What BMI does and doesn't tell you
Body Mass Index is your weight in kilograms divided by your height in metres squared. It's a fast population-level screen, but it can't tell muscle from fat — so very muscular people often read as "overweight" while looking lean. Use it alongside, not instead of, measures like waist circumference and body composition.
The standard categories
Under 18.5 is underweight, 18.5–24.9 is a healthy weight, 25–29.9 is overweight, and 30+ is classed as obese. If you want to change where you land, start with your calorie target.
How BMI is calculated
Body Mass Index is a simple formula: weight (kg) divided by height (m) squared (in imperial units: 703 × pounds ÷ inches²). The World Health Organization uses these adult ranges:
- Below 18.5 — underweight
- 18.5 to 24.9 — normal weight
- 25 to 29.9 — overweight
- 30 or more — obesity (classes I, II and III from 30, 35 and 40)
Worked example
A person at 80 kg (176 lb) and 1.78 m (5'10"): 1.78 × 1.78 = 3.17; 80 ÷ 3.17 = BMI 25.2, just into the overweight range. At 65 kg (143 lb) and 1.65 m (5'5"): 65 ÷ 2.72 = BMI 23.9, normal weight. The first person would need only about 1.2 kg (2.6 lb) less to reach the normal range — or none at all, if that weight is muscle (see below).
Where BMI gets it right and where it doesn't
BMI was designed to study populations, not to diagnose individuals. Across large groups, a high BMI is linked to more diabetes, high blood pressure and heart disease. For a single person, though, it has important blind spots:
- It can't tell muscle from fat. A strength athlete at 90 kg and 1.80 m has a BMI of 27.8 ("overweight") with little body fat.
- It doesn't show where fat sits. Abdominal fat matters more for health than fat on the hips and thighs, and BMI can't see it.
- It shifts with age and ethnicity. Older adults lose muscle and can have a "normal" BMI with high body fat; in Asian populations risk starts at lower values.
A measurement to pair with it: your waist
Waist circumference is cheap and says a lot. Measure at navel height with a tape, without squeezing. As a general guide, more than 94 cm / 37 in (men) and 80 cm / 31.5 in (women) signals increased risk, and more than 102 cm / 40 in and 88 cm / 35 in, high risk. Another rule of thumb: your waist should be less than half your height.
What to do with your result
If your BMI is above 25 and your waist is high too, a moderate calorie deficit is the first step — the deficit calculator shows how much to eat. If you lift and the high BMI comes from muscle, focus on body composition rather than the scale: the body recomposition calculator helps you lose fat while keeping muscle. And if your BMI is below 18.5, talk to a health professional before changing your diet.
Frequently asked questions
Is BMI accurate for athletes?
Often not — extra muscle inflates the number. Lifters and athletes should lean on body-fat and performance measures instead.
Does BMI differ for men and women?
The formula and adult categories are the same. Body-fat percentage, which differs by sex, is a more nuanced measure.