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How Weight-for-Age Percentiles Work (2-20 Years)

This calculator uses CDC growth reference data for children and adolescents ages 2 to 20 years. The CDC growth charts are based on national survey data collected from 1963 to 1994 and represent how American children actually grew during that period.

The calculator uses the LMS method (Lambda-Mu-Sigma), a statistical technique that summarizes the distribution of weight at each age using three parameters. Lambda (L) accounts for the skewness of the distribution, Mu (M) is the median, and Sigma (S) is the coefficient of variation.

Understanding Percentiles

A percentile indicates the percentage of children who weigh less than your child. For example, if your child is at the 75th percentile, 75% of children of the same age and sex weigh less.

  • 5th to 85th percentile is considered the healthy weight range
  • Below the 5th percentile may indicate underweight
  • 85th to 95th percentile may indicate overweight
  • Above the 95th percentile may indicate obesity
  • The trend over time matters more than a single measurement

Example Calculation

A 10-year-old boy (120 months) weighing 32.53 kg would be approximately at the 50th percentile (Z-score near 0), since the CDC median weight for boys at 120 months is about 32.53 kg.

Frequently Asked Questions

What age range does this calculator cover?

This calculator uses CDC growth charts for children and adolescents from 2 to 20 years (24 to 240 months). For children under 2 years, use one of our infant growth calculators.

Should I use my child’s weight in kg or pounds?

You can use either unit. The calculator converts pounds to kilograms automatically. For the most accurate result, use the weight recorded at your pediatrician’s office.

How is this different from the WHO calculators?

The CDC growth charts are reference charts based on how American children actually grew, while WHO charts are standards based on how children should grow under optimal conditions. The CDC recommends using WHO standards for children under 2 and CDC charts for ages 2-20.

Are these the same charts my doctor uses?

Yes. In the United States, the CDC recommends these growth charts for clinical use in children and adolescents ages 2 to 20 years.

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