What is BMR?
BMR stands for basal metabolic rate: the energy your body needs at complete rest just to stay alive. If you lay still in a comfortable room all day without eating or moving, your BMR is roughly what you would burn.
It is not a small number. For most people, BMR accounts for 60–70% of total daily calorie burn. Your heart, liver, kidneys, and brain run continuously, and that costs energy long before you take a single step.
The Mifflin-St Jeor formula
Published in 1990, Mifflin-St Jeor is the equation most dietitians reach for first. It uses four inputs — weight, height, age, and sex — and tends to be more accurate for modern populations than the older alternatives.
For men
Weight in kg, height in cm, age in years. Result is calories per day.
For women
Identical to the male equation except for the final constant: −161 instead of +5.
Worked example — women
Take a 30-year-old woman weighing 65 kg at a height of 165 cm.
Female, 30 years, 65 kg, 165 cm
Using the Mifflin-St Jeor equation for women.
- 1
Weight term
10 × 65 = 650
- 2
Height term
6.25 × 165 = 1,031.25
- 3
Age term
5 × 30 = 150
- 4
Combine the terms
650 + 1,031.25 − 150 = 1,531.25
- 5
Apply the female constant
1,531.25 − 161 = 1,370.25
Result
BMR ≈ 1,370 calories per day
Worked example — men
Now a 30-year-old man weighing 80 kg at a height of 180 cm.
Male, 30 years, 80 kg, 180 cm
Using the Mifflin-St Jeor equation for men.
- 1
Weight term
10 × 80 = 800
- 2
Height term
6.25 × 180 = 1,125
- 3
Age term
5 × 30 = 150
- 4
Combine the terms
800 + 1,125 − 150 = 1,775
- 5
Apply the male constant
1,775 + 5 = 1,780
Result
BMR ≈ 1,780 calories per day
The Harris-Benedict formula
The Harris-Benedict equationdates from 1919 and was revised in 1984. You'll still see it in older calculators and textbooks. It uses the same four inputs but different coefficients, and it tends to overestimate BMR by around 5% for many people.
Applied to the same 30-year-old man above (80 kg, 180 cm), Harris-Benedict returns roughly 1,853calories against Mifflin-St Jeor's 1,780 — a difference of about 73 calories a day. Small, but it compounds over months.
The Katch-McArdle formula
Katch-McArdle takes a different approach entirely. Rather than adjusting for sex and age, it works from lean body mass— the weight of everything in your body that isn't fat. Since lean tissue drives resting energy use, this is more precise when you have a reliable body fat measurement.
Body fat percentage expressed as a whole number, e.g. 20 for 20%.
80 kg at 20% body fat
Katch-McArdle applies equally to any sex — only lean mass matters.
- 1
Calculate the fat-free fraction
1 − (20 ÷ 100) = 0.80
- 2
Calculate lean body mass
80 × 0.80 = 64 kg LBM
- 3
Apply the lean mass coefficient
21.6 × 64 = 1,382.4
- 4
Add the constant
370 + 1,382.4 = 1,752.4
Result
BMR ≈ 1,752 calories per day
Which formula should you use?
| Formula | Inputs needed | Best for |
|---|---|---|
| Mifflin-St Jeor | Weight, height, age, sex | Almost everyone — the current default recommendation |
| Katch-McArdle | Weight, body fat % | People with an accurate body composition measurement |
| Harris-Benedict | Weight, height, age, sex | Comparing against older sources; tends to run high |
If you're unsure, use Mifflin-St Jeor. The gap between these equations is typically 50–150 calories a day, which matters far less than whether you accurately track what you eat against the target you set.
What changes your BMR
- Lean muscle mass. The single biggest lever you can actually pull. Muscle burns more energy at rest than fat, so resistance training raises BMR over time.
- Body size. A larger body has more tissue to maintain, which is why weight and height feature in every equation.
- Age. BMR declines gradually through adulthood, largely tracking the loss of lean mass rather than age itself.
