, after 10 to 12 hours without food. This BMR calculator estimates it from your measurements with three published equations, so you can see how closely they agree.

- Mifflin-St Jeor, the revised Harris-Benedict and Katch-McArdle side by side
- Katch-McArdle appears when you enter your body fat
- The gap between the equations, in kcal and percent
- Kilograms or pounds, with the answer in kcal and kJ

## How does this BMR calculator work?

Enter your sex, age, weight and height, and your body fat if you know it. The calculator runs each equation on the same numbers and shows the results side by side, with the gap between the highest and lowest. Mifflin-St Jeor is the headline figure because a systematic review found it the most reliable.

### 1. Enter your measurements

Mifflin-St Jeor and Harris-Benedict need only your sex, age, weight and height. Use kilograms and centimetres, or pounds, feet and inches; the calculator converts imperial figures to metric before it does any maths, at 0.4536 kg to the pound and 2.54 cm to the inch.

- For adults 18 and over
- Weight 30–300 kg, height 120–230 cm

### 2. Add your body fat if you have it

Katch-McArdle works from lean body mass, your weight minus your fat, so it needs a body fat percentage. Leave the field empty and the calculator shows the other two equations only. Enter it and you also see your lean mass in kilograms.

- Katch-McArdle is only as good as the body fat figure you give it
- No figure yet? The body fat calculator gives an estimate

### 3. Compare the equations

Each result is rounded to the nearest 10 kcal a day. The gap line shows how far apart the lowest and highest results are, in kcal and as a percentage of the lowest. A small gap means the equations agree about someone of your build; a large one means more uncertainty.

- Shown in kcal a day, with kilojoules for Mifflin-St Jeor

### 4. Turn it into daily calories

BMR is only the resting part of your day. Multiply it by a physical activity level to get your total daily energy expenditure, which is what the TDEE calculator does, then split that into protein, fat and carbohydrate with the macro calculator.

- BMR is 45 to 70% of an adult’s daily energy, depending on age and lifestyle (FAO/WHO/UNU)

## Which BMR formulas does the calculator use?

Three published equations. Mifflin-St Jeor and the revised Harris-Benedict use sex, age, weight and height, with one version for men and one for women. Katch-McArdle uses only lean body mass, so it needs your body fat. All three give kilocalories a day from weight in kilograms and height in centimetres.

- Mifflin-St Jeor, men: `BMR = 10 × W + 6.25 × H − 5 × A + 5`
- Mifflin-St Jeor, women: `BMR = 10 × W + 6.25 × H − 5 × A − 161`
- Harris-Benedict revised, men: `BMR = 88.362 + 13.397 × W + 4.799 × H − 5.677 × A`
- Harris-Benedict revised, women: `BMR = 447.593 + 9.247 × W + 3.098 × H − 4.330 × A`
- Katch-McArdle: `BMR = 370 + 21.6 × LBM, where LBM = W × (1 − BF ÷ 100)`
- Kilojoules: `kJ = kcal × 4.184`

- BMR: basal (resting) metabolic rate in kcal a day
- W: weight in kg; H: height in cm; A: age in years
- BF: body fat in percent; LBM: lean body mass in kg
- Pounds × 0.4536 = kg; inches × 2.54 = cm

## Where do the three equations come from?

Each was fitted to the measured resting energy of a group of people, and the groups differ in size, age, body weight and era. Harris and Benedict measured 239 adults a century ago; Mifflin and St Jeor measured 498, nearly half of them with obesity. That is why one person gets three slightly different answers.

| Equation | Built from | Needs | Keep in mind |
| --- | --- | --- | --- |
| Harris-Benedict, original (1918) | 239 healthy adults measured by indirect calorimetry | Sex, age, weight, height | In Mifflin’s 1990 study it overestimated measured resting energy by 5% |
| Harris-Benedict, revised (Roza and Shizgal, 1984) | The original data plus 98 more of Benedict’s subjects over a wider age range | Sex, age, weight, height | Its authors put its precision at about ±14% for well-nourished people |
| Mifflin-St Jeor (1990) | 498 healthy adults aged 19–78: 264 of normal weight, 234 with obesity | Sex, age, weight, height | Within 10% of measured in more people than other equations (2005 review) |
| Katch-McArdle (370 + 21.6 × lean mass) | The general equation in Cunningham’s 1991 review of lean mass and resting energy | Weight and body fat | Ignores age, sex and height; a rough body fat figure gives a rough answer |

Sources: Harris and Benedict 1918; Roza and Shizgal 1984; Mifflin et al. 1990; Cunningham 1991; Frankenfield et al. 2005. The calculator uses the revised Harris-Benedict, not the 1918 original.

