BMI calculator
Calculate your body mass index (BMI) from height and weight. Separately for men and women, with body-fat estimate and healthy-weight range.
Where and how to measure your wrist?
Measure your wrist circumference just above the wrist bone (closer to the hand), where the wrist is narrowest. A tailor's tape or a piece of string against a ruler works fine. Don't pull tight, just wrap around. Wrist size helps estimate how much of your weight is skeletal frame.
Enter your height and weight – the result appears instantly.
What BMI is and how it's calculated
BMI (body mass index) is a simple number that relates weight to height and gives a rough indication of whether a person's weight is appropriate. It is calculated as weight in kilograms divided by height in meters squared:
BMI = weight (kg) ÷ height (m)²
Example: a person 175 cm (1.75 m) tall weighing 70 kg has a BMI of 70 ÷ 1.75² = 70 ÷ 3.0625 ≈ 22.9, which falls in the normal-weight range.
BMI categories
The World Health Organization (WHO) divides adult BMI into these ranges:
| Category | BMI |
|---|---|
| Underweight | below 18.5 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25 – 29.9 |
| Obesity class 1 | 30 – 34.9 |
| Obesity class 2 | 35 – 39.9 |
| Obesity class 3 | 40 and above |
Does BMI differ for men and women?
The formula itself and the category boundaries are the same for men and women – 25 kg/m² is the overweight threshold for both. The difference is in body composition: at the same BMI, women on average have a higher body-fat percentage than men, who carry more muscle mass. That's why, when you enter your age in the calculator, it shows a body-fat estimate – and switching sex shows how much it differs even though the BMI stays the same.
Body-fat percentage estimate
Body-fat percentage describes body composition better than BMI alone. The calculator estimates it from BMI, age and sex using the Deurenberg equation:
% fat = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4
(sex = 1 for male, 0 for female). This is only a rough population-level estimate, not a measurement. Actual body fat is measured by a device – a bioimpedance scale or, more accurately, a DEXA scan. For muscular people the equation tends to overestimate fat (muscle counts the same as in less-trained people), and underestimate it for very lean people.
Body frame from wrist circumference
The Deurenberg equation has no idea how robust your skeleton is – and a person with a sturdier frame carries more lean mass at the same BMI. That's why you can enter your wrist circumference: the ratio of height to wrist size gives a rough estimate of frame size (small, medium, large), and the fat estimate is adjusted slightly. The wrist works well because it's barely affected by fat – it essentially measures the skeleton. There are three body-frame types:
- Ectomorph narrow frame, leaner build – fat estimate adjusted slightly up
- Mesomorph medium, athletic build – no adjustment
- Endomorph sturdy frame – fat estimate adjusted slightly down
This is a simplification – actual somatotype is a spectrum and is measured with more dimensions. But even this simple correction shows why BMI alone says little about body composition.
Limitations of BMI
BMI is a population-level indicator, not a diagnosis. It doesn't account for body composition, fat distribution, age, ethnicity, or training level. For athletes, pregnant women, children and the elderly it has limited value. To assess health it's worth adding other measures (waist circumference, body-fat percentage) and consulting a doctor.
Related
Note: the BMI calculator is for informational purposes only and does not replace professional medical examination. If you have concerns about your weight, consult a doctor.