How To Calculate BMR: Step-by-Step Guide
If you already know what BMR represents, this guide skips the theory and goes straight to the math: the exact formula used by most modern calculators, a full worked example, and the mistakes that most often throw the result off.
What Information Do You Need?
- Your age in years.
- Your biological sex.
- Your height in centimeters.
- Your current weight in kilograms.
Use your most recent weight — a figure from six months ago will skew the result more than most people expect.
The Mifflin-St Jeor Formula
Most reliable online calculators, including this site's, use the Mifflin-St Jeor equation, which replaced the older Harris-Benedict formula in most clinical settings because it tends to match real-world measurements more closely.
For men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
For women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161
Worked Example — Man
Age 30, height 175 cm, weight 70 kg:
(10 × 70) + (6.25 × 175) − (5 × 30) + 5
= 700 + 1093.75 − 150 + 5 = 1648.75
This man's estimated BMR is approximately 1649 calories per day — the energy his body needs before any movement is counted.
Worked Example — Woman
Age 28, height 162 cm, weight 60 kg:
(10 × 60) + (6.25 × 162) − (5 × 28) − 161
= 600 + 1012.5 − 140 − 161 = 1311.5
This woman's estimated BMR is approximately 1312 calories per day.
Why Two Different Formulas for Men and Women?
The −5 versus −161 adjustment exists because, on average, men carry a higher proportion of muscle mass at the same height and weight, and muscle burns more calories at rest than fat does. The formula bakes this average difference in rather than requiring a separate muscle-mass measurement.
Common Calculation Mistakes
Using outdated weight. If you've gained or lost more than a few kilograms since your last calculation, the result will drift from reality — recalculate after any meaningful weight change.
Mixing up units. Entering height in inches into a centimeter-based formula (or the reverse) produces a wildly wrong number. Convert first, or use a calculator that lets you pick your unit.
Treating BMR as your eating target. The number this formula produces is what your body needs at complete rest. It is not how many calories to eat — that figure comes from multiplying BMR by an activity factor to get TDEE.
Rounding too early. Rounding height or weight before plugging them into the formula compounds small errors. Use your exact measurements and round only the final result.
Older Formulas You Might Still See
Some calculators still use the original 1919 Harris-Benedict equation or its 1984 revision. Both remain reasonably accurate but tend to overestimate BMR slightly compared to Mifflin-St Jeor, particularly in people with higher body fat percentages. If two calculators give you different numbers, check which formula each one uses before assuming either is wrong.
What to Do With Your BMR Number
On its own, BMR only describes resting energy needs. To plan actual daily eating, multiply it by an activity multiplier — 1.2 for a mostly sedentary day up to around 1.9 for very heavy physical work — to get your Total Daily Energy Expenditure (TDEE). That TDEE figure, not the raw BMR, is what most nutrition plans should be built around.
Frequently Asked Questions (FAQ)
Which formula is most accurate — Mifflin-St Jeor or Harris-Benedict?
Mifflin-St Jeor is generally considered the more accurate of the two for most adults and is the version most current calculators default to.
Do I need a lab test to get an accurate BMR?
No. Formula-based estimates are close enough for everyday nutrition planning. Lab-based indirect calorimetry exists but is rarely necessary outside clinical or research settings.
What if my calculated BMR seems too low or too high?
Double-check that your height and weight were entered in the correct units and that your age is current. If the number still seems off after that, individual metabolic variation is normal and a healthcare provider can help interpret it.
Does the formula change for people over 60?
The same equation is used, though its accuracy can decrease slightly in older adults due to natural changes in muscle mass that the formula doesn't directly measure.
How precise is "approximately" in these calculations?
Formula-based BMR is typically within about 10% of measured resting metabolic rate for most healthy adults — close enough to plan around, but not a clinical diagnosis.
Conclusion
The Mifflin-St Jeor formula turns four simple measurements into a usable daily calorie baseline in under a minute. Get your height, weight, and age right, apply the correct version for your sex, and you have a solid starting point — just remember the result is your resting number, not your eating target.
Medical Disclaimer
This article is intended for educational purposes only and should not be considered medical advice, diagnosis, or treatment. BMR calculations provide estimates and cannot account for every individual factor or medical condition. Always consult a qualified healthcare professional or registered dietitian before making significant changes to your diet, exercise routine, or weight management plan.