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BMI Calculator — Step-by-Step Solutions

The BMI Calculator computes your Body Mass Index using the standard formula BMI = weight / height². It supports both metric (kg/cm) and imperial (lb/ft/in) units and classifies your result according to WHO categories.

Enter your weight and height to see your BMI, category classification, and healthy weight range. The gauge visualisation shows where you fall on the BMI scale.

Your calculations will appear here

How to Use the BMI Calculator

  1. Select your unit system: metric (kg and cm) or imperial (lbs and feet/inches).
  2. Enter your weight in the appropriate field.
  3. Enter your height in the appropriate field.
  4. Click "Calculate" to see your BMI value and the corresponding WHO category.

Formula and Theory

The Body Mass Index is calculated as: BMI = weight (kg) / height (m)^2. In imperial units: BMI = (weight (lbs) * 703) / height (inches)^2. It provides a simple numeric measure of a person's body weight relative to their height.

The WHO classifies BMI into categories: Underweight (below 18.5), Normal weight (18.5 to 24.9), Overweight (25.0 to 29.9), Obesity Class I (30.0 to 34.9), Obesity Class II (35.0 to 39.9), and Obesity Class III (40.0 and above).

BMI is a screening tool, not a diagnostic one. It does not account for muscle mass, bone density, age, sex, or fat distribution. Athletes with high muscle mass may have a high BMI without being overweight. Waist circumference and body fat percentage provide complementary information.

Worked Examples

Example 1: Normal weight

Problem: Calculate the BMI for a person weighing 70 kg and 175 cm tall.

Solution: BMI = 70 / (1.75)^2 = 70 / 3.0625 = 22.86.

Answer: BMI = 22.9 (Normal weight)

Example 2: Overweight

Problem: Calculate the BMI for a person weighing 95 kg and 180 cm tall.

Solution: BMI = 95 / (1.80)^2 = 95 / 3.24 = 29.32.

Answer: BMI = 29.3 (Overweight)

Example 3: Imperial units

Problem: Calculate the BMI for a person weighing 154 lbs and 68 inches tall.

Solution: BMI = (154 * 703) / (68)^2 = 108,262 / 4,624 = 23.42.

Answer: BMI = 23.4 (Normal weight)

Example 4: Real-world: tracking progress

Problem: A person at 90 kg and 170 cm loses 5 kg. How does their BMI change?

Solution: Before: BMI = 90 / 1.70^2 = 31.1 (Obese I). After: BMI = 85 / 1.70^2 = 29.4 (Overweight).

Answer: BMI drops from 31.1 to 29.4, moving from Obese I to Overweight

Frequently Asked Questions

What is a healthy BMI?
A BMI between 18.5 and 24.9 is classified as normal weight by the WHO. However, health depends on many factors beyond BMI, including diet, exercise, body composition, and family history.
Is BMI reliable for everyone?
BMI does not distinguish between muscle and fat. It can overestimate body fat in muscular individuals and underestimate it in older people who have lost muscle mass. It is less accurate for pregnant women, children, and highly trained athletes.
What are alternatives to BMI?
Waist-to-hip ratio, body fat percentage, and waist circumference are additional indicators of health risk. These measures can provide a more complete picture when used alongside BMI.
Does BMI differ by age or sex?
The standard BMI formula does not adjust for age or sex. Some health organisations use age-specific and sex-specific percentile charts, particularly for children and adolescents, where growth patterns vary significantly.

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