How to Calculate BMR (Mifflin-St Jeor Formula)

Most people never calculate their BMR by hand, but it’s simpler than it looks, and understanding the formula helps you trust the number. This guide walks through the three main BMR equations, shows worked examples for a man and a woman, and explains which one to use. If you’d rather skip the arithmetic, the BMR calculator does all three instantly.

New to the topic? Start with what BMR is, then come back for the math.

The Mifflin-St Jeor equation (the modern standard)

This is the formula almost every calculator uses today, because research shows it’s the most accurate for the average person. It needs four things: weight in kilograms, height in centimetres, age in years, and sex.

Men: BMR = (10 x weight kg) + (6.25 x height cm) - (5 x age) + 5

Women: BMR = (10 x weight kg) + (6.25 x height cm) - (5 x age) - 161

The only difference between the two is the last number: men add 5, women subtract 161. That constant accounts for the average difference in body composition between the sexes.

Worked example: a woman

Let’s take a 30-year-old woman who weighs 65 kg and is 165 cm tall.

• 10 x 65 = 650
• 6.25 x 165 = 1,031
• 5 x 30 = 150
• BMR = 650 + 1,031 - 150 - 161 = 1,370 calories a day

Worked example: a man

Now a 30-year-old man at 80 kg and 180 cm.

• 10 x 80 = 800
• 6.25 x 180 = 1,125
• 5 x 30 = 150
• BMR = 800 + 1,125 - 150 + 5 = 1,780 calories a day

That’s the whole method. If your measurements are in pounds and inches, convert first: divide pounds by 2.205 for kilograms, and multiply inches by 2.54 for centimetres.

Converting your units first

Since the formula runs on metric, here’s the quick conversion you’ll usually need.

You have	Multiply by	To get
Pounds (lb)	0.4536	Kilograms (kg)
Inches (in)	2.54	Centimetres (cm)
Feet	30.48	Centimetres (cm)
Stone	6.35	Kilograms (kg)

So 176 pounds is 176 x 0.4536, which is about 80 kg, and 5 feet 11 inches is 71 inches x 2.54, which is about 180 cm.

The Harris-Benedict equation (the older one)

Harris-Benedict came first, back in 1919, and was revised in 1984. It’s still used, though it tends to run slightly high for many people.

Men: BMR = 88.36 + (13.40 x weight kg) + (4.80 x height cm) - (5.68 x age)

Women: BMR = 447.59 + (9.25 x weight kg) + (3.10 x height cm) - (4.33 x age)

For the same 30-year-old woman above, Harris-Benedict gives about 1,435 calories, roughly 65 more than Mifflin-St Jeor. That small gap is why the newer formula is now preferred.

The Katch-McArdle formula (if you know your body fat)

The other two only use height, weight, age, and sex. Katch-McArdle takes a different route: it’s based on your lean body mass, so it can be more accurate if you know your body fat percentage.

Lean body mass (LBM) = weight kg x (1 - body fat %)

BMR = 370 + (21.6 x LBM)

Say you weigh 80 kg at 20 percent body fat. Your lean mass is 80 x 0.8, which is 64 kg. Your BMR is 370 + (21.6 x 64), which is about 1,752 calories. Because it ignores sex and age and focuses on muscle, it’s popular with lifters who’ve measured their body fat.

Which BMR formula should you use?

Here’s the quick guide.

Formula	Best for	Needs body fat?
Mifflin-St Jeor	Almost everyone, the default	No
Harris-Benedict	Comparison, older references	No
Katch-McArdle	Lean or muscular people who know body fat	Yes

For most people, Mifflin-St Jeor is the right call, and it’s what the BMR calculator uses by default. Remember these are estimates: real metabolism varies by around 10 percent between people, so use your result as a starting point, not a medical fact. Calcowa’s health tools are for general guidance and aren’t medical advice.

Common mistakes when calculating BMR

If your number looks off, it’s usually one of these.

• Forgetting to convert units. The formulas run on kilograms and centimetres. If you plug in pounds and inches by mistake, your BMR comes out wildly wrong. Convert first.
• Confusing BMR with TDEE. BMR is your resting burn. It’s normal for it to feel low, because it doesn’t include any activity yet. You add that separately to get your daily total.
• Using Harris-Benedict and expecting the same answer. The older formula runs a little high, so a small difference between calculators is normal, not an error.
• Guessing your height or weight. Small input errors change the result, so measure rather than estimate if you can.
• Treating the number as exact. Two people with identical stats can have BMRs that differ by 200 calories. The formula gives an average, so verify it against your real-world results over a couple of weeks.

Once you’ve avoided those, your calculated BMR is a dependable starting point for planning calories.

What to do with your BMR

Your BMR is the resting baseline. To get the calories you actually burn in a day, you multiply it by an activity factor, which gives your TDEE. That’s the number you build a diet around, and it’s covered in BMR vs TDEE. You can also let the TDEE calculator do it for you.