BMI Calculator
Calculate your Body Mass Index and see where you fall on the WHO scale.
Step 1 of 1
Enter your height and weight
How This Calculator Works
The BMI Formula
Body Mass Index is calculated using a simple formula established by Adolphe Quetelet in the 1830s and adopted by the WHO as a population-level screening tool:
BMI = weight (kg) ÷ height (m)²
Imperial: BMI = (weight in lbs × 703) ÷ (height in inches)²
What Your BMI Means
The WHO classifies BMI into categories based on population studies linking body weight to health outcomes. A higher BMI is associated with increased risk of type 2 diabetes, cardiovascular disease, and certain cancers, while a very low BMI can indicate malnutrition or other health concerns.
Limitations to Consider
BMI does not measure body fat directly. It cannot distinguish between muscle, bone, and fat mass, which means muscular individuals may receive an elevated BMI despite having a healthy body composition. Factors like age, sex, ethnicity, and fitness level all influence what a healthy weight looks like. For a more complete assessment, consider combining BMI with waist circumference and body fat percentage measurements.
Frequently Asked Questions
What is BMI and how is it calculated?
BMI (Body Mass Index) is calculated by dividing your weight in kilograms by your height in meters squared: BMI = weight (kg) / height (m)². For example, a person weighing 70 kg who is 1.75 m tall has a BMI of 70 / (1.75 × 1.75) = 22.9. The World Health Organization (WHO) uses BMI as a simple screening tool for weight categories.
Is BMI accurate for everyone?
BMI has significant limitations. It does not distinguish between muscle mass and fat mass, meaning athletes or muscular individuals may be classified as overweight despite having low body fat. BMI also does not account for age, sex, ethnicity, or body fat distribution. Research published in the International Journal of Obesity (2016) found that nearly 75 million US adults were misclassified by BMI alone. It is a useful screening tool but not a definitive measure of health.
What are the WHO BMI categories?
The World Health Organization defines six BMI categories: Underweight (below 18.5), Normal weight (18.5–24.9), Overweight (25.0–29.9), Obese Class I (30.0–34.9), Obese Class II (35.0–39.9), and Obese Class III (40.0 and above). These ranges were established based on population studies linking BMI to health outcomes and mortality risk.
What is a healthy BMI range?
A BMI between 18.5 and 24.9 is considered normal weight by the WHO. According to a 2019 meta-analysis in The BMJ, the lowest all-cause mortality risk is associated with a BMI of approximately 22–23. However, optimal BMI can vary by age and ethnicity — for example, the WHO suggests lower cutoff points for Asian populations (23 for overweight, 27.5 for obese).
How often should I check my BMI?
For most adults, checking BMI every few months is sufficient unless you are actively working on weight management. BMI is best used as one data point among many — combine it with waist circumference, body fat percentage, blood pressure, and other health markers for a more complete picture. The KCALM app can help you track your weight trends over time.
Does BMI apply to children and teenagers?
This calculator is designed for adults aged 20 and older. For children and teens (ages 2–19), BMI is calculated the same way but interpreted using age- and sex-specific percentile charts from the CDC. A child’s BMI percentile shows how their BMI compares to other children of the same age and sex.
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