Basal metabolic rate, and why three formulas disagree
BMR is the energy your body uses at complete rest. Three established formulas give different answers for the same person, which is the most honest thing this calculator shows you.
What this calculator does
- Estimates basal metabolic rate using the Mifflin-St Jeor formula.
- Shows the Harris-Benedict result alongside it for comparison.
- Adds the Katch-McArdle estimate where a body fat percentage is supplied.
- Reports resting energy use per hour.
How the calculation works
Basal metabolic rate is what the body uses at complete rest, before any movement at all — keeping organs running, maintaining temperature, everything involuntary. It is not the same as the total energy you use in a day, which is BMR multiplied by an activity factor. All three formulas here are regressions fitted to population data, taking height, weight, age and sex as inputs. Mifflin-St Jeor is used as the headline figure because it is generally the most accurate of the three for the general population. Harris-Benedict is older and tends to read higher. Katch-McArdle works from lean body mass instead, so it only appears when a body fat percentage is supplied, and it is often the better estimate for people whose body composition is far from average. Showing all three is deliberate: they can disagree by a couple of hundred calories for the same person, and presenting one figure alone would imply a precision that none of them has.
The rule
Mifflin-St Jeor: BMR = 10 × weight in kg + 6.25 × height in cm − 5 × age, then −161 for women or +5 for men.
Step by step
- Take weight in kilograms, height in centimetres and age in years.
- Apply the Mifflin-St Jeor regression to give the headline BMR.
- Apply Harris-Benedict to the same inputs for comparison.
- If a body fat percentage was given, apply Katch-McArdle from lean mass.
- Divide the headline figure by 24 to give resting energy use per hour.
Worked example
A 34-year-old woman, 165 cm tall and 68 kg, with a moderate activity level and no body fat percentage entered.
What was entered
| Sex | Female |
|---|---|
| Age | 34 |
| Height | 165 cm |
| Weight | 68 kg |
| Activity level | Moderate |
| Body fat (%) if known | Not given |
The arithmetic
- Mifflin-St Jeor gives 10 × 68 + 6.25 × 165 − 5 × 34 − 161 = 1,380.25 calories a day.
- Harris-Benedict gives 1,440.34 for the same person — about 60 calories higher.
- Katch-McArdle is unavailable, because no body fat percentage was entered.
- The headline figure works out at about 57.51 calories an hour at complete rest.
- The 60-calorie gap between two established formulas is the useful signal: these are estimates with real uncertainty, not measurements.
What the calculator returns
| Basal metabolic rate | 1,380.25 |
|---|---|
| Harris-Benedict estimate | 1,440.34 |
| Calories per hour at rest | 57.51 |
| Headline formula | Mifflin-St Jeor |
Key assumptions
- The formulas are population regressions applied to the details entered.
- The sex option reflects the basis on which the formulas were originally derived.
- The calculator is intended for adults aged 18 and over.
Limitations
- This is BMR, not the total energy you use in a day. Daily need is BMR multiplied by an activity factor, and that multiplier is itself a rough approximation.
- Individual metabolic rates vary considerably around the population average, and genetics, medical conditions, medication and body composition all move them.
- The formulas were derived on particular populations and are less reliable for people far from those averages — very muscular, very lean, or at the extremes of height and weight.
- It is not suitable for children, and not appropriate during pregnancy or breastfeeding.
- This is information, not medical or dietary advice. A GP or registered dietitian should be involved before making significant changes, and anyone who has experienced an eating disorder should speak to a GP first.
Common questions
Is this how many calories I should eat?
Why do the formulas disagree?
Should I enter my body fat percentage?
Why does the calculator ask for sex?
Related calculators
- BMI Calculator — BMI describes body size relative to height, a different question from energy use.
- TDEE Calculator — Body composition measures add what a weight-based estimate cannot see.
Official sources
Every figure in this guide was checked against the sources below. Where a source could not confirm a figure, it is marked as requiring verification rather than presented as settled.
- Understanding calories — NHSHow calorie needs relate to activity and body size
- Overweight and obesity management (NG246): identifying and assessing overweight, obesity and central adiposity — NICEHealthy weight 18.5–24.9, overweight 25–29.9, obesity from 30. For South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds, overweight is 23–27.4 and obesity 27.5 or above