Every BMR calculator on the internet — including ours — is a machine for hiding the math. You type in your numbers, a result appears, and the formula that produced it stays invisible. This article is the opposite of that: every BMR equation that matters, written out in full, with real worked examples — for men and women, in metric and imperial — plus an honest look at which formula you should actually be using and why.
By the end you'll be able to calculate your own BMR with a pocket calculator and a napkin — and you'll understand what each part of the equation is actually doing.
What BMR is (in 60 seconds)
Your Basal Metabolic Rate is the energy your body burns in 24 hours doing nothing at all — not digesting, not walking, not thinking hard. Just staying alive: heart, lungs, brain, kidneys, liver, and the constant cellular repair work that never switches off. For most people it's 60–75% of total daily calorie burn, which is why getting it right matters more than any workout.
One distinction worth knowing: BMR is measured under strict lab conditions (fasted, rested, lying still, thermoneutral room). RMR — Resting Metabolic Rate — is measured under looser conditions and runs slightly higher. The formulas below technically estimate RMR, but everyone (including every calculator and textbook) calls the result BMR. Don't let anyone trip you up with that pedantry.
The five formulas at a glance
| Formula | Year | Inputs | Best for |
|---|---|---|---|
| Mifflin-St Jeor | 1990 | Weight, height, age, sex | Almost everyone — the modern default |
| Harris-Benedict (original) | 1919 | Weight, height, age, sex | Historical interest; runs high |
| Harris-Benedict (revised) | 1984 | Weight, height, age, sex | Still baked into hospital charts |
| Katch-McArdle | — | Lean body mass | Anyone who knows their body fat % |
| Cunningham | 1980 | Lean body mass | Athletes and very muscular people |
Two "families": the first three guess your metabolism from weight, height, age and sex; the last two skip the guessing and work from lean body mass directly. That single difference explains almost everything about which one to pick.
Formula 1: Mifflin-St Jeor — the modern default
In 1990, two researchers at the University of Nevada ran resting metabolism measurements on roughly 500 people and built the simplest useful equation the field had seen:
Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161
In 2005 the American Dietetic Association compared the major equations against actually measured metabolism and concluded Mifflin-St Jeor was the most accurate for the general population — within 10% of measured values more often than any rival. That's the study that made it the default in most nutrition tools today, including our calculator.
Worked example — a 35-year-old man, 82 kg, 180 cm
BMR = (10 × 82) + (6.25 × 180) − (5 × 35) + 5
BMR = 820 + 1,125 − 175 + 5 = 1,775 kcal/day
Worked example — a 28-year-old woman, 63 kg, 167 cm
BMR = (10 × 63) + (6.25 × 167) − (5 × 28) − 161
BMR = 630 + 1,043.75 − 140 − 161 = 1,373 kcal/day
Mifflin-St Jeor in pounds and inches
Same formula, converted units (1 kg = 2.2046 lb, 1 cm = 0.3937 in):
Men: BMR = (4.536 × weight in lb) + (15.88 × height in in) − (5 × age) + 5
Women: BMR = (4.536 × weight in lb) + (15.88 × height in in) − (5 × age) − 161
Check it against the example man: 82 kg = 180.8 lb, 180 cm = 70.9 in → (4.536 × 180.8) + (15.88 × 70.9) − 175 + 5 = 820 + 1,125 − 175 + 5 = 1,775. Same answer, different units.
What each part of the formula is doing
- 10 × weight — the big engine. Most of your BMR scales with how much of you there is, so weight gets the largest coefficient.
- 6.25 × height — taller people carry more tissue per kilogram of scale weight (bigger frame, longer organs), so height adds a smaller bonus.
- − 5 × age — metabolism drifts down slowly with age, roughly 5 kcal per year of life. Not because aging is magic, but mostly because of gradual muscle loss — which is why a muscular 55-year-old can out-burn a sedentary 25-year-old.
- + 5 / − 161 — the sex adjustment. Not a statement about souls: it's a blunt statistical correction for the fact that, at equal weight and height, female bodies on average carry less metabolically expensive lean mass and more metabolically quiet fat mass. The −161 is also why short women so often end up with BMRs that surprise them.
