BMR calculator — Mifflin-St Jeor, Harris-Benedict and Katch-McArdle
Estimate your basal metabolic rate with three published equations side by side, see the working, and learn which one fits your body composition.
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These equations were built from male and female reference groups — pick the one closest to your physiology.
5 ft 10.1 in
172 lb · 12 st 4 lb
These equations are validated for adults aged roughly 18 to 80.
Unlocks the Katch-McArdle equation, which uses lean mass instead of total weight.
Predictive equations land within about 10 percent of a measured BMR for most people, but individual metabolic rate varies by roughly ±200 kcal at the same height, weight and age. Use the number as a starting point, not a verdict.
Mifflin-St Jeor (1990)
1,748 kcalThe current default in dietetics. Validated on a modern population and usually the closest of the three for people of average body composition.
Harris-Benedict (revised 1984)
1,817 kcalThe classic equation, re-fitted by Roza and Shizgal in 1984. Tends to read a few percent high for sedentary adults because the 1919 sample was leaner and more active.
The formula, with your numbers
- Mifflin-St Jeor
- (10 × 78) + (6.25 × 178) − (5 × 30) + 5 = 1748 kcal/day
- Harris-Benedict
- 88.362 + (13.397 × 78) + (4.799 × 178) − (5.677 × 30) = 1817 kcal/day
Add a body fat percentage to unlock Katch-McArdle, which is usually the most accurate equation for lean or very muscular people.
| Activity level | × | Calories / day |
|---|---|---|
| SedentaryDesk job, mostly sitting, no deliberate exercise beyond moving around the house. | 1.2 | 2,097 |
| Lightly activeLight exercise 1–3 days a week, or a job that keeps you on your feet for part of the day. | 1.375 | 2,403 |
| Moderately activeModerate exercise 3–5 days a week — gym sessions, running, team sport, hard cycling commutes. | 1.55 | 2,709 |
| Very activeHard exercise 6–7 days a week, or a physical job on top of regular training. | 1.725 | 3,014 |
| Extra activeTwice-a-day training, manual labour, or an athlete in a heavy training block. | 1.9 | 3,320 |
What this means
Your body burns roughly 1,748 kcal a day doing nothing at all. That covers breathing, circulation, brain activity, kidney and liver function and keeping your temperature steady — about 60 to 70 percent of most people’s total daily burn.
The three equations differ by 70 kcal here, which is normal. They were fitted to different populations decades apart, and none of them can see your thyroid function, your muscle mass or how much you fidget. Pick one, stick with it, and let two weeks of real weight data tell you whether it was right.
BMR is never a calorie target. Even a completely sedentary day costs about 20 percent more than BMR once you add digestion and moving around.
How to use this BMR calculator
- Choose metric or imperial — the toggle is shared across every calculator on the site.
- Enter sex, height, weight and age. Every equation on this page needs all four.
- If you know your body fat percentage, switch on the toggle. That unlocks Katch-McArdle, which works from lean mass and is usually the most accurate of the three for lean or muscular people.
- Compare the three results, then look at the activity table underneath to see what each one becomes over a real day.
What BMR actually measures
Basal metabolic rate is the energy your body spends staying alive with no activity at all: breathing, pumping blood, running the brain and kidneys, synthesising proteins and holding body temperature steady. A true BMR measurement means lying still in a thermoneutral room after an overnight fast, awake but completely at rest, with gas exchange measured directly.
For most adults BMR is 60 to 70 percent of total daily energy expenditure. The brain alone accounts for roughly 20 percent of it. That is why cutting calories hard does not produce proportional weight loss — the majority of your burn is not something you can consciously change.
Resting metabolic rate (RMR) is the same idea under less strict conditions and typically lands 5 to 10 percent higher. Predictive equations sit somewhere between the two, and in everyday use the terms are used interchangeably.
