The four WHO weight categories
Every BMI result falls into one of four bands defined by the World Health Organization. The bands are the same for adult men and women aged 20 and over:
Under 18.5
Underweight
18.5 – 24.9
Normal weight
25 – 29.9
Overweight
30 and above
Obese
Drifting a point or two inside a band means very little. Crossing a boundary is the signal worth paying attention to, because that is where the associated health research actually changes. The healthy weight range this calculator shows is simply the span of weights that would keep you between 18.5 and 24.9 at your current height.
Working the formula by hand
BMI is your weight in kilograms divided by your height in metres squared. The squaring is the part people get wrong — it is height × height, not height × 2.
Someone 1.70 m tall weighing 65 kg
- Height squared
- 1.70 × 1.70 = 2.89
- Weight ÷ height²
- 65 ÷ 2.89
- Category
- Normal weight
BMI = 22.5
In imperial units the same relationship needs a conversion factor: pounds divided by inches squared, multiplied by 703. The calculator above handles either system, and converts your healthy weight range back into whichever units you entered.
Where BMI misleads
BMI was built to describe populations, not individuals, and it only knows two things about you. That makes it a fast first screen and a poor final verdict. The situations below are where it most often gets a person wrong:
Muscular builds get flagged as overweight
Muscle is denser than fat, so athletes and regular lifters often land in the overweight or obese bands while carrying very little fat. Body fat percentage is the better measure here.
Older adults can look healthier than they are
Muscle mass falls with age while fat can rise. A BMI in the normal band may hide a body composition that has shifted considerably, which is why waist measurement is a useful companion.
It was never designed for children or pregnancy
Children are assessed against age-and-sex percentile charts instead, and BMI is not meaningful during pregnancy. Neither group should read the adult bands.
The same BMI means different things by sex
At an identical BMI, women typically carry more body fat than men. The bands do not adjust for this, which is another argument for reading BMI alongside other measures.
What to pair it with
Two measurements cover most of what BMI misses, and both are quick. Waist circumference captures abdominal fat, which carries more health risk than fat elsewhere. Body fat percentage separates the muscle from the fat that BMI lumps together. Tracked over months, the direction those numbers move tells you far more than any single reading — and if a result concerns you, a healthcare professional can interpret it against your full history rather than two numbers.