## What does a worked example look like?

Take a 30-year-old man who weighs 80 kg and is 180 cm tall. Mifflin-St Jeor gives 10 × 80 + 6.25 × 180 − 5 × 30 + 5, which is 1,780 kcal a day. The revised Harris-Benedict gives about 1,850 kcal, so the two equations are roughly 70 kcal, or 4%, apart.

- Revised Harris-Benedict: 88.362 + 13.397 × 80 + 4.799 × 180 − 5.677 × 30 = 1,853.6 kcal
- With 20% body fat his lean mass is 64 kg, and Katch-McArdle gives 370 + 21.6 × 64 = 1,752 kcal
- With all three, the gap is about 100 kcal, or 5.8% of the lowest
- A 30-year-old woman, 60 kg and 165 cm: about 1,320 kcal by Mifflin-St Jeor and 1,380 by Harris-Benedict
- In kilojoules, 1,780 kcal is 1,780 × 4.184, about 7,450 kJ

## What is the difference between BMR and RMR?

They describe the same thing measured under different rules. BMR is measured lying down, awake, after 10 to 12 hours without food and eight hours of physical rest. Resting metabolic rate (RMR) asks for some hours without food, stimulants or hard exercise, then a short rest. Calculators use the two terms almost interchangeably.

|  | Basal metabolic rate (BMR) | Resting metabolic rate (RMR) |
| --- | --- | --- |
| Before the test | 10–12 hours without food and 8 hours of physical rest | No food, alcohol, caffeine, nicotine or hard exercise for some hours |
| During the test | Lying down, awake and relaxed, in a room neither warm nor cool enough to change it | 10–20 minutes of rest in a comfortable room, then about 10 minutes of measuring |
| Where you see it | The FAO/WHO/UNU energy reports and the Harris-Benedict studies | Clinics, and equations such as Mifflin-St Jeor |

Sources: FAO/WHO/UNU 2004, chapter 2; Compher et al. 2006, a systematic review of how to measure resting metabolic rate.

## What affects your BMR?

Mostly your fat-free mass: muscle, organs and bone. In a study of 150 adults in Scotland, fat-free mass explained 63% of the differences in BMR between people, fat mass 6% and age 2%, with about a quarter unexplained. Sex had no significant effect once body composition was counted.

- More lean mass means a higher BMR, which is why Katch-McArdle works from lean mass alone
- Every equation here lowers BMR with age: Mifflin-St Jeor by 5 kcal a day for each year
- Thyroid hormone counts: in the same study, thyroxine explained a quarter of the unexplained differences in men
- Illness, malnutrition, pregnancy and some medicines change resting energy in ways the equations do not capture

## Why do the equations give different answers?

Each equation fits the people it was built from. Harris and Benedict measured mostly young adults of normal weight in the 1910s; Mifflin and St Jeor included many people with obesity and adults up to 78. Katch-McArdle ignores age, sex and height. Your result shifts with how closely you resemble each group.

- Mifflin-St Jeor came within 10% of measured resting energy in more people, lean or obese, than other common equations (Frankenfield 2005)
- The same review found older adults and US ethnic minorities under-represented when these equations were built and tested
- Katch-McArdle can suit lean, muscular people, because it counts lean mass directly
- The gap line in the calculator shows how much the choice of equation matters for you

## How accurate is a BMR calculator?

Close enough to start from, not close enough to trust blindly. The 2005 systematic review found Mifflin-St Jeor the most reliable equation, yet still noted large errors for some individuals. The only way to know your own BMR is to measure it by indirect calorimetry, a breathing test some clinics and laboratories offer.

- Use the estimate as a starting point and judge your intake by your weight trend
- The equations are for healthy adults, not for illness, pregnancy or teenagers
- Katch-McArdle inherits any error in your body fat figure
- Roza and Shizgal found the Harris-Benedict equations unreliable in malnourished patients

## How do coaches use BMR with their clients?

A client’s BMR is the first step towards their maintenance calories and a check that a calorie target is not set too low. In Trainaq you set calorie and macro targets on the client’s profile from a date you choose, and their food log, weight and check-ins show whether the targets work.