The three equations
| Equation | Men | Women | Inputs |
|---|---|---|---|
| Mifflin-St Jeor (1990) | 10W + 6.25H − 5A + 5 | 10W + 6.25H − 5A − 161 | weight kg, height cm, age |
| Harris-Benedict (revised 1984) | 88.362 + 13.397W + 4.799H − 5.677A | 447.593 + 9.247W + 3.098H − 4.330A | weight kg, height cm, age |
| Katch-McArdle (1996) | 370 + 21.6 × lean body mass in kg | weight kg, body fat % | |
Mifflin-St Jeor
Published in 1990 and validated on 498 healthy adults, it is the equation the Academy of Nutrition and Dietetics recommends for non-obese and obese adults alike. In validation studies it predicts measured RMR within 10 percent for about 82 percent of people, which is the best record of any general equation. If you only look at one number on this page, look at this one.
Harris-Benedict
The original 1919 study by James Harris and Francis Benedict is the ancestor of every equation here. Its sample was leaner and more physically active than a modern population, so the original coefficients over-predict by around 5 percent today. Roza and Shizgal re-fitted them in 1984, and those revised numbers are what this calculator uses. It still tends to read a little high for sedentary adults.
Katch-McArdle
This one ignores your total weight and works only from lean body mass, on the reasoning that fat tissue is metabolically cheap while muscle and organs are expensive. That makes it the best choice at both extremes: a bodybuilder at 8 percent body fat and someone at 45 percent are both poorly served by weight-only equations. Its catch is that it is only as good as your body fat estimate.
How accurate is any of this?
Two people of identical sex, age, height and weight can have measured BMRs 200 to 300 kcal apart. Thyroid function, organ size, genetics, past dieting history and even brown fat activity all move the number, and no equation can see any of them.
Treat the output as a starting hypothesis. Set your intake from it, track weight for two to three weeks, and adjust. That feedback loop will get you a personal number within about 50 kcal — better than any laboratory equation applied blind.
Things that genuinely change your BMR
- Muscle mass. Skeletal muscle burns roughly 13 kcal per kg per day at rest. Adding 5 kg of muscle is worth about 65 kcal a day — real, but far less than fitness marketing implies.
- Body size in general. Bigger bodies of any composition cost more to run. This is why BMR falls as you lose weight and diets get harder over time.
- Age. BMR declines roughly 1 to 2 percent per decade after about 20, mostly through lost lean mass rather than a metabolic slowdown per se.
- Prolonged dieting. Sustained large deficits can suppress BMR by 10 to 15 percent beyond what weight loss alone predicts — adaptive thermogenesis. Diet breaks at maintenance help.
- Thyroid function and some medications. Both can shift resting metabolism substantially, which is one reason a doctor is the right person to talk to about an unexpectedly low number.
BMR is not a calorie target
Eating at your BMR means eating as if you were in a coma. Even a fully sedentary day costs about 20 percent more once you add the thermic effect of food and simply moving around a house. To get a target you can actually eat to, multiply BMR by an activity factor — that is what the TDEE calculator does, and it feeds straight into the calorie and macro calculators.
Questions
What is the difference between BMR and RMR?
BMR is measured after an overnight fast, lying still in a thermoneutral room, fully rested. RMR is measured under looser conditions and usually lands about 5 to 10 percent higher. These equations predict BMR, and in practice people use the number for both.
Which equation should I use?
Mifflin-St Jeor is the default for most people — it was validated on a modern population and typically lands within about 10 percent of measured BMR. If you know your body fat percentage, Katch-McArdle is usually better, because it works from lean mass rather than total weight.
Why is Harris-Benedict higher than Mifflin-St Jeor?
The original Harris-Benedict data was collected in 1919 on a leaner, more active sample, so it tends to over-predict by roughly 5 percent for today’s average adult. The revised 1984 coefficients used here narrow the gap but do not close it.
Can I eat only my BMR calories?
BMR is what your body burns doing nothing at all — it does not include digestion, walking or exercise. Eating at BMR is a large deficit for most people. Use the TDEE calculator to get a realistic daily target.