To calculate your BMR by hand, use Mifflin-St Jeor: for women, BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161; for men, end with + 5 instead of − 161. A 35-year-old woman at 70 kg and 165 cm works out to 1,395 calories a day.
You do not need a formula to get your number — a free bmr calculator does it instantly. But working it out once by hand means you understand what the number means, you can spot when an app has given you a wrong one, and you can recalculate in seconds whenever your weight changes. This guide does exactly that, with one worked example carried through three different formulas.
What You Need Before You Start
You need four numbers, and they must be in the right units, because the formulas only work in metric:
- Weight in kilograms. If you weigh yourself in pounds, divide by 2.2046. Our example woman weighs 70 kg, which is 70 × 2.2046 = 154.3 lb.
- Height in centimetres. If you measure in inches, multiply by 2.54. She is 165 cm tall, which is 165 ÷ 2.54 = 65 inches (5 feet 5 inches).
- Age in years. Use your age today, not the age you were when you last checked. The formula subtracts 5 calories for every year, so a birthday does move the result.
- Sex. The Mifflin-St Jeor and Harris-Benedict formulas have separate versions for men and women, because average body composition differs. The Katch-McArdle formula does not need your sex — but it does need your body fat percentage.
Weigh yourself in the morning, after using the bathroom and before eating, for the most consistent number. Measure your height once properly — standing straight against a wall, without shoes — and reuse it.
Step 1: Calculate BMR with Mifflin-St Jeor
Mifflin-St Jeor, published in 1990, is the formula most dietitians use first. A 2005 review by the Academy of Nutrition and Dietetics found it was the equation most often within 10 percent of measured resting energy. The two versions are:
- Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161
- Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
The only difference is the final constant: −161 for women, +5 for men. That 166-calorie gap reflects average differences in muscle mass and body composition between the sexes.
Work it through for our example — a 35-year-old woman, 70 kg, 165 cm:
- Weight term: 10 × 70 = 700
- Height term: 6.25 × 165 = 1,031.25
- Age term: 5 × 35 = 175
- Combine: 700 + 1,031.25 − 175 − 161 = 1,395.25, or 1,395 calories a day
That is her BMR: the calories her body burns at rest. If you want the background on what that number actually represents, read what BMR is first — then come back here for the arithmetic.
Step 2: Cross-Check with Harris-Benedict
Harris-Benedict is the oldest of the three, first published in 1919 and revised in 1984. It usually runs 50 to 150 calories higher than Mifflin-St Jeor, which makes it a useful cross-check rather than a first choice. The original version for women is:
BMR = 655 + (9.563 × weight in kg) + (1.850 × height in cm) − (4.676 × age)
For the same woman:
- Base: 655
- Weight term: 9.563 × 70 = 669.41
- Height term: 1.850 × 165 = 305.25
- Age term: 4.676 × 35 = 163.66
- Combine: 655 + 669.41 + 305.25 − 163.66 = 1,466 calories a day
That is 71 calories above her Mifflin-St Jeor result of 1,395 — the kind of small gap you should expect. If two formulas disagree by 300 calories or more, recheck the weight conversion first: entering pounds where the formula wants kilograms roughly doubles the weight term.
Step 3: Cross-Check with Katch-McArdle (If You Know Your Body Fat)
Katch-McArdle is the only common formula that uses body fat percentage instead of total weight, which makes it the best choice for very lean or very muscular people, whose BMR depends on lean mass rather than scale weight. It needs just two numbers:
BMR = 370 + (21.6 × lean body mass in kg), where lean body mass = weight × (1 − body fat percentage)
Our example woman has 28 percent body fat:
- Lean mass: 70 × (1 − 0.28) = 70 × 0.72 = 50.4 kg
- BMR: 370 + 21.6 × 50.4 = 370 + 1,088.64 = 1,458.64, or 1,459 calories a day
Her three formulas now read 1,395 (Mifflin-St Jeor), 1,466 (Harris-Benedict), and 1,459 (Katch-McArdle). A free basal metabolic rate calculator shows all three side by side, and if you know your body fat percentage, the same page’s body fat calculator result feeds straight into the Katch-McArdle figure. Use Mifflin-St Jeor as your headline number unless you are notably lean or muscular — in that case, trust Katch-McArdle more.
