BMI Calculator
Calculate body mass index with metric and imperial units.
How to Use the BMI Calculator
Body Mass Index (BMI) is a simple numerical measure derived from a person's weight and height. It's widely used as a screening tool to categorize individuals as underweight, normal weight, overweight, or obese, and to identify potential health risks associated with these weight categories.
Metric vs. Imperial
Click the Metric tab to enter height in centimeters and weight in kilograms. Click Imperial to enter height in feet and inches, and weight in pounds. The calculator automatically converts imperial measurements to metric before computing BMI.
BMI Formula
The formula is: BMI = weight(kg) / height(m)². For imperial: weight must be converted from pounds to kilograms (1 lb = 0.453592 kg) and height from inches to meters (1 inch = 0.0254 m).
BMI Categories
- Below 18.5 — Underweight: May indicate nutritional deficiency or other health issues.
- 18.5–24.9 — Normal weight: Associated with the lowest health risks for most people.
- 25.0–29.9 — Overweight: Moderate increased risk for some health conditions.
- 30.0 and above — Obese: Significantly increased risk for cardiovascular disease, type 2 diabetes, and other conditions.
Limitations of BMI
BMI is a useful screening tool but is not a diagnostic measure. It does not account for muscle mass (athletes often have high BMI despite low body fat), bone density, age, sex, or ethnicity. A person can have a "normal" BMI but high body fat percentage, or a high BMI due to muscle mass rather than fat. Always consult a healthcare professional for a complete assessment of your health status.
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Frequently Asked Questions
What is a healthy BMI?
According to the World Health Organization (WHO), a BMI between 18.5 and 24.9 is considered a healthy weight range for adults. A BMI below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obese. However, these thresholds may not be appropriate for all ethnic groups — some health organizations suggest lower thresholds for Asian populations.
Is BMI accurate for athletes and muscular people?
BMI is less accurate for people with high muscle mass. Since muscle weighs more than fat per unit volume, a very muscular person (bodybuilder, football player) can have a BMI in the "overweight" or "obese" range while having a low percentage of body fat. In these cases, other measures like body fat percentage or waist circumference provide a more accurate health assessment.
Does BMI apply to children?
BMI is used for children and teens but is interpreted differently — as a BMI-for-age percentile rather than absolute thresholds. Because children's body composition changes with age and differs between boys and girls, the same BMI number can mean different things at different ages. This calculator uses adult BMI thresholds only.
How is the healthy weight range calculated?
The healthy weight range is calculated by finding the weights that correspond to BMI values of 18.5 (lower bound) and 24.9 (upper bound) for your entered height. The formula used is: weight = BMI × height(m)². For example, for a height of 1.70m, the range is 18.5 × 1.70² = 53.5 kg to 24.9 × 1.70² = 71.9 kg.
Should I be concerned if my BMI is slightly outside the normal range?
A BMI slightly above or below the normal range doesn't necessarily indicate a health problem. BMI is a population-level screening tool, not an individual diagnostic. Overall lifestyle factors — diet quality, physical activity, sleep, stress management, and non-smoking — are stronger predictors of health than BMI alone. If your BMI is significantly outside the normal range, consult a healthcare provider for a comprehensive evaluation.