How this BMI calculator works
Body Mass Index (BMI) is a simple ratio of your weight to your height. It was designed as a quick way to screen large populations for weight ranges that may be associated with health risks. The formula is the same everywhere: take your weight in kilograms and divide it by your height in metres squared.
If you enter imperial units, the calculator first converts them: pounds are multiplied by 0.453592 to get kilograms, and your total height in inches (feet × 12 + inches) is multiplied by 0.0254 to get metres. It then uses the same kg/m² formula, so metric and imperial entries give the same result.
Disclaimer: This tool is for general information and education only. It is not medical advice and does not diagnose, treat, or provide a measure of overall health. BMI does not distinguish muscle from fat and may not suit athletes, pregnant people, children, or older adults. For guidance about your weight or health, consult a qualified healthcare professional.
Frequently asked questions
- How is BMI calculated?
- BMI is your weight in kilograms divided by the square of your height in metres (kg/m²). For imperial units, convert pounds to kilograms (lb × 0.453592) and height to metres (total inches × 0.0254), then apply the same formula. For example, 70 kg at 1.75 m is 70 ÷ (1.75 × 1.75) = 22.9.
- What is a healthy BMI?
- The World Health Organization considers a BMI from 18.5 to 24.9 to be in the normal or healthy range for most adults. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is classed as obese.
- Is BMI accurate for athletes?
- Not always. BMI only uses height and weight, so it cannot tell muscle from fat. Very muscular people such as athletes and bodybuilders may have a high BMI while carrying little body fat, which makes BMI a poor measure for them.
- Does BMI differ by age or sex?
- The standard adult categories are the same regardless of age or sex, but interpretation can vary. Older adults often carry more body fat at the same BMI, women generally have more body fat than men at the same BMI, and children and teens use age- and sex-specific percentile charts instead of fixed thresholds.
- What are the BMI categories?
- Using WHO thresholds: underweight is a BMI below 18.5, normal or healthy weight is 18.5 to 24.9, overweight is 25 to 29.9, and obese is 30 or above. These apply to most adults aged 20 and over.
- Should I see a doctor about my BMI?
- This calculator is for general information only and is not medical advice, diagnosis, or treatment. BMI is a screening tool, not a measure of health on its own. If you have concerns about your weight or health, speak with a qualified healthcare professional who can consider your full circumstances.