1. What is BMI Z-Score?

The BMI z-score (standard deviation score) indicates how many units of standard deviation a child's BMI is above or below the average BMI for their age and gender. It's used to determine BMI percentile in growth charts.

2. How Does the Calculator Work?

The calculator uses the z-score equation:

Where:

• \( BMI \) — Body Mass Index (kg/m²)
• \( M \) — Median BMI for specific age and gender (from growth charts)
• \( S \) — Standard deviation for specific age and gender

Explanation: The z-score shows how far and in what direction the child's BMI deviates from the population mean, converted to standard deviation units.

3. Importance of BMI Percentile

Details: BMI percentile is the most common indicator to assess size and growth patterns in children. It helps identify underweight (<5th percentile), healthy weight (5th-85th), overweight (85th-95th), and obesity (≥95th).

4. Using the Calculator

Tips: Enter the child's BMI (calculated as weight in kg divided by height in meters squared), the median BMI (M) for their exact age and gender from CDC/WHO growth charts, and the corresponding standard deviation (S).

5. Frequently Asked Questions (FAQ)

Q1: Where can I find M and S values?
A: CDC growth charts provide LMS values (L=skewness, M=median, S=variation) for each age and gender.

Q2: How does z-score relate to percentile?
A: Z-score of 0 = 50th percentile, +1 = ~84th, +2 = ~98th, -1 = ~16th, -2 = ~2nd percentile.

Q3: Why use z-scores instead of percentiles?
A: Z-scores are better for statistical calculations and tracking changes in extreme BMI values (>97th or <3rd percentiles).

Q4: What's the age range for this method?
A: Typically used for children and adolescents aged 2-20 years.

Q5: How often should BMI be monitored?
A: For children, BMI should be calculated and plotted at least annually, more frequently if tracking growth concerns.