- Sex. Men typically carry more lean mass at the same body weight, which is what the
+5and−161constants are approximating. - Prolonged aggressive dieting. Extended severe restriction can push resting expenditure down as the body adapts — one reason crash diets stall.
- Thyroid function and genetics.Real, but largely outside your control. If your measured energy needs are far from predicted, that's worth raising with a doctor.
From BMR to a calorie target
Your BMR alone isn't a calorie goal — it excludes every calorie you burn walking, working, exercising, and digesting. To get a number you can actually eat to, multiply your BMR by an activity factor. That gives you TDEE, your total daily energy expenditure.
- Read the TDEE guide → for the full activity multiplier table and how to set a deficit or surplus.
- Read the BMI guide → if you also want to see where your current weight sits relative to your height.
Frequently asked questions
How do I calculate my BMR?+
The most widely recommended method is the Mifflin-St Jeor equation. Multiply your weight in kilograms by 10, add your height in centimetres multiplied by 6.25, then subtract your age in years multiplied by 5. Add 5 if you are male, or subtract 161 if you are female.
What is a normal BMR?+
Most adult women fall roughly between 1,200 and 1,600 calories per day, and most adult men between 1,500 and 2,000. Your own figure depends on your size, age, sex, and how much muscle you carry, so treat these as broad reference ranges rather than targets.
Which BMR formula is the most accurate?+
For the general population, Mifflin-St Jeor is considered the most reliable of the equations that use only height, weight, age, and sex. If you know your body fat percentage from a reliable measurement, Katch-McArdle is usually more accurate still because it works from lean body mass.
What is the difference between BMR and RMR?+
BMR is measured under strict conditions — fully rested, fasted, and at a comfortable temperature. RMR, resting metabolic rate, is measured under more relaxed conditions and typically comes out around 10 percent higher. In everyday use the two terms are often used interchangeably.
Should I eat fewer calories than my BMR?+
Generally no. Your BMR covers only the energy needed to keep your organs running at rest, before any movement is accounted for. Eating below it for extended periods risks nutrient shortfalls and muscle loss. Build a deficit from your TDEE instead, which already includes your daily activity.
Does BMR decrease with age?+
Yes, gradually. Every BMR equation subtracts a term for age, largely because people tend to lose lean muscle mass over time. Resistance training slows that decline by preserving muscle, which is the tissue that drives resting energy use.
Can I increase my BMR?+
To a modest degree. Building muscle raises resting energy expenditure because muscle is more metabolically active than fat, and staying adequately fed prevents the downward adaptation that comes with prolonged aggressive dieting. The effect is real but gradual, not dramatic.
What is the BMR formula for women?+
Using Mifflin-St Jeor: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161. For a 30-year-old woman weighing 65 kg at 165 cm, that works out to 650 + 1,031.25 − 150 − 161 = about 1,370 calories per day.
What is the BMR formula for men?+
Using Mifflin-St Jeor: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5. For a 30-year-old man weighing 80 kg at 180 cm, that gives 800 + 1,125 − 150 + 5 = about 1,780 calories per day. The only difference from the female equation is the final constant.
How do I calculate BMR in pounds and feet?+
Convert to metric first, since every standard BMR equation expects kilograms and centimetres. Divide pounds by 2.205 to get kilograms, and multiply total inches by 2.54 to get centimetres. So 176 lb at 5 ft 11 in becomes roughly 80 kg and 180 cm, which you then feed into the Mifflin-St Jeor formula.
Why do different BMR calculators give me different numbers?+
Because they use different equations. Mifflin-St Jeor, Harris-Benedict, and Katch-McArdle apply different coefficients, and Harris-Benedict in particular tends to run about 5 percent higher. A spread of 50 to 150 calories between calculators is normal and matters far less than tracking your intake consistently.
Does BMR include exercise?+
No. BMR covers only the energy needed to keep you alive at complete rest — it excludes walking, training, and even digesting food. To account for movement you multiply BMR by an activity factor, which gives you TDEE, the number you actually eat to.
Keep learning
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