The Three Formulas Compared
| Formula | Needs | Our example (35F, 70 kg, 165 cm) | Best for |
|---|---|---|---|
| Mifflin-St Jeor (1990) | Weight, height, age, sex | 1,395 | Most adults — the default |
| Harris-Benedict (1919, rev. 1984) | Weight, height, age, sex | 1,466 | Cross-checking; older charts |
| Katch-McArdle | Weight, body fat % | 1,459 | Lean or muscular people |
The practical lesson: the formula you pick matters far less than most people think. The spread here is 71 calories — about 5 percent — while your activity level moves the final answer by hundreds.
Step 4: Turn BMR into Daily Calories (TDEE)
BMR is what you would burn lying in bed all day. Real life adds movement, so multiply BMR by an activity factor to get your total daily energy expenditure (TDEE) — the number you actually plan meals around:
| Activity level | What it looks like | Multiplier | Her TDEE (BMR 1,395) |
|---|---|---|---|
| Sedentary | Desk job, little or no exercise | 1.2 | 1,674 |
| Lightly active | Light exercise 1–3 days a week | 1.375 | 1,918 |
| Moderately active | Moderate exercise 3–5 days a week | 1.55 | 2,163 |
| Very active | Hard exercise 6–7 days a week | 1.725 | 2,407 |
| Extremely active | Physical job plus daily training | 1.9 | 2,651 |
Her lightly active TDEE is 1,395 × 1.375 = 1,918.47, or 1,918 calories a day. Eat about that and her weight stays roughly stable; eat 500 less (about 1,418) and she loses roughly a pound a week, since a pound of body fat stores about 3,500 calories. Be honest about the activity level — most people pick one tier too high, which inflates the target and quietly erases the deficit.
A Second Worked Example: A 30-Year-Old Man
The same steps work for men, with the +5 constant. Take a 30-year-old man weighing 80 kg and 180 cm tall:
- Weight term: 10 × 80 = 800
- Height term: 6.25 × 180 = 1,125
- Age term: 5 × 30 = 150
- Combine: 800 + 1,125 − 150 + 5 = 1,780 calories a day
His lightly active TDEE is 1,780 × 1.375 = 2,447.5, or 2,448 calories a day — about 530 more than the woman in the first example, simply because he is larger and male. That gap is why copying someone else’s calorie target never works. For context on the other half of the picture, check your body mass index with our free BMI calculator.
Five Mistakes That Give You the Wrong BMR
- Using pounds instead of kilograms. The formulas want kilograms. Entering 154 lb as if it were 154 kg adds 840 phantom calories to the weight term alone.
- Using feet and inches instead of centimetres. Convert first: 5 feet 5 inches is 65 inches, and 65 × 2.54 = 165 cm. Entering 65 where the formula wants centimetres collapses the height term.
- Using the wrong sex constant. The gap between +5 and −161 is 166 calories — enough to erase a small deficit on its own.
- Treating BMR as your eating target. BMR is the floor, not the target. Plan from TDEE, and stay above the 1,200-calorie (women) or 1,500-calorie (men) floor unless a doctor advises otherwise. Our guide to BMR vs TDEE explains the difference in full, and our calorie calculator turns your TDEE into a weight-loss target.
- Forgetting to recalculate. Every kilogram lost cuts BMR by about 10 calories a day. After losing 10 kg, your BMR is roughly 100 calories lower — recalculate with your new weight or the scale will seem to stall for no reason.
FAQ: How to Calculate BMR
What is the easiest way to calculate BMR by hand?
Use Mifflin-St Jeor: for women, (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161; for men, the same formula ending in + 5. A 35-year-old woman at 70 kg and 165 cm gets 1,395 calories a day.
Which BMR formula is most accurate?
Mifflin-St Jeor is generally considered the most accurate for most adults. Katch-McArdle is better if you are very lean or muscular and know your body fat percentage.
How do I calculate BMR without knowing my body fat percentage?
Use Mifflin-St Jeor or Harris-Benedict — neither needs body fat. Only Katch-McArdle requires it. If you want the lean-mass version, estimate your body fat first or use the Katch-McArdle result on our calculator page.
Why does my BMR differ between calculators?
Different calculators use different formulas, and Harris-Benedict typically runs 50 to 150 calories above Mifflin-St Jeor. Small gaps are normal. A gap of several hundred calories usually means a unit error — check that weight is in kilograms and height in centimetres.
How often should I recalculate my BMR?
Recalculate whenever your weight changes by more than about 5 kg, or once every few months while you are actively losing or gaining weight. Age alone moves BMR by only about 5 calories a year, so birthdays matter far less than weight changes.
Do I need a calculator, or can I just use a formula?
The formula is the calculator — a free BMR calculator simply runs the same arithmetic and adds the TDEE step. Use the tool for speed, and use the hand calculation here to check that the tool’s answer makes sense.