BMI Calculator
Body Mass Index compares your weight to your height. It is a quick screening number — useful for spotting a trend, but blind to how much of your weight is muscle.
Reading the number
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
The formula is simply weight in kilograms divided by height in metres squared. It was designed in the 19th century to describe populations, and that is still what it does best.
Where BMI gets it wrong
BMI knows your weight. It has no idea what that weight is made of. A lean, muscular athlete routinely reads "overweight" while carrying very little fat. Someone with low muscle mass can sit in the healthy band while carrying more fat than is good for them — sometimes called normal-weight obesity.
Add one measurement and BMI gets far more useful. Waist circumference at the navel: above 94 cm (37 in) for men or 80 cm (31.5 in) for women is the point where risk starts climbing, whatever the BMI says. Two numbers beat one.
What to do with it
Use BMI as a starting flag, not a diagnosis. If it sits outside the healthy band, the useful next steps are a body-fat estimate and your daily calorie needs — those tell you what to change. Try the body fat calculator and the calorie calculator.
And track the direction, not the decimal. A BMI moving from 29 to 27 over three months is real progress, regardless of which label either number carries.
Frequently asked questions
Does BMI work for athletes?
Poorly. Muscle is denser than fat, so heavily trained people often read overweight while being genuinely lean. Use body fat percentage and waist measurement instead.
Is BMI valid for children?
Not in this form. Children and teens are assessed on age-and-sex percentile charts, not the fixed adult categories above.
What is a good BMI?
For most adults, 18.5 to 24.9. But a stable weight, a healthy waist measurement and good bloodwork matter more than landing on a specific decimal.