BMI Calculator

BMI (Body Mass Index) is a value derived from a person's weight and height. It is calculated as weight (kg) divided by height squared (m²). BMI is used as a screening tool to categorise weight status.

The standard BMI categories are: Underweight (BMI below 18.5), Normal weight (18.5–24.9), Overweight (25–29.9), and Obese (30 and above).

BMI is a useful screening tool but not a direct measure of body fat or health. It does not account for muscle mass, bone density, age, or fat distribution. Athletes, for example, may have a high BMI due to muscle rather than fat.

The metric formula is BMI = weight (kg) ÷ height² (m²). The imperial formula is BMI = 703 × weight (lbs) ÷ height² (inches²). Both give the same result.

For most adults, a BMI between 18.5 and 24.9 is considered healthy. A BMI below 18.5 may indicate undernutrition with associated health risks such as weakened immunity and bone loss. A BMI of 25–29.9 is overweight, and 30 or above is classified as obese with increased risk of heart disease, type 2 diabetes, and other conditions.

BMI decreases when you lose body weight. The most effective approach combines a moderate caloric deficit (eating 300–500 kcal fewer per day than you burn), regular aerobic exercise (150+ minutes per week), strength training to preserve muscle mass, and adequate sleep. Rapid crash diets are counterproductive as they often cause muscle loss and metabolic slowdown.

Yes. Obesity is further divided into three classes: Class I (BMI 30–34.9) — moderate health risk; Class II (BMI 35–39.9) — high health risk; Class III or "severe obesity" (BMI 40+) — very high health risk. Class III obesity is associated with significantly elevated rates of cardiovascular disease, sleep apnea, and type 2 diabetes.

The same BMI scale applies to both men and women. However, women naturally have a higher percentage of body fat than men at the same BMI. A BMI of 22 may represent 18% body fat in a man but 25% in a woman. This is why body fat percentage measurement provides more nuanced information than BMI alone.

BMI tends to underestimate health risks in older adults because muscle mass naturally declines with age (sarcopenia), while fat mass increases — without the total weight necessarily changing. Many researchers suggest that a BMI of 22–27 may be more appropriate for adults over 65, and waist circumference is a useful additional marker.

No. For children and teenagers (ages 2–19), BMI is interpreted differently using BMI-for-age percentiles on growth charts. A child at the 85th–95th percentile is considered overweight, and above the 95th percentile is obese. Adult BMI categories do not apply to children.