BMI Calculator
Check your Body Mass Index instantly
What is BMI Calculator?
Body Mass Index (BMI) is a widely used screening tool that estimates body fatness by relating a person's weight to their height. Calculated as weight in kilograms divided by height in metres squared (kg/m²), it provides a quick numerical indicator of whether someone falls into an underweight, normal weight, overweight, or obese category.
The World Health Organisation (WHO) defines the standard BMI ranges as: Underweight (below 18.5), Normal weight (18.5–24.9), Overweight (25–29.9), Obesity Class I (30–34.9), Obesity Class II (35–39.9), and Obesity Class III (40 and above). For children and teenagers, age and sex-specific percentile charts are used instead of fixed ranges.
While BMI is a useful population-level screening tool, it has recognised limitations — it does not directly measure body fat percentage, and can misclassify muscular athletes as overweight or underweight elderly individuals as normal. For a comprehensive assessment of health, BMI should be considered alongside other measurements such as waist circumference, blood pressure, and cholesterol levels. Altairys's BMI Calculator supports both metric (kg/cm) and imperial (lb/in) inputs and shows your result alongside the WHO classification scale so you can contextualise where you sit.
How to Use BMI Calculator
- Enter your height
Input your height in centimetres (metric) or feet and inches (imperial).
- Enter your weight
Input your weight in kilograms (metric) or pounds (imperial).
- See your BMI
Your BMI is calculated and displayed immediately with your WHO classification.
- Understand your result
Read what your BMI category means and what ranges are considered healthy.
Key Benefits
Switch between metric (kg/cm) and imperial (lb/ft) without re-entering data.
See where you fall on the WHO BMI scale with a clear visual indicator.
BMI updates in real time as you adjust height or weight values.
Get clear explanations of what each BMI range means for health.
Frequently Asked Questions
According to the WHO, a BMI between 18.5 and 24.9 is considered normal weight for adults. Below 18.5 is underweight; 25–29.9 is overweight; 30 and above is obese.
BMI can overestimate body fat in athletes who have high muscle mass. A muscular person may have a high BMI while having very low body fat percentage.
The standard BMI formula is the same, but women naturally have a higher body fat percentage than men at the same BMI. Some health professionals use sex-adjusted reference values.
No. Children and teenagers should use age and sex-specific BMI percentile charts, not the adult ranges. Consult a paediatrician for assessment.
BMI is a screening tool, not a diagnostic one. If your BMI is elevated, consult a doctor who can assess other health markers including waist circumference, blood pressure, and fitness level.