BMR calculator: your basal metabolic rate, three ways
Compare Mifflin-St Jeor, Harris-Benedict and Katch-McArdle side by side. See each equation, when each one fits, and why they give different numbers.
How do I calculate my BMR?
Use the Mifflin-St Jeor equation: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age, then add 5 for men or subtract 161 for women. A 30-year-old woman who is 168 cm and 68 kg has a BMR of about 1,420 kcal a day. If you know your body fat percentage, the Katch-McArdle equation is an alternative.
How to use the calculator
- Enter sex, age, height and weightChoose metric or US units and fill in each field. These four numbers drive Mifflin-St Jeor and Harris-Benedict.
- Add body fat if you know itBody fat is optional. Enter it and the Katch-McArdle row fills in; leave it blank and that row asks for it.
- Compare the three resultsThe headline is Mifflin-St Jeor, rounded to the nearest 10 kcal. The other two sit under it for comparison.
- Check the activity rowsThe lower list multiplies your Mifflin-St Jeor BMR by five activity factors, from Sedentary to Extra active. Each row is an estimated daily burn.
Basal metabolic rate is the energy your body uses at complete rest: breathing, circulation, keeping warm, running your brain and organs. For most people it is the biggest part of the day’s burn, far bigger than exercise. It is measured lying still, fasted, after a night’s sleep. Almost nobody gets measured, so equations estimate it from body size. More on the idea in what BMR is.
The three formulas
- Mifflin-St Jeor (men): BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
- Mifflin-St Jeor (women): BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
- Harris-Benedict (men): BMR = 88.362 + 13.397 × weight (kg) + 4.799 × height (cm) − 5.677 × age
- Harris-Benedict (women): BMR = 447.593 + 9.247 × weight (kg) + 3.098 × height (cm) − 4.330 × age
- Katch-McArdle: BMR = 370 + 21.6 × lean mass (kg), where lean mass = weight × (1 − body fat % ÷ 100)
The Harris-Benedict version is the one revised by Roza and Shizgal (1984); the original equations date from 1919. Katch-McArdle ignores sex and age. It only needs lean mass, which is why it needs your body fat.
A worked example
Take the calculator’s default person, a woman of 30, 168 cm and 68 kg, and say she has 30% body fat. Her lean mass is 68 × 0.7 = 47.6 kg.
| Equation | Working | Result |
|---|---|---|
| Mifflin-St Jeor | 680 + 1,050 − 150 − 161 | 1,420 kcal |
| Harris-Benedict | 447.593 + 628.8 + 520.5 − 129.9 | 1,470 kcal |
| Katch-McArdle | 370 + 21.6 × 47.6 | 1,400 kcal |
The spread is 70 kcal, about 5%. Enter 22% body fat instead and Katch-McArdle jumps to 1,520 kcal, higher than both. Eight points of body fat moved it by about 120 kcal. That equation is only as good as your body-fat number.
you want the default. It was published in 1990 from measurements of about 500 healthy adults, and it is the equation most dietitians use when nobody has measured you.
you know your body fat from a reliable method, such as a DEXA scan, and you are very lean or very muscular. Lean mass drives resting burn, and this is the only equation of the three that sees it.
you want to compare against an older plan or app that uses it. It tends to read a little higher than Mifflin-St Jeor.
Why the equations disagree
Each one was fit to a different group of people, in a different era, from different inputs. Harris-Benedict came from people measured more than a century ago. Mifflin-St Jeor came from a later sample that included people with obesity. Katch-McArdle swaps height, age and sex for lean mass. None of them can see how much of your weight is muscle unless you give it a body-fat figure.
Sex is a stand-in for that missing number. At the same height, weight and age, Mifflin-St Jeor puts a man 166 kcal a day above a woman, mostly because men carry more lean mass on average. A muscular woman or a sedentary man can sit on the wrong side of that gap. Katch-McArdle avoids the guess, if your body-fat figure is good.
How accurate is it?
A 2005 review by Frankenfield and colleagues found Mifflin-St Jeor came within 10% of measured resting energy for more people than the other common equations. That still leaves people it misses by more. Body-fat scales that use bioimpedance can be off by several points, and that error feeds straight into Katch-McArdle. Use the three numbers as a range, not a verdict.
What to do with your BMR
BMR on its own is not an eating target. It is what you would burn lying still all day. Multiply it by an activity factor to get your daily burn. The rows under the result do that for you, or the TDEE calculator picks one level and adds targets for cutting and gaining. Then track food against that number in a macro tracker, and adjust once you have two weeks of weigh-ins.
FAQ
What is a normal BMR?
There is no single normal; size, sex and age drive it. By Mifflin-St Jeor, a 30-year-old woman at 168 cm and 68 kg comes out near 1,420 kcal a day, and a 30-year-old man at 180 cm and 80 kg near 1,780 kcal.
Does BMR slow down with age?
The equations take off a little for every year, partly because people tend to lose muscle as they age. Strength training helps you hold on to it.
Can I raise my BMR?
Adding muscle raises it a little, and so does carrying more weight in general. No food or supplement changes it enough to matter for your weight.
Is BMR the same as RMR?
Nearly. BMR is measured under strict conditions: fasted, rested, lying down after sleep. RMR is measured under looser ones and reads slightly higher. Calculators treat them as the same number.
Should I eat my BMR to lose weight?
Usually not. Your BMR is what you burn lying still, so eating that little leaves a large deficit on any normal day. Start from your TDEE and subtract a modest deficit instead.