BMI Calculator — Check Your Body Mass Index Instantly

What is BMI?

Body Mass Index (BMI) is a simple numerical value derived from your weight and height. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, BMI is widely used by healthcare professionals as a quick screening tool to categorize individuals into weight groups — underweight, normal weight, overweight, or obese. While it does not measure body fat directly, BMI serves as a reliable indicator of potential health risks associated with weight.

The World Health Organization (WHO) and national health agencies around the world use BMI as a standard metric for population-level health assessments. It remains one of the simplest and most accessible tools for adults to monitor their weight status relative to their height.

BMI Formula — Metric and Imperial

BMI is calculated using one of two formulas depending on the unit system:

Example (Metric): A person weighing 70 kg and 170 cm (1.70 m) tall has a BMI of 70 / (1.70)² = 70 / 2.89 = 24.2 (Normal weight).

Example (Imperial): A person weighing 154 lbs and 67 inches tall has a BMI of (154 × 703) / (67)² = 108,262 / 4,489 = 24.1 (Normal weight).

BMI Categories Chart

The WHO defines the following BMI categories for adults (18 years and older):

• Underweight — BMI less than 18.5. May indicate malnutrition, eating disorders, or other health conditions. Associated with weakened immune system, bone loss, and anemia.
• Normal Weight — BMI 18.5 to 24.9. Associated with the lowest risk of weight-related health problems. The ideal BMI midpoint is approximately 21.7.
• Overweight — BMI 25 to 29.9. Indicates excess weight that may increase risk of cardiovascular disease, type 2 diabetes, and joint problems.
• Obese Class I — BMI 30 to 34.9. Moderate obesity with significantly increased health risks including hypertension, sleep apnea, and metabolic syndrome.
• Obese Class II — BMI 35 to 39.9. Severe obesity with high risk of serious health complications.
• Obese Class III — BMI 40 and above. Morbid obesity with very high risk of life-threatening conditions. Medical intervention is often recommended.

Healthy Weight Range

Your healthy weight range depends on your height. It is calculated by finding the weight values that correspond to a BMI of 18.5 and 24.9 for your height. For example, a person who is 170 cm tall has a healthy weight range of approximately 53.5 kg to 72.0 kg. Staying within this range reduces the risk of chronic diseases and promotes overall well-being.

Limitations of BMI

While BMI is a useful and widely accepted screening tool, it has several important limitations:

• Does not differentiate muscle from fat — Athletes and bodybuilders may be classified as overweight or obese despite having low body fat percentage.
• Does not account for body fat distribution — Abdominal (visceral) fat is more dangerous than fat stored in hips and thighs. BMI cannot distinguish between the two.
• Age and gender differences — Older adults tend to have more body fat than younger adults at the same BMI. Women naturally carry more body fat than men.
• Ethnic variations — Some populations (e.g., South Asian, East Asian) may have higher health risks at lower BMI values. The WHO suggests lower BMI cutoffs for Asian populations.
• Not suitable for children — BMI-for-age percentile charts should be used for individuals under 18 years old.

For a more comprehensive health assessment, combine BMI with other measurements such as waist circumference, waist-to-hip ratio, body fat percentage, and blood pressure/cholesterol levels.