Formula 2: Harris-Benedict — the 1919 classic (and its 1984 facelift)
The original basal metabolism equation, published by J. Arthur Harris and Francis G. Benedict in 1919 from careful lab measurements on volunteers:
Men (1919): BMR = 66.5 + (13.75 × kg) + (5.003 × cm) − (6.75 × age)
Women (1919): BMR = 655.1 + (9.563 × kg) + (1.850 × cm) − (4.676 × age)
In 1984, Roza and Shizgal re-evaluated the original data and published a revised version that's what most people today actually mean when they say "Harris-Benedict":
Men (1984): BMR = 88.362 + (13.397 × kg) + (4.799 × cm) − (5.677 × age)
Women (1984): BMR = 447.593 + (9.247 × kg) + (3.098 × cm) − (4.330 × age)
Worked examples (1984 version)
Same man (82 kg, 180 cm, 35): 88.362 + 1,098.55 + 863.82 − 198.70 = 1,852 kcal/day
Same woman (63 kg, 167 cm, 28): 447.593 + 582.56 + 517.37 − 121.24 = 1,426 kcal/day
Notice both came out higher than Mifflin-St Jeor (1,775 and 1,373). That's the formula's known bias: Harris-Benedict overestimates by about 5% on average, more so in people with above-average body fat. It overestimates by more today than in 1919 for a simple reason — the volunteers it was built on were leaner than today's average person. It survives in hospital dietetics and old software, but there's rarely a reason to choose it now.
Formula 3: Katch-McArdle — the one that uses lean mass
Here's the problem with everything above: a 100 kg bodybuilder and a 100 kg sedentary person get the same answer from Mifflin-St Jeor. Muscle burns roughly 13 kcal/kg/day at rest; fat burns about 4.5. Body composition is the variable the weight-height formulas can't see.
Katch-McArdle sees it:
BMR = 370 + (21.6 × lean body mass in kg)
Your lean body mass (LBM) is everything that isn't fat:
LBM = weight × (1 − body fat %)
Worked examples
Our example man at 82 kg with 20% body fat: LBM = 82 × 0.80 = 65.6 kg
BMR = 370 + (21.6 × 65.6) = 370 + 1,417 = 1,787 kcal/day — remarkably close to his Mifflin number, because he's near the average build the formula assumes.
Our example woman at 63 kg with 30% body fat: LBM = 63 × 0.70 = 44.1 kg
BMR = 370 + (21.6 × 44.1) = 370 + 953 = 1,323 kcal/day — 50 kcal below her Mifflin number for the same reason in reverse.
The magic of Katch-McArdle is what happens at the extremes. Give that 100 kg bodybuilder 12% body fat: LBM = 88 kg → BMR = 2,271 kcal. Give the 100 kg sedentary person 35% body fat: LBM = 65 kg → BMR = 1,774 kcal. Same weight, same height, same age — a 500 kcal/day difference the weight-based formulas are blind to.
The catch: you need a body fat number, and most body fat scales and gym caliper readings have a wide error bar. A wrong body fat percentage just moves the error from one place to another. If your estimate is a guess, stick with Mifflin-St Jeor.
Formula 4: Cunningham — the athlete's version
RMR = 500 + (22 × lean body mass in kg)
Structurally identical to Katch-McArdle with two knobs turned: a higher constant (500) and a higher lean-mass coefficient (22 vs 21.6). It was derived largely from data on trained subjects, and it tends to run 3–6% above Katch-McArdle — which is why endurance athletes and heavy lifters often find it matches their measured metabolism best, and why everyone else finds it slightly generous.
Our example man: 500 + (22 × 65.6) = 1,943 kcal/day. Our example woman: 500 + (22 × 44.1) = 1,470 kcal/day.
Honorable mentions: Schofield and Oxford
Two more equation families exist, and you'll run into them in research papers: the WHO/Schofield equations (1985) and the Oxford equations (2005, built from over 10,000 measurements). They use sex and age-band tables with different coefficients for each population group. They were designed for epidemiology and population nutrition at scale, not for one specific person choosing a dinner — and they tend to overestimate individuals, especially in hotter climates where they were calibrated. Worth knowing they exist; not worth using for yourself.
All five formulas, same two people
This is the table every comparison article should show and most don't: every formula applied to the same man (82 kg, 180 cm, 35 y, 20% BF) and the same woman (63 kg, 167 cm, 28 y, 30% BF):
| Formula | Man (kcal) | Woman (kcal) | Reading |
|---|---|---|---|
| Mifflin-St Jeor | 1,775 | 1,373 | The sensible default |
| Harris-Benedict 1919 | 1,858 | 1,436 | +83 / +63 vs MSJ |
| Harris-Benedict 1984 | 1,852 | 1,426 | +77 / +53 vs MSJ |
| Katch-McArdle | 1,787 | 1,323 | Near MSJ for average builds |
| Cunningham | 1,943 | 1,470 | The athlete's premium |
The spread between the highest and lowest estimate for the same person is roughly 170 kcal — about a slice and a half of pizza, every single day. That's the honest precision limit of BMR formulas, and it's why this site keeps repeating one piece of advice: treat your BMR as a starting hypothesis, then let 2–3 weeks of real-world weight data correct it.
Which BMR formula should you use?
- You're an average adult, no body fat data? Mifflin-St Jeor. It's the validated default, full stop.
- You lift seriously, or you know your body fat % reasonably well? Katch-McArdle.
- You're a competitive/endurance athlete with known lean mass? Cunningham.
- Your hospital or old app hands you a Harris-Benedict number? Fine — just know it may run ~5% high, and shave accordingly.
- Comparing several? The truth is usually closest to the Mifflin number unless your build is unusual.
From BMR to real life: the activity multipliers
BMR is a lab number — you burn it doing nothing. To estimate total daily energy expenditure (TDEE), multiply by an activity factor:
| Lifestyle | Multiplier |
|---|---|
| Sedentary — desk job, no exercise | × 1.2 |
| Lightly active — light exercise 1–3 days/week | × 1.375 |
| Moderately active — 3–5 days/week | × 1.55 |
| Very active — hard training 6–7 days/week | × 1.725 |
| Extra active — hard training + physical job | × 1.9 |
One honest warning: people systematically overestimate their activity factor. If you're unsure, pick the level below your instinct — then adjust from real data. (Our example man at 1,775 BMR with a desk job and 3 gym days: 1,775 × 1.375 ≈ 2,441 kcal to maintain.)
Seven mistakes that make your BMR wrong
- Pounds into a metric formula. The #1 error in the history of BMR calculating. 10 × 180 (pounds) is not 10 × 82 (kg). The result isn't slightly wrong — it's absurd. If you think in pounds, use the imperial versions above.
- Stale weight. A formula fed last year's weight gives last year's metabolism. Recalculate every 3–4 kg of change.
- Adding exercise calories twice. If you already multiplied by 1.55, your gym sessions are in there. BMR + multiplier is the whole picture.
- Using "goal weight" instead of current weight. Tempting, dishonest, and the math will not negotiate.
- Trusting the first body fat reading. Bioimpedance scales swing several points with hydration. Average three morning readings before feeding Katch-McArdle.
- Forgetting the formula's own error bar. ±10% on 1,775 kcal is ±178 kcal. No formula is a measurement.
- Panicking at a low number. A 1,250 kcal BMR isn't broken — it's usually just a smaller, older, or less muscular body being described honestly. We wrote a whole piece about why your BMR is lower than you think it should be.
Frequently asked questions
What is the most accurate BMR formula?
For the general population, Mifflin-St Jeor — it was the top performer in the ADA's 2005 comparison of major equations. For lean or muscular people who know their body fat percentage, Katch-McArdle is typically more accurate because it works from lean mass instead of guessing it from weight, height, age and sex.
Is there a BMR formula in pounds and inches?
Yes — the imperial Mifflin-St Jeor: men, (4.536 × lb) + (15.88 × in) − (5 × age) + 5; women, the same but − 161. Harris-Benedict has imperial versions too (men: 66 + 6.24 × lb + 12.7 × in − 6.8 × age). Just never mix units — pick one system and stay in it.
Why do women subtract 161 in Mifflin-St Jeor?
It's a statistical correction for body composition: at the same weight and height, female bodies on average carry less lean mass (the metabolically expensive tissue) and more fat mass. The −161 adjusts for that average. It's not a penalty, and it's not personal — individual body composition beats any population average.
Does BMR include exercise?
No. BMR is the doing-nothing baseline. Exercise is accounted for by multiplying BMR by an activity factor (1.2–1.9) to get TDEE. That's also why training doesn't change your BMR as much as people hope — though building muscle does, slowly, because muscle is metabolically active tissue.
How many calories does BMR burn per hour?
Divide your daily BMR by 24. Our example man's 1,775 kcal is about 74 kcal/hour — roughly one calorie every 49 seconds, just existing.
Can I eat below my BMR to lose weight faster?
You can, and for short periods it's common — but it's a blunt tool. We unpack the trade-offs honestly in Eating Below Your BMR: The Real Math.
Do BMR formulas work for children?
No. Every formula on this page is for adults (18+). Children and teens have different equations entirely — ask a pediatrician or dietitian rather than applying adult formulas to growing bodies.
Why did my BMR change when I recalculated?
Because one of its inputs changed: you're older (−5 kcal/year), you weigh something different, or your body fat estimate moved. Small shifts are the formula working as designed — see ' + '' + '' + t.title + '' + '' + t.desc + '' + ''; }).join(''); return '
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