What is BMI (Body Mass Index)?
Body Mass Index (BMI) is a numerical value calculated from a person's weight and height. It is a widely used screening tool to categorize individuals into weight categories — underweight, normal weight, overweight, and obese — which may indicate potential health risks. The BMI formula divides a person's weight in kilograms by the square of their height in meters (kg/m²).
BMI was developed by Belgian mathematician Adolphe Quetelet in the 19th century and has been used by healthcare professionals worldwide for decades as a quick, inexpensive screening method. While it has limitations, it remains a useful first indicator when assessing weight-related health risks at the population level.
BMI Formula
- Metric: BMI = Weight (kg) ÷ Height² (m²)
- Imperial: BMI = [Weight (lbs) ÷ Height² (inches²)] × 703
BMI Categories (WHO Standard)
- Below 18.5 — Underweight: May indicate malnutrition, eating disorders, or other health conditions. Medical consultation is recommended.
- 18.5 to 24.9 — Normal Weight: Associated with the lowest health risks for most adults. Maintain through balanced diet and regular exercise.
- 25.0 to 29.9 — Overweight: Elevated risk for certain conditions like type 2 diabetes, hypertension, and heart disease. Lifestyle changes may be beneficial.
- 30.0 to 34.9 — Obese Class I: High risk. Medical consultation recommended to discuss weight management strategies.
- 35.0 to 39.9 — Obese Class II: Very high risk. Medical intervention is typically recommended.
- 40.0 and above — Obese Class III (Morbid Obesity): Extremely high risk. Immediate medical attention is strongly advised.
Limitations of BMI
While BMI is a useful general screening tool, it has several well-known limitations that you should be aware of:
- Does not distinguish muscle from fat: Athletes and bodybuilders with high muscle mass may have a high BMI but very low body fat percentage.
- Does not account for age: Older adults naturally lose muscle and gain fat, so the same BMI may indicate different health risks at different ages.
- Does not account for gender: Women naturally carry more body fat than men at the same BMI level.
- Does not consider fat distribution: Where fat is stored (abdomen vs hips) matters greatly for health risk, which BMI does not capture.
- Not accurate for children: Children should use age and gender-specific BMI percentile charts, not standard adult BMI categories.
- Ethnicity differences: Health risks associated with BMI levels can differ across ethnic groups. South Asian and East Asian populations may face higher risks at lower BMI values.
Frequently Asked Questions
Is BMI an accurate measure of health?+
BMI is a useful screening tool but not a direct measure of health or body fat percentage. It is best used as one of several indicators alongside waist circumference, blood pressure, cholesterol, blood sugar levels, and overall lifestyle factors. Always consult a healthcare professional for a complete health assessment.
What is the ideal BMI for an adult?+
According to the World Health Organization (WHO), a BMI between 18.5 and 24.9 is considered normal and associated with the lowest health risks for most adults. However, "ideal" BMI can vary by age, sex, ethnicity, and individual health factors.
Can I use this BMI calculator for children?+
No. This calculator uses adult BMI categories. For children and teenagers (ages 2–19), BMI must be interpreted using age and sex-specific growth charts. Please consult a pediatrician for accurate assessment of a child's weight status.
Should I use metric or imperial units?+
Use whichever unit system you are most comfortable with — both give identical results. Select Metric if you know your weight in kilograms and height in centimeters. Select Imperial if you know your weight in pounds and height in feet and inches.
I am an athlete with high muscle mass. Is BMI accurate for me?+
BMI is not accurate for athletes and bodybuilders. Muscle weighs more than fat, so athletes often have a high BMI despite having very low body fat. For athletes, methods like DEXA scans, skinfold measurements, or hydrostatic weighing give more accurate body composition data.
How much should I weigh for my height?+
Our BMI calculator shows your healthy weight range based on a BMI of 18.5 to 24.9. This gives you a target weight range corresponding to normal BMI for your specific height. The result is displayed after you calculate your BMI.