BMI (Body Mass Index) is a numerical value derived from dividing a person's weight by the square of their height. It is used as a general screening tool for weight categories.
How BMI Works
The formula is: BMI = weight (kg) ÷ height (m)² or equivalently BMI = weight (lbs) × 703 ÷ height (in)². The result places you in one of four categories:
- Underweight: BMI below 18.5
- Normal: BMI 18.5–24.9
- Overweight: BMI 25.0–29.9
- Obese: BMI 30.0 and above
Limitations of BMI
BMI does not distinguish between muscle and fat. A muscular athlete can have a "overweight" BMI despite having low body fat. BMI also does not account for fat distribution — abdominal fat is more dangerous than fat carried elsewhere. For a more complete picture, consider body fat percentage, waist circumference, or waist-to-hip ratio.
Real-World Example
A person who is 5'10" (70 inches) and weighs 175 lbs: BMI = 175 × 703 ÷ (70)² = 122,925 ÷ 4,900 = 25.1 — just into the "overweight" category, though this person may be perfectly healthy depending on body composition.
Frequently Asked Questions
Is BMI accurate?
BMI is a useful population-level screening tool but has significant individual limitations. It overestimates body fat in muscular people and underestimates it in older adults who have lost muscle mass. It should be one data point among several.
What is a healthy BMI?
The "normal" range is 18.5-24.9. However, health depends on many factors beyond BMI, including fitness level, diet quality, blood pressure, cholesterol, and family history. A BMI of 26 in a fit person may be healthier than a BMI of 22 in a sedentary person.
Is BMI different for children?
Yes. For children and teens (ages 2-20), BMI is plotted on age-and-sex-specific growth charts and expressed as a percentile rather than an absolute number, because body composition changes significantly during growth.