What is BMI?
Body Mass Index (BMI) is a single number that places your weight in context against your height. Doctors and public-health agencies use it as a quick, low-cost screen for weight-related health risk in adults — not as a diagnosis, but as a signal of when a closer look may be worthwhile. It is calculated as:
BMI = weight (kg) ÷ height² (m²)
Example: BMI for a 70 kg, 175 cm adult
Take someone who weighs 70 kg and stands 175 cm (1.75 m). Square the height: 1.75 × 1.75 = 3.0625 m². Then divide: 70 ÷ 3.0625 = 22.9. A BMI of 22.9 sits inside the 18.5–24.9 band, so this person falls in the normal weight range.
BMI Categories
The World Health Organization defines the following adult BMI categories:
Underweight — BMI below 18.5
May indicate nutritional deficiency or an underlying health condition. Consult a healthcare professional.
Normal weight — BMI 18.5 to 24.9
Associated with the lowest health risk for most adults.
Overweight — BMI 25 to 29.9
Increased risk of developing cardiovascular disease, type 2 diabetes, and other conditions.
Obese — BMI 30 and above
Higher risk of serious health conditions. Class I (30–34.9), Class II (35–39.9), and Class III (40+) indicate increasing severity.
Limitations of BMI
BMI is a screening tool, not a diagnostic measure. Because it works only from height and weight, it cannot tell muscle from fat: it may flag a muscular athlete as overweight, or rate an older adult with little muscle as normal weight even when body-fat is high. It also says nothing about where fat is stored (abdominal fat carries more risk), and it is less reliable for children, pregnant people, the very tall or very short, and across different ethnic groups. Treat your result as a starting point, not a verdict — and confirm any concern with a healthcare provider who can weigh in factors BMI ignores.
Frequently Asked Questions
What is BMI?
BMI (Body Mass Index) is a number calculated from your height and weight. It is used as a screening tool to identify possible weight problems in adults.
What is a healthy BMI?
A BMI between 18.5 and 24.9 is considered normal weight for most adults. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obese.
Is BMI accurate?
BMI is a useful screening tool but has limitations. It does not directly measure body fat or account for muscle mass, bone density, age, or sex differences in fat distribution. Consult a healthcare professional for a full health assessment.