BMI Calculator

What is BMI?

Body Mass Index (BMI) is a measure that uses your height and weight to estimate body fat. It was developed in the 19th century and is widely used by health professionals as a screening tool. BMI provides a simple numeric value that helps categorize weight status. It is not a direct measure of body fat but correlates with body fat in most adults. BMI is most useful for identifying potential weight-related health risks in the general population.

How to Use the Calculator

Select your preferred unit system (metric or imperial). Enter your weight and height in the appropriate fields. For metric: use kilograms for weight and centimeters for height. For imperial: use pounds for weight, and feet and inches for height. Click Calculate to see your BMI and classification. The result shows your BMI value (rounded to one decimal) and the WHO category for adults.

The Formula

BMI is calculated using the formula:

BMI = weight (kg) / height² (m)

For metric inputs: height in centimeters is converted to meters by dividing by 100. For imperial inputs: weight in pounds is converted to kg (÷ 2.20462), and height in feet and inches is converted to meters using (ft × 12 + in) × 0.0254.

Examples

Example 1: Metric

A person who weighs 70 kg and is 175 cm tall: BMI = 70 / (1.75)² = 70 / 3.0625 = 22.9. Result: Normal weight.

Example 2: Imperial

A person who weighs 154 lb and is 5'9" (5 ft 9 in): Height = (5 × 12 + 9) × 0.0254 = 1.75 m. Weight = 154 / 2.20462 = 69.85 kg. BMI = 69.85 / (1.75)² = 22.8. Result: Normal weight.

Example 3: Overweight

A person who weighs 90 kg and is 170 cm tall: BMI = 90 / (1.70)² = 31.1. Result: Obese (Class I).

Frequently Asked Questions

What is a healthy BMI?

According to WHO guidelines for adults, a BMI between 18.5 and 24.9 is considered normal or healthy weight. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obese.

Does BMI apply to athletes?

BMI may not accurately reflect body composition in athletes or very muscular individuals, as muscle weighs more than fat. Athletes with high muscle mass may have a high BMI despite having low body fat.

What are the input limits?

We validate inputs to reasonable ranges: height 50–250 cm (or equivalent in ft/in), weight 2–300 kg (or 4–660 lb). These limits help avoid errors and improve user experience.

Is BMI accurate for everyone?

BMI is a screening tool, not a diagnostic. It does not account for age, sex, muscle mass, bone density, or ethnic differences. Children and adolescents use different BMI-for-age charts. Consult a healthcare provider for personalized assessment.

What standard does this calculator use?

We use WHO (World Health Organization) adult BMI classification standards. The categories are: Underweight (<18.5), Normal (18.5–24.9), Overweight (25–29.9), Obese (≥30).