- Targets cover calories, protein, carbs, fat and fibre, with their history
- Weight, body fat and other measurements sit in the client’s progress view with their weight trend
- Weekly check-ins can ask for weight and waist, so you see the trend before changing targets

[Nutrition coaching in Trainaq](https://trainaq.com/features/nutrition)

## Sources

1. [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.](https://doi.org/10.1093/ajcn/51.2.241)
2. [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.](https://doi.org/10.1093/ajcn/40.1.168)
3. [Harris JA, Benedict FG. A biometric study of human basal metabolism. Proc Natl Acad Sci U S A. 1918;4(12):370–373.](https://doi.org/10.1073/pnas.4.12.370)
4. [Cunningham JJ. Body composition as a determinant of energy expenditure: a synthetic review and a proposed general prediction equation. Am J Clin Nutr. 1991;54(6):963–969.](https://doi.org/10.1093/ajcn/54.6.963)
5. [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.](https://doi.org/10.1016/j.jada.2005.02.005)
6. [Johnstone AM, Murison SD, Duncan JS, et al. Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine but not sex, circulating leptin, or triiodothyronine. Am J Clin Nutr. 2005;82(5):941–948.](https://doi.org/10.1093/ajcn/82.5.941)
7. [Compher C, Frankenfield D, Keim N, et al. Best practice methods to apply to measurement of resting metabolic rate in adults: a systematic review. J Am Diet Assoc. 2006;106(6):881–903.](https://doi.org/10.1016/j.jada.2006.02.009)
8. [Pavlidou E, Papadopoulou SK, Seroglou K, Giaginis C. Revised Harris-Benedict equation: new human resting metabolic rate equation. Metabolites. 2023;13(2):189. (A review that tables the original and revised Harris-Benedict samples and coefficients.)](https://doi.org/10.3390/metabo13020189)
9. [FAO, WHO, UNU. Human energy requirements: report of a joint FAO/WHO/UNU expert consultation. Rome, 2004. Chapter 2.](https://www.fao.org/4/y5686e/y5686e04.htm)

## FAQ

### Is BMR the same as TDEE?

No. BMR is what you burn at complete rest; TDEE, total daily energy expenditure, adds everything else in your day, from walking and work to training and digesting food. The FAO/WHO/UNU report puts BMR at 45 to 70% of daily energy for adults. The TDEE calculator turns your BMR into daily calories.

### Should I eat only my BMR to lose weight?

No. Your BMR is what you would burn lying still all day, so eating only that much leaves you short on every day you get up and move. Work from your TDEE instead and eat a little less than it. Talk to a doctor or registered dietitian before going lower than your BMR.

### Which BMR equation is the most accurate?

For most healthy adults, Mifflin-St Jeor: a 2005 systematic review found it within 10% of measured resting energy in more people than other common equations. Katch-McArdle can suit lean, muscular people whose body fat is measured well. No equation is right for every individual; only measuring your resting energy is.

### Why is my BMR different on other calculators?

Other calculators may use the original 1918 Harris-Benedict instead of the 1984 revision, Cunningham’s older lean-mass formula of 500 + 22 × lean mass, or different rounding. Check which equation a calculator names; with the same equation and the same inputs, the answer should match ours to within rounding.

### Does BMR go down with age?

In every equation here, yes. Mifflin-St Jeor takes off 5 kcal a day for each year of age, and the revised Harris-Benedict takes off 5.7 for men and 4.3 for women. In a 2005 study of 150 adults, though, age explained only 2% of the differences in BMR; fat-free mass explained 63%.

### Who should not rely on a BMR calculator?

Anyone under 18, anyone pregnant or breastfeeding, and anyone with a condition or medicine that affects metabolism, such as a thyroid disorder. The equations were built from healthy adults. In those cases ask a doctor or registered dietitian, who can measure your resting energy or adjust the estimate for you.

## Related

- [TDEE calculator](https://trainaq.com/tools/tdee-calculator): Turn your BMR into daily calories
- [Macro calculator](https://trainaq.com/tools/macro-calculator): Split your calories into protein, fat and carbs
- [Body fat calculator](https://trainaq.com/tools/body-fat-calculator): A body fat estimate for Katch-McArdle
- [Nutrition coaching in Trainaq](https://trainaq.com/features/nutrition): Calorie and macro targets for every client

Source: https://trainaq.com/tools/bmr-calculator