1. What is BMI?

Body Mass Index (BMI) is a measure that uses height and weight to work out if weight is healthy. For children and teens, BMI is age- and sex-specific and is often referred to as BMI-for-age.

2. How Does the Calculator Work?

The calculator uses the BMI formula:

Where:

• \( weight \) — Weight in kilograms (kg)
• \( height \) — Height in meters (m)

For children: The calculated BMI should be plotted on a BMI-for-age growth chart to determine the percentile.

3. Importance of BMI Calculation

Details: BMI is a screening tool to identify possible weight problems in children and teens. It's not a diagnostic tool but can indicate if further assessment is needed.

4. Using the Calculator

Tips: Enter weight in kg, height in meters, and optionally age and gender for children. All values must be valid (weight > 0, height > 0).

5. Frequently Asked Questions (FAQ)

Q1: What is a healthy BMI for children?
A: For children, BMI is interpreted using age- and sex-specific percentiles. Generally:

• Underweight: <5th percentile
• Healthy weight: 5th to <85th percentile
• Overweight: 85th to <95th percentile
• Obese: ≥95th percentile

Q2: Why is BMI different for children?
A: Children's body composition varies with age and between boys and girls, so their BMI needs to be compared against growth charts.

Q3: When should BMI be measured in children?
A: The NHS recommends measuring BMI annually from age 2 as part of routine growth monitoring.

Q4: Are there limitations to BMI?
A: BMI doesn't distinguish between fat and muscle mass. Athletic children may have high BMI without excess fat.

Q5: Where can I find NHS growth charts?
A: NHS growth charts are available from your GP or school nurse, and online through official NHS resources.