How to Calculate BMR: Harris-Benedict vs Mifflin-St Jeor

I Spent Way Too Long Staring at Two Formulas

So here's what happened. I was trying to figure out how many calories I actually need — not some generic "2,000 calories a day" number from a nutrition label, but my number. And I kept running into these two formulas: Harris-Benedict and Mifflin-St Jeor. They both claim to calculate your BMR (basal metabolic rate), which is basically how many calories your body burns just existing. Like, if you laid in bed all day and did absolutely nothing, that's your BMR.

The problem? They give you different numbers.

I stared at both equations for longer than I'd like to admit, plugged in my stats, got two results that were like 80-100 calories apart, and honestly had no idea which one to trust. It took me a while to figure out why they're different and which one most dietitians and researchers actually prefer these days. So I'm going to walk you through both, show you the actual math, and tell you what I landed on.

What BMR Actually Is (and Why You Should Care)

BMR stands for basal metabolic rate. That's the number of calories your body needs to keep your organs running, your blood pumping, your lungs breathing — all the stuff that happens whether you're awake or asleep. It's not the same as your TDEE (total daily energy expenditure), which factors in walking around, exercise, and everything else you do. BMR is just the baseline. The floor.

Why does it matter? Because if you're trying to lose weight, gain muscle, or even just understand why you feel tired all the time, knowing your BMR gives you a starting point. I'd been guessing at my calorie needs for years, and it turns out I was undershooting by a pretty significant margin, which explained a lot about my energy levels.

Your BMR accounts for roughly 60-75% of the calories you burn in a day. That's a huge chunk.

The Two Formulas, Side by Side

Alright, here's where we get into the actual math. Both formulas use your weight, height, age, and sex. But they were developed decades apart and they weight those variables a little differently.

The Harris-Benedict equation was originally published in 1919 (yeah, over a hundred years ago) and then revised in 1984 by Roza and Shizgal. The revised version is what most people mean when they say "Harris-Benedict" today. The Mifflin-St Jeor equation came along in 1990, and it was specifically designed to be more accurate for modern populations — people who are, on average, heavier and less active than folks in 1919.

💡 THE FORMULAS

Mifflin-St Jeor:
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

Revised Harris-Benedict:
Men: BMR = (13.397 × weight in kg) + (4.799 × height in cm) − (5.677 × age) + 88.362
Women: BMR = (9.247 × weight in kg) + (3.098 × height in cm) − (4.330 × age) + 447.593

Weight = body weight in kilograms · Height = height in centimeters · Age = age in years

Let me run through a quick example so you can see how these actually play out. Say you're a 35-year-old male, 80 kg (about 176 lbs), 178 cm tall (roughly 5'10").

Mifflin-St Jeor:
(10 × 80) + (6.25 × 178) − (5 × 35) + 5
= 800 + 1112.5 − 175 + 5
= 1,742.5 calories/day

Revised Harris-Benedict:
(13.397 × 80) + (4.799 × 178) − (5.677 × 35) + 88.362
= 1071.76 + 854.22 − 198.70 + 88.362
= 1,815.6 calories/day

So there's about a 73 calorie difference right there. That might not sound like much, but over a week that's roughly 500 calories — which is basically an extra meal you're either accounting for or not.

The American Dietetic Association (now called the Academy of Nutrition and Dietetics) started recommending Mifflin-St Jeor back in 2005 as the most reliable equation for estimating BMR in non-obese and obese individuals. That was kind of the deciding factor for me. If the professional dietitians picked one, I'll go with that.

But honestly? Neither formula is perfect. They're estimates. Your actual BMR depends on things like muscle mass, genetics, thyroid function, and a bunch of other stuff these equations can't account for. The gold standard is something called indirect calorimetry, where they literally measure the oxygen you breathe in and the carbon dioxide you breathe out. Most of us aren't doing that.

From BMR to Something Actually Useful

Here's the thing — your BMR by itself isn't super actionable. You don't just lie in bed all day (I hope). To get your actual daily calorie needs, you multiply your BMR by an activity factor. This gives you your TDEE, and that's the number you use for meal planning, cutting, bulking, or whatever your goal is.

So our example guy with a BMR of about 1,743 — if he's moderately active, he'd need around 2,700 calories a day to maintain his weight. That's a real, usable number. Want to lose weight? Subtract 300-500 from that. Want to gain? Add 300-500. It's not rocket science, but you need that BMR starting point or you're just guessing.

You can also use our TDEE calculator to skip the manual multiplication, or check out the calorie intake calculator if you want something that factors in your specific goals. And if you're tracking macros alongside calories, the macro calculator breaks things down into protein, carbs, and fat.

For body composition stuff, a body fat percentage calculator can give you a better picture of where you stand, and the BMI calculator is there too (though I have my opinions about BMI, which is a whole other conversation).

So Which Formula Should You Actually Use?

Mifflin-St Jeor. That's my answer.

It's newer, it's been validated more thoroughly against modern populations, and the professional nutrition organizations prefer it. Harris-Benedict isn't wrong exactly — it's just a bit more generous with its estimates, and if you're trying to lose weight, an overestimate is the last thing you want. I used Harris-Benedict for months before switching and I think that slight overcount was quietly undermining my progress, which was frustrating because I thought I was doing everything right.

That said, both formulas lose accuracy at the extremes — very muscular people, very lean people, older adults, and individuals with significantly higher body fat percentages. If you fall into any of those categories, treat the number as a rough starting point and adjust based on what actually happens over 2-3 weeks. The scale and the mirror will tell you more than any equation.

If you want to explore related health numbers, we've got an ideal weight calculator and a lean body mass calculator that pair well with BMR tracking. And for the fitness-minded folks, the one rep max calculator is a fun one too.

Is BMR the same as resting metabolic rate (RMR)?

Not exactly, though people use them interchangeably all the time (I did for years). BMR is measured under very strict conditions — you're supposed to be in a completely fasted state, in a dark room, after a full night's sleep. RMR is a little more relaxed in terms of testing conditions, so it tends to run about 10-20% higher than BMR. For practical purposes, the difference is small enough that most online calculators treat them as roughly the same thing.

Can I increase my BMR?

Yes! Building muscle is the most reliable way. Muscle tissue burns more calories at rest than fat tissue does — somewhere in the ballpark of 6 calories per pound of muscle per day versus about 2 calories per pound of fat. That doesn't sound dramatic, but add 10 pounds of muscle and you're burning an extra 40-ish calories daily just by existing. Over a year, that adds up.

Why do the two formulas give different results?

They were built from different study populations decades apart. Harris-Benedict used data from the early 1900s, Mifflin-St Jeor from 1990. People's body compositions, diets, and activity levels changed a lot in those 70+ years, so the newer equation better reflects current populations. The coefficients (the numbers multiplied by your weight, height, and age) are weighted differently in each formula, which is where the gap comes from.