BMI
Body mass index estimate.
What does BMI mean?
Body Mass Index (BMI) is a simple screening measure that estimates body fat based on your weight and height. It is widely used by healthcare professionals as a quick way to categorize individuals as underweight, normal weight, overweight, or obese. While BMI does not directly measure body fat, it correlates with more direct measures and is a useful starting point for assessing health risks.
How to calculate BMI
The imperial formula is: BMI = (weight in lbs × 703) / (height in inches)². For example, a person who weighs 160 lbs and is 5'8" (68 inches) has a BMI of (160 × 703) / (68²) = 24.3, which falls in the normal weight range. The standard BMI categories are: Underweight (below 18.5), Normal weight (18.5–24.9), Overweight (25–29.9), and Obese (30 and above).
FAQ
BMI has limitations. It does not distinguish between muscle and fat, so muscular athletes may be classified as overweight despite having low body fat. It also does not account for age, sex, bone density, or fat distribution. It works best as a general screening tool for the average adult population.
The WHO categories are: Underweight (BMI below 18.5), Normal weight (18.5–24.9), Overweight (25.0–29.9), and Obese (30.0 and above). Obesity is further divided into Class I (30–34.9), Class II (35–39.9), and Class III (40+).
For children and teens (ages 2–20), BMI is calculated the same way but interpreted differently using age- and sex-specific percentile charts. A child in the 85th–94th percentile is considered overweight, and at or above the 95th percentile is considered obese.
A BMI between 18.5 and 24.9 is considered normal weight and is generally associated with the lowest health risks. However, BMI alone does not determine health — factors like diet, exercise, waist circumference, and family history also matter.
The metric formula is BMI = weight (kg) / height (m)². To convert: 1 lb = 0.4536 kg, 1 inch = 0.0254 m. For example, 160 lbs = 72.6 kg and 68 inches = 1.727 m, giving BMI = 72.6 / 1.727² = 24.3.
Related calculators
- BMR— Basal metabolic rate.
- Calorie Needs (TDEE)— Total daily energy expenditure.
- Body Fat %— Estimated fat proportion.
- Heart Rate Zone— Target